ArticlePDF Available

Mechanisms of biogenic gas migration revealed by seep carbonate paragenesis, Panoche Hills, California

Authors:

Abstract and Figures

A comprehensive study of seep carbonates at the top of the organic-rich Maastrichtian to Danian Moreno Formation in the Panoche Hills (California) reveals the mechanisms of generation, expulsion, and migration of biogenic methane that fed the seeps. Two selected outcrops show that seep carbonates developed at the tip of sand dykes intrude up into the Moreno Formation from deeper sandbodies. Precipitation of methane-derived cements occurred in a succession of up to 10 repeated elementary sequences , each starting with a corrosion surface followed by dendritic carbonates, botryoidal aragonite, aragonite fans, and finally laminated micrite. Each element of the sequence reflects three stages. First, a sudden methane pulse extended up into the oxic zone of the sediments, leading to aerobic oxidation of methane and carbonate dissolution. Second, after consumption of the oxygen, anaerobic oxidation of methane coupled with sulfate reduction triggered carbonate precipitation. Third, progressive diminishment of the methane seepage led to the deepening of the reaction front in the sediment and the lowering of precipitation rates. Carbonate isotopes, with d 13 C as low as-51‰ Peedee belemnite, indicate a biogenic origin for the methane, whereas a one-dimensional basin model suggests that the Moreno Formation was in optimal thermal conditions for bacterial methane generation at the time of seep carbonate precipitation. Methane pulses are interpreted to reflect drainage by successive episodes of sand injection into the gas-generating shale of the Moreno Formation. The seep carbonates of the Panoche Hills can thus be viewed as a record of methane production from a biogenic source rock by multiphase hydraulic fracturing.
Content may be subject to copyright.
Mechanisms of biogenic gas
migration revealed by seep
carbonate paragenesis, Panoche
Hills, California
Jean-Philippe Blouet, Patrice Imbert, and
Anneleen Foubert
ABSTRACT
A comprehensive study of seep carbonates at the top of the
organic-rich Maastrichtian to Danian Moreno Formation in the
Panoche Hills (California) reveals the mechanisms of generation,
expulsion, and migration of biogenic methane that fed the seeps.
Two selected outcrops show that seep carbonates developed at
the tip of sand dykes intrude up into the Moreno Formation from
deeper sandbodies. Precipitation of methane-derived cements
occurred in a succession of up to 10 repeated elementary se-
quences, each starting with a corrosion surface followed by
dendritic carbonates, botryoidal aragonite, aragonite fans, and
nally laminated micrite. Each element of the sequence reects
three stages. First, a sudden methane pulse extended up into the
oxic zone of the sediments, leading to aerobic oxidation of
methane and carbonate dissolution. Second, after consumption
of the oxygen, anaerobic oxidation of methane coupled with
sulfate reduction triggered carbonate precipitation. Third,
progressive diminishment of the methane seepage led to the
deepening of the reaction front in the sediment and the lowering
of precipitation rates. Carbonate isotopes, with d
13
Caslow
as -51Peedee belemnite, indicate a biogenic origin for the
methane, whereas a one-dimensional basin model suggests that the
Moreno Formation was in optimal thermal conditions for bacterial
methane generation at the time of seep carbonate precipitation.
Methane pulses are interpreted to reect drainage by successive
episodes of sand injection into the gas-generating shale of the
Moreno Formation. The seep carbonates of the Panoche Hills can
thus be viewed as a record of methane production from a biogenic
source rock by multiphase hydraulic fracturing.
AUTHORS
Jean-Philippe Blouet ~Department of
Geosciences, University of Fribourg, Chemin
du Mus´
ee 6, 1700 Fribourg, Switzerland;
jean-philippe.blouet@unifr.ch
Jean-Philippe Blouet is a research geoscientist
with more than 10 years of experience in
carbonate concretions. He has a double M.Sc.
degree in petroleum geology awarded from
the University of Nancy (France). He worked at
Totals headquarters from 2012 to 2014. He
is currently researching the petroleum
signicance of seep carbonates in an industry-
funded Ph.D. project with Total at the
University of Fribourg (Switzerland).
Patrice Imbert ~Centre Scientique et
Technique Jean F´
eger, Total, Avenue
Larribau, 64000 Pau, France; patrice.
imbert@total.com
Patrice Imbert, Ph.D., is a geologist at Total
Exploration and Production, Research and
Development division. He specialized rst in
turbidite sedimentology and seismic
interpretation in Totals headquarters and
occupied various positions overseas in
exploration and reservoir geology. His
current research interests are uid ow in
sedimentary basins, with a special focus on
the seismic expression of hydrocarbon
migration and expulsion, present and past.
Anneleen Foubert ~Department of
Geosciences, University of Fribourg, Chemin
du Mus´
ee 6, 1700 Fribourg, Switzerland;
anneleen.foubert@unifr.ch
Anneleen Foubert has been an associate
professor of carbonate sedimentology at the
University of Fribourg (Switzerland) since
February 2013. Previously, she worked as
researcher at the University of Leuven
(Belgium) and as carbonate sedimentologist
at Total (Pau, France). Trained as a marine
geologist at Ghent University (Belgium), she
specializes in the study of carbonate mound
systems through space and time.
ACKNOWLEDGMENTS
We thank Total Exploration and Production
for the nancing of Jean-Philippe Blouet, for
granting permission to publish this research,
Copyright ©2016. The American Association of Petroleum Geologists. All rights reserved. Gold Open
Access. This paper is published under the terms of the CC-BY license.
Manuscript received February 16, 2016; provisional acceptance February 16, 2016; revised manuscript
received August 5, 2016; nal acceptance October 17, 2016.
DOI:10.1306/10171616021
AAPG Bulletin, v. nn, no. nn (nn 2016), pp. 131 1
INTRODUCTION
Seep carbonates result from hydrocarbon migration up to the
seabed at sites called cold seeps (Paull et al., 1984; Boetius and
Wenzh¨
ofer, 2013). They precipitate during the biocatalyzed
anaerobic oxidation of methane (AOM) and by sulfate reduction
in the sulfatemethane transition zone (SMTZ; Boetius et al.,
2000). The rst direct observations of seep carbonates pre-
cipitating on the sea oor originated from Hovland et al. (1985)
and Ritger et al. (1987). Later, Roberts (2001) demonstrated that
seep carbonate facies were controlled by the hydrocarbon ux dy-
namics and by the depth of the SMTZ in the sediment (Roberts,
2001). Regnier et al. (2011) showed that seep carbonates could
precipitate at the seabed if the methane ux were sufcient to
exceed the rate of sulfate diffusion into the sediment or within
the subseaoor at diffusion rates. Paull and Ussler (2008) also re-
ported that migrating gas could shift the SMTZ upward and pre-
cipitate a greater amount of carbonate. Furthermore, the abundance
of carbonate decreases and carbonate depth below seabed increases
away from the venting site. In particular, where the SMTZ is close to
the seabed, it can support the establishment of a chemosyn-
thesis-based megafauna (Gay et al., 2006; Judd and Hovland,
2007) dominated by mollusks, annelids, and crustaceans (Kiel,
2010). Gas seepage intensity varies over various time scales
(Naudts et al., 2010; Ho et al., 2012) as a function of migration
mechanisms and pathways, typically faults, hydrofractures, or
sand injectites (Mazzini et al., 2003; Duranti and Mazzini, 2005;
Jonk et al., 2005; Hurst and Cartwright, 2007; Ho et al., 2016).
The Panoche giant injection complex (California) is one of the
worlds largest sand injection complexes. It is an ensemble of
intrusive sedimentary features produced by elevation of pore
pressure in a sand reservoir leading to hydrofracturation of a
sealing strata, uidization, and injection of the sand slurry (Hurst
et al., 2011). The Panoche giant injection complex developed
from the sandy turbidites of the Upper Cretaceous Panoche
Formation through the overlying Maastrichtian to Danian low
porosity Moreno Formation along a vertical section of 1200 m
(0.75 mi) and over at least 300 km
2
(115 mi
2
). The Panoche giant
injection complex has been extensively studied by many authors
(Vigorito et al., 2008; V´
etel and Cartwright, 2010; Vigorito and
Hurst, 2010; Scott et al., 2013). Schwartz et al. (2003) and
Minisini and Schwartz (2007) described seep carbonates at the
top of the Moreno Formation. The authors correlated seep car-
bonate distributions over several kilometers to the presence of
underlying sand dykes emanating from the Panoche Formation,
thereby providing evidence that the injectites served as hydro-
carbon migration pathways. However, the source rock was not
identied, and the migration mechanisms were not detailed in
these publications. In general terms, although seep carbonates are
and for many insightful discussions, in
particular with geoscientists of the
sedimentology and geochemistry research
teams. The eld campaign in 2013 was
greatly facilitated by Ivano Aiello, who
granted access to the Moss Landing Marine
Laboratories, and Hilde Schwartz is
acknowledged for providing access to the
library of the University of Santa Cruz. Both
Hilde and Ivano greatly helped to guide the
early eld reconnaissance. Sutieng Ho is
greatly thanked for assisting the completion
of this manuscript and for her useful advice
and stimulating discussions of the work. We
thank Kristin and Everett Meagher Robinson
for their warm welcome in California. We are
grateful to Giovanni Aloisi, who offered
fruitful insights into the subject in its early
stages. Thin Section Lab and its personnel are
thanked for providing access to their facilities.
We are grateful to Robin Fentimen for his
correction of the English language. Reviews of
the original manuscript by Martin Hovland
and an anonymous reviewer were
constructive and benecial.
EDITORSNOTE
Color versions of Figures 115 can be seen in
the online version of this paper.
2Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
obvious hydrocarbon indicators, examples of seep
carbonates used as a tool to reconstruct a complete
petroleum system still remain scarce. A petroleum
system, as dened by Magoon and Dow (1994), is the
concept unifying the essential elements of petroleum
geology (source rock, reservoir, seal, overburden),
processes (generation, migration, accumulation), and
that includes shows and seeps.
Basedondetailedstudyoftwooutcropsinthe
Panoche Hills, identication of a recurrent paragenetic
sequence, and distribution of a chemosynthetic fossil
fauna, this study evaluates the paleouid ux. It
also addresses the issue of hydrocarbon generation
using one-dimensional (1-D) thermal modeling of
the organic-rich Moreno Formation and its potential
as a biogenic source rock. By combining an assessment
of the seepage style at the sea oor and the fracturing
mechanisms of the source rock at depth, this paper
proposes a comprehensive model for the generation,
expulsion, and migration processes of methane in the
Panoche Hills during the earliest Paleocene.
GEOLOGICAL SETTING
The Panoche Hills are located in the Coast Range
along the western edge of the San Joaquin Valley in
central California, approximately 200 km (125 mi)
southeast of San Francisco. The San Joaquin Valley
constitutes the southern part of the Great Valley,
anorthwestsoutheast elongated basin, 700 km
(435 mi) long and 100 km (62 mi) wide (Figure 1).
Geodynamic Context
From the Late Jurassic to the Oligocene, the Great
Valley was a forearc basin, located between the Sierra
Nevada magmatic arc to the west and the Franciscan
subduction complex to the east (Dickinson and Seely,
1979; Ingersoll, 2008). It is lled with up to 9 km
(6 mi) of sediment, mostly composed of shale and
turbidite (Bailey et al., 1964; Bartow and Nilsen, 1990).
During the Neogene, tectonic plate motion switched
from convergent to strike slip, leading to the em-
placement of the San Andreas Fault and associated
uplift of the western side of the Great Valley (Atwater
and Molnar, 1973; Graham et al., 1984; Norris and
Webb, 1990). As a consequence, the western edge of
the Great Valley is at present tilted approximately
40° toward the axis of the San Joaquin Valley, and
a nearly complete sedimentary inll from the
ophiolitic bedrock up to Oligocene strata is ex-
posed (Figure 2).
Stratigraphy
The oldest exposed sedimentary rocks, in faulted
contact with the Franciscan complex, belong to the
Panoche Formation (Anderson and Pack, 1915). This
6.5-km (4-mi)-thick formation is dated Campanian
to Maastrichtian and comprises conglomeratic sand-
stone in the lower part, transitioning upward to chan-
nelized turbiditic sandstone interbedded with shale. It
corresponds to deep-sea fan deposits sourced from the
eroded Sierra Nevada (Ingersoll, 1979; McGuire,
1988). The overlying Maastrichtian to Danian Moreno
Formation is 450 to 650 m (1475 to 2130 ft) thick in
the study area (Scott et al., 2013) and is composed of
ne-grained units divided into ve Members: the
Dosados, Tierra Loma Shale, Marca Shale, Dos Palos
Shale, and Cima Sandstone Lentil (Payne, 1951). The
Dos Palos Shale Member contains the Cima Sandstone
Lentil. The dominant lithology of the formation is
clayey, with diatomite in the Marca Shale Member.
The Cima Sandstone Lentil is silty to sandy. From the
Dosados to the Marca Shale Members, laminated
facies dominate, and benthic biotas are rare, thus in-
dicating deposition in anoxic conditions, whereas
suboxic conditions are suggested in the nonlaminated
Dos Palos Shale Member (McGuire, 1988). Bio-
stratigraphic analysis of the series by Martin (1964)
showed a progressive overall shoaling throughout the
Moreno Formation, which Martin (1964) interpreted
as a transition from the base of slope to a shelfal
environment.
Concretions interpreted as seep carbonates
(Schwartz et al., 2003) are present in the Dos Palos
Shale Member, up to the Cima Sandstone Lentil
stratigraphic level (Minisini and Schwartz, 2007).
However, they are isolated and rare in the Dos Palos
Shale Member and are abundant only in the Cima
Sandstone Lentil. Minisini and Schwartz (2007)
distinguished three types of concretions based on
their overall geometry. They dened (1) nodules as
subspherical masses with diameters ranging from
a few inches to several yards; (2) mounds as irregular,
roughly conical bodies a few yards in diameter; and
(3) crusts as stratiform bodies.
BLOUETETAL. 3
The Moreno Formation is unconformably overlain
by Paleocene and Eocene marine deposits. During
the Oligocene and Neogene, the northern part of
the San Joaquin Valley was uplifted, resulting in de-
position of continental to very shallow water deposits
(Bartow, 1991).
The Panoche Giant Injection Complex and Its
Relation to Seepage
Extensively studied over the past two decades (Hurst
and Cartwright, 2007), the Panoche Hills host one of
the best exposed sand injection complexes in the
world. Sands, from the Panoche Formation, intruded
more that 1 km (0.6 mi) upward (not decompacted)
into the overlying low-permeability seal constituted
by the Moreno Formation. Most injectites termi-
nate below the Dos Palos Shale Member, with a few
reaching the Cima Sandstone Lentil (Weberling,
2002; Scott et al., 2013). Based on sandstone grain
composition and possible sandvolcano morphologies,
Vigorito et al. (2008) interpreted sandstone beds in
the Cima Sandstone Lentil to be at least partially of
extrusive origin. Therefore, sand extrusion onto the
sea oor dates back to the Danian. The Panoche giant
injection complex thus exposes all three genetic units
involved in the sand remobilization systems: the sand
reservoir unit, the intruded unit, and the extrusive
unit (Figure 2).
Minisini and Schwartz (2007) conducted a sta-
tistical study of the densities of injectites and the
density of overlying seep carbonates over a 20-km
(12-mi)-long outcrop, suggesting that the injectites
acted as a hydrocarbon pathway up to the sea oor.
The distribution of seep carbonates over a thick
sedimentary column suggests that seepage and in-
jectite emplacement spread over a substantial time
interval before sand extrusion on the sea oor
(Weberling, 2002).
Figure 1. Geodynamic setting
of the Panoche Hills outcrop. (A)
Location of the study area along
the western margin of the North
American continent. (B) Simpli-
ed geological map of California.
(C) Schematic cross section of the
western American plate during
the early Paleocene. (D) Recent
geological prole (modied from
Dickinson and Seely, 1979, with
permission of AAPG).
4Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
Source-Rock Properties
The alternation of coarse turbidites and organic-poor
shale of the Panoche Formation is not considered
a source rock for potential hydrocarbons (He et al.,
2014). In contrast, the Moreno Formation is the only
known Cretaceous source rock in the region (McGuire,
1988; Peters et al., 2007). Peters et al. (2007) de-
termined an original total organic carbon (TOC)
value of the Moreno Formation as high as 3.7%
based on 326 samples from 11 wells. In particular,
from well Exxon 1 Chounet Ranch, located 11 km
(7 mi) westward from the studied outcrop,
McGuire (1988) reported an average TOC of 3%
for the laminated lower part of the Moreno For-
mation (i.e., the Dosados Member, the Tierra Loma
Shale Member, and the Marca Shale Member) and
1.2% TOC for the Dos Palos Shale Member. Bio-
marker analyses by Bac (1990) on cores of Exxon 1
Chounet Ranch indicate a marine origin for the
organic matter.
MATERIALS AND METHODS
Carbonate concretions, widespread in the Cima Sand-
stone Lentil, have been observed in detail along
a 4.5-km (2.8-mi)-long, nearly continuous, outcrop
in the central part of the Panoche Hills, from the
Dosados Canyon to Panoche Creek (Figure 3). This
paper distingushes the three types of concretions
recognized by Minisini and Schwartz (2007) (nodule,
mound, crust).
Polished slabs of rock were observed in natural light
and 365-nm ultraviolet light (UV); 50 thin sections were
examined with optical microscopy using plane-polarized
light, cross-polarized light, and cathodoluminescence
(CL). The CL was generated by a CITL system, model
CCL 8200 ink4 (12 kV, 450 mA).
Scanning electron microscope (SEM) observa-
tions were carried out on fresh, fractured surfaces
coated with gold and on thin sections coated with
carbon. Energy dispersive spectrometry (EDS) was
used to determine the qualitative elemental com-
position of carbon-coated samples.
For x-ray diffraction (XRD), 27 samples were
crushed manually in an agate mortar. The powders
were analyzed with a Rigaku Ultima IV diffrac-
tometer system equipped with a Cu x-ray tube, op-
erated at 40 kV and 40 mA, and with a D-Tex linear
detector. Scans were run from 5° to 70°2q,withastep
interval of 0.01°2qand a goniometry speed of 120 s/°2q.
The identication of all minerals was performed
Figure 2. General stratigraphy
and lithology of the Panoche Hills
in the EscarpadoRight Angle
Canyon area (after Scott et al.,
2013, with permission of AAPG).
Total organic carbon (TOC) curve
from McGuire (1988) and He
et al. (2014). Fm. =Formation.
BLOUETETAL. 5
using the Rigaku PDXL2 software package and the
ICDD Powder Diffraction File 2014 database
(International Centre for Diffraction Data).
For stable oxygen and carbon isotope analysis, 78
samples of carbonate were selected. They were taken
from polished blocks and on rock chips using a
handheld microdrill under an alternation of natural
and UV light. Samples were then analyzed using
a Kiel III automated carbonate preparation device
coupled to a Finnigan MAT 252 isotope-ratio mass
spectrometer. Carbonate material reacts with 100%
phosphoric acid for 10 min at 70°C. The CO
2
prod-
uct is then passed through the isotope-ratio mass
spectrometer for masses 44, 45, and 46 measure-
ments alternately with the measurement of a cali-
brated reference CO
2
gas. Instrumental precision is
monitored by analysis of NBS 18, NBS 19, and in
some instances LSVEC reference material. Pre-
cisions are 0.05for carbon and 0.14for
oxygen. Isotope results are given relative to the
Peedee belemnite standard.
RESULTS
At least two major clusters of carbonate concretions
were observed in direct connection with dykes (Figure
3), and these are the main focus of this paper. They
are referred to as outcrop Aand outcrop Bhereafter.
Description of Outcrops A and B
Outcrop A is located in the Dos Palos Shale Member,
several tens of yards below the Cima Sandstone Lentil
(Figure 3). It consists of a sandstone dyke surrounded
in its uppermost 10 m (32 ft) by carbonate nodules,
a few tens of centimeters (<1 ft) in diameter. The dyke
Figure 3. Geological map of
the southeastern Panoche Hills
showing the locations of the
studied outcrops (topographic
map from the US Geological
Survey, 1956). Fm. =Formation.
6Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
tip is capped by a 2-m (7-ft)-diameter mound (Figure
4A). Above outcrop A, The Cima Sandstone Lentil is
poor in concretions and contains only a few nodules up
to 1 m (3.2 ft) in diameter, crosscut by some thin dykes.
Outcrop B is located within the Cima Sand-
stone Lentil (Figure 3). It is composed of a cluster of
ve mounds (»1to3m[3to10ft]indiameter)and
many nodules overlying a dyke swarm (Figure 4B).
Two sets of subvertical dykes, respectively oriented
N070 and N140, are visible over several tens of
meters. Four of the mounds occur at the same
stratigraphic level, and one is located approximately
15 m (50 ft) below the others. Above the mounds,
discontinuous stratiform concretions (crusts, oblate
nodules) are widely distributed in this part of the
Cima Sandstone Lentil.
Authigenic Carbonates
This section focuses on the mesoscopic characteristics
of the carbonate concretions, whereas the petrogra-
phy and geochemistry are analyzed in the Petrogra-
phy of Carbonates and Stable Isotope Geochemistry
sections, respectively.
Mounds
The mounds of outcrops A and B are characterized by
a mottled carbonate fabric (Figure 5A) comprising light-
gray patches of micrite in a brown to greenish micro-
sparitic matrix. The boundary between the two colors
ranges from smooth to sharp (Figure 5B, C). In places,
brecciated pieces of the gray lithology are included in
the brown one. The mounds are crosscut by randomly
oriented septarian cracks up to 50 cm (20 in.) long.
Mounds differ from other carbonate concretions by
the presence of numerous cylindrical tubes often coated
by a cortex of light-gray carbonate, lled with sev-
eral generations of cements (Figure 5C). The tubes are
approximately 1.5 cm (0.6 in.) in diameter, randomly
oriented and occasionally branched (Figure 5D). They
are identied as cemented Thalassinoides burrows.
Nodules
The nodules, which are spatially associated with
the mounds of outcrops A and B, are smooth and
subspherical reaching approximately 1 ft (0.3 m) in
diameter. Nodules consist of gray to yellow micrite
organized in concentric layers. The layers are con-
tinuous and isopachous in the central part of the
nodules and commonly discontinuous in the external
part (Figure 5E).
Dykes
Geometry
Two sets of dykes are present in outcrop B. The rst
consists of four subvertical dykes trending N140, and
the other comprises approximately 10 subvertical
dykes trending N70. The latter extend upward to
the highest stratigraphic level, a few yards below the
lowest mound. A dyke oriented N70 offsets one of
the dykes oriented N140 (Figure 4), indicating that
the second dyke set postdates the rst. The maximum
thickness observed for the dykes is approximately
30 cm (12 in.); their length ranges from a few meters
to 150 m (500 ft). They are characterized by abun-
dant en echelon segmentation (Figure 6A), a char-
acteristic described as common in the area by V´
etel
and Cartwright (2010).
Outcrop A contains a single subvertical dyke
trending N55. The thickness of the dyke decreases
regularly from 20 cm (8 in.) at the bottom of the
outcrop (~20 m [65 ft] beneath the mound) to 2 cm
(0.8 in.) 1 m [39 in.] below it. Dyke walls are smooth
or locally covered by ute casts, with the tail pointing
upward (Figure 6B), indicating an upward propaga-
tion of the sand slurry.
Petrography
Samples from dykes underlying concretions are com-
posed of ne-grained quartzo-feldspathic sandstone
(<500 mm) cemented by equant calcite with un-
dulating extinction and locally replaced by iron oxides
(Figure 6C).
Fauna
Fossils in the Dos Palos Shale and Cima Sandstone
Lentil are rare except for wood fragments commonly
heavily perforated by xylophagous bivalves.
Bivalves,tubeworms,andThalassinoides are
present in outcrop B (Figure 7A), whereas outcrop
AonlyshowsThalassinoides.Thalassinoides are
common in mounds of the Panoche Hills but are
BLOUETETAL. 7
Figure 4. View of the two studied outcrops showing the relationship between dykes and overlying seep carbonates. Observed dykes are
indicated by solid lines, whereas inferred dykes are represented as dotted lines. (A) Outcrop A. The upper part of the dyke is surrounded by
nodules, and its tip is directly capped by a mound. The dyke is observed as fragments, and two excavations show it in situ. (B) Outrcop B:
a cluster of mounds and nodules in the Cima Sandstone Lentil overlying a dyke swarm (directions of the dykes indicated by white arrows).
Away from the dyke swarm, the concretions of the Cima Sandstone Lentil comprise only crusts and nodules.
8Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
Figure 5. Macroscopic characteristics of mounds and nodules. (A) Typical structure of a mound, protruding from the sandy Cima
Sandstone Lentil. The brown/gray mottled fabric is clearly visible. (B) Two types of contact between the gray micrite and the brown
microsparite: brecciated (bottom of the picture) and smooth (top of the picture). (C) Two tubes lled with cements. They are hosted in
a light-gray micrite surrounded by a brown microsparite. The contact between the gray and the brown matrices is smooth. (D) An isolated
Thalassinoides burrow cemented by carbonate and gypsum. (E) A large nodule. The external layer is partially covering the inner layers. A
neomorphosed crust of gypsum coats the nodule. (F) Cross section of a nodule. Concentric growth layers are visible, the inner layers are
continuous and isopachous, whereas the external layer is discontinuous. This nodule is partially epigenized in gypsum. Note: A color version
can be seen in the online version.
BLOUETETAL. 9
Figure 6. Dykes in the Moreno Formation. (A) A discontinuous dyke with overlapping segments (outcrop B). (B) Flute casts with tips
orientated upward (outcrop A). (C) Thin section of sandstone dyke composed of quartz, feldspar, and some shale clasts cemented by
calcite.
10 Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
rare within the shale or other types of concretions
(crust and nodules).
In outcrop B, the distribution of fauna is strictly
restricted to a roughly 50-m (165-ft)-wide area
around the mounds, from a few meters (a few yards)
under the tip of the highest dyke up to the top of the
outcrop (Figure 4). The most common clams have
been identied by Kiel (2013) as the lucinids
Nymphalucina panochensis. In addition, rare, possi-
ble mytilids and other species of bivalves are
present (Figure 7B, C). Rare tubes found as rela-
tively straight broken segments at least 10 cm
(0.3 ft) long and exhibiting perfectly circular sec-
tions less than 1 cm (0.4 in.) in diameter (enlarging
progressively through the segments) are attrib-
uted to the Vestimentifera by Schwartz et al.
(2003). The density of fossils is approximately 1/m
2
(1/11 ft
2
). Fossils are undeformed in mounds and
are generally found compacted in the surrounding
shale.
PETROGRAPHY OF CARBONATES
Mounds and nodules from outcrops A and B have
very similar microscopic petrography, so they are
described together. A detailed petrographic de-
scription of all minerals is summarized in Table 1,
following the layout and style of Campbell et al.
(2002).
Based on the observed succession of cements
and on petrographic and geochemical analyses, the
events of the paragenetic sequence have been
grouped into early and late diagenetic stages. A
similar succession of events can be observed in
nodules and concretions of both outcrops A and B,
although some cements can be missing locally. The
cements are therefore numbered according to the
complete (composite) sequence from N°1 to N°19.
The succession of cements N°3 to N°6 can repeat
several times and dene a specicstratigraphic
pattern.
Figure 7. Fauna of outcrop B. (A) A lucinid bivalve in life position and a tube worm. (B) Close-up view of the lucinid Nymphalucina
panochensis. (C) An uncommon clam, possibly a mytilid.
BLOUET ET AL. 11
Table 1. Petrographic Characteristics of the Diagenetic Minerals
Mineral Phase Given Name
and Symbol Mineralogy Petrographic Character Crystal Morphology Crystal Size
Ultraviolet
Fluorescence Cathodoluminescence
Shale and sandstone Clay, Qtz, Fsp Anhedral ~3mm None None
Micrite (N°1) HMC, Qtz, Fsp, Dol,
Py
Brown, siliciclastic-rich micrite;
sparse presence of framboidal
pyrite
Anhedral ~3mm Zonation yellow to
brown
None or very dull
orange
Peloid-rich micrite
(N°2)
(Impossible to
isolate for XRD)
Geopetal deposits of micritic peloids
embedded in micritic matrix;
peloidsdiameters range from 50
to 500 mm (rarely 1 mm); locally
epigenised in gypsum
Anhedral ~3mm Yellow to brown None
Micritic rim (N°3) HMC, Arg, Brt Dark-brown micrite with sparse
isolated sparite; at SEM scale,
appears composed of a vast
diversity of crystal morphology;
locally epigenised in gypsum
Anhedral, radial
brous, subhedral,
dendritic
Maximal ber
length of a few
tens of microns
Orange Mottled, bright to dull
orange; sparite
nonluminescent
Botryoidal aragonite
(N°4)
Arg (?) (impossible
to isolate for XRD)
Light-yellow, radiating fans with
radial extinction, growth lines
marked by thin bands; very rich
in micron-sized uid inclusions;
locally epigenised in gypsum
Botryoidal Up to 300-mm-thick
botryoids
Color overprinted
by adjacent large
fans of aragonite
(N°5)
None
Fans of aragonite
(N°5)
Arg Fans of aragonite needles with
radial extinction; locally
epigenised in gypsum
Needles with
hexagonal
transverse section
Up to 1.5-mm-thick
fans
Strong
uorescence,
white to slightly
orange
None
Laminated micrite
(N°6)
HMC Laminated micrite or microsparite
with very rare siliciclastic
elements, presence of
framboidal pyrite;
nonisopachous and meniscus,
rarely as coating of a nucleus
Anhedral From 3 to 70 mm Light-brown to gray
or none
Dull to moderately
bright orange;
brightness variations
follow lamination
(continued)
12 Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
Table 1. Continued
Mineral Phase Given Name
and Symbol Mineralogy Petrographic Character Crystal Morphology Crystal Size
Ultraviolet
Fluorescence Cathodoluminescence
Microsparite (N°7) Cal Dark-brown mosaic of
microsparite; presence of
siliciclastic elements
Anhedral Up to 100 mmNone Dullorange
HMC
Qtz
Fsp
Sparite (N°8) Cal Undulose extinction, sometimes
radial
Subhedral ~75 mm Blue Homogeneous to
zoned, moderately
bright orange
Dyke cement (N°9) HMC Undulose extinction Anhedral ~100 mm None None
Septarian calcite
(N°10)
Cal Fans divided in different segments
that show undulose extinction
under crossed nicols; contain
thin layers of gray micrite
Elongated crystals,
with jagged
boundary
between each
other
0.2-mm-thick layer Strong
uorescence,
white to greenish
Moderately bright
orange
Septarian calcite
(N°11)
Cal Same features as the previous
cement (N°10) but without
micrite inclusions
Same features as
the previous
cement (N°10)
1- to 2-mm-thick
layer
Weak uorescence,
dirtywhite
None
Septarian micrite
(N°12)
Cal Anisopach layer of gray,
laminated, and clotted micrite
Anhedral ~3mm Moderately dirty
white
Mottled to
homogeneous,
moderately bright
orange
Septarian calcite
(N°13)
Cal Radial brous fans with undulose
extinction; extremely rich in
micron-sized, monophased
liquid uid inclusions; rare
presence of a thin gas bubble;
locally epigenised in Qtz
Needles 800-mm layer None Homogeneous,
moderately bright
orange
Septarian hematite (N°14) Hem Opaque crystals Euhedral 200 mm None None
(continued)
BLOUET ET AL. 13
Table 1. Continued
Mineral Phase Given Name
and Symbol Mineralogy Petrographic Character Crystal Morphology Crystal Size
Ultraviolet
Fluorescence Cathodoluminescence
Septarian micrite
(N°15)
Cal Gray-brown clotted micrite Anhedral None Homogeneous,
moderately bright
orange
Septarian calcite
(N°16)
Cal Colorless in thin section; yellow at
sample scale
Euhedral rhombs 5 mm None Dark to dull orange
Septarian microsparite
(N°17)
Cal Microstalagmitic morphology,
with stalagmite up to 8 mm in
height; gray micrite
Anhedral ~10 mm None or very
slightly brown
Homogeneous,
moderately bright
orange
Septarian quartz (N°18) Qtz Colorless Euhedral hexagonal
prism
1 mm None None
Gypsum (N°19) Gp Ubiquitous in the Moreno Fm.;
brous coating along cracks and
lithological interface
Fibrous Up to several-
centimeters-thick
layers
None None
Abbreviations: Arg 5aragonite; Brt 5barite; Cal 5calcite; Dol 5dolomite; Fm. 5Formation; Fsp 5feldspar; Gp 5gypsum; Hem 5hematite; HMC 5high-Mg calcite; Py 5pyrite; Qtz 5quartz; SEM 5scanning electron
microscope; XRD 5x-ray diffraction.
14 Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
Early Diagenesis
The volumetrically major phase is a silty micrite (N°1).
Macroscopically, it corresponds to the nodules and to
the gray areas in the mounds. The micrite (N°1) is
characterized by a strong yellow to brown UV uo-
rescence. In the mounds, colored zoning can appear as
spotty, or locally layered, often organized as concentric
rims, especially in cortex of burrows and in spherical
concretions (Figure 8A, B).
Different cement generations are present in the
cavities and burrows of the micrite (N°1).
The inll usually starts with a geopetal layer of
a peloid-rich micrite (N°2). The diameters of the
peloids are heterogeneous and range from 50 mmto
1 mm (Figure 8C). Laterally to the peloid-rich micrite
deposits, the sides of the burrows are lined by a
micritic rim (N°3) covered by botryoidal aragonite
(N°4) and then, in syntaxial overgrowth, by fans of
aragonite (N°5) (Figure 8D).
The XRD analyses of the micritic rim (N°3) in-
dicate that the volumetrically major phases are high-
Mg calcite and aragonite. In addition, SEM coupled to
EDS observations reveal a mix of carbonates with high
variability of minor cations, including Mg, Mn, Sr,
and Fe, and the presence of rare crystals of barite
(Figure 8E). The morphology of the carbonate crystals
at the SEM scale is also variable: micritic, radial
brous (Figure 8F), subhedral (Figure 8G), and
dendritic (Figure 8G, H). Two different morphol-
ogies of dendrites have been identied: 5-mm-long,
randomly oriented, elongated crystals or hieroglyphic
crystals several tens of microns in total length.
In the mound of outcrop B, irregular layers of lam-
inated micrite (N°6) may cover the aragonite fans (N°5)
(Figure 9A, B). The coating is meniscate and nongeopetal.
It can be locally strongly developed and lls voids. It is
locally recrystallized into microsparite (N°7). Rarely, the
laminae can develop around a nucleus (Figure 9B).
The sequence, consisting of micritic rim (N°3),
botryoidal aragonite (N°4), aragonite fans (N°5), and
laminated micrite (N°6), may repeat up to approximately
10 times (Figure 9C). This repetitive sequence will
hereafter be referred to as the elementary sequence (ES).
A trend can be observed from the earliest to the
latest ES: the micritic rims (N°3) decrease in thickness,
whereas the aragonite fans (N°5) thicken, and the
laminated micrite (N°6) that is often absent in the
earliest ES becomes more common (Figure 9A, C).
The boundary between two successive ESs can be
marked by a truncation of the minerals showing
dissolution features (Figure 9D, E).
Late Diagenesis
The microsparite (N°7) constitutes a mosaic of crystals
(Figure 9C). It does not uoresce and thus is distin-
guishable from all earlier cements (Figure 8B). In places,
the microsparite (N°7) contains cracks, a few milli-
meters to centimeters long, lled with sparite (N°8).
In the mounds, at the macroscopic scale, this
microsparite (N°7) constitutes the brown to greenish
areas. It is volumetrically the most important phase
of the mounds together with micrite (N°1). The mi-
crosparite (N°7) appears to surround the concretions
of micrite (N°1), cementing the mound (Figure 5).
Locally, angular clasts of micrite (N°1) are embedded
in the microsparite (N°7), resulting in a breccia tex-
ture (Figures 5C, 10A).
Randomly oriented septarian cracks up to 50 cm
(20 in.) long crosscut all earlier cements of the
mounds (Figure 10A). In one representative and
thick septarian crack from outcrop A, we observed
four generations of sparite (N°10, N°11, N°13,
N°16), two generations of micrite (N°12, N°15), one
generation of hematite (N°14), one generation of
microsparite (N°17), and one generation of quartz
(N°18). The microsparite (N°17) has a micro-
stalagmitic morphology, with the largest observed
stalagmite up to 8 mm (0.3 in.) in height (Figure 10B).
STABLE ISOTOPE GEOCHEMISTRY
Stable Isotopes
The results of stable carbon and oxygen isotope ana-
lyses of the studied carbonate phases from outcrops A
and B are summarized in the Appendix and plotted in
Figure 11.
The material analyzed corresponds to the volu-
metrically most important carbonate phases: mi-
crite (N°1), peloid-rich micrite (N°2), aragonite
fans (N°5), laminated micrite (N°6), microsparite
(N°7), sparite (N°8), and all carbonate cements of
the septarian cracks (N°10 to N°17). Volumetri-
cally minor phases of carbonate cements were
BLOUET ET AL. 15
16 Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
impossible to isolate because of the limited ability
of the microdrill to recover very small amounts;
however, a mix of some minor phases was obtained
and analyzed. Two samples of cements of the
sandstone dykes were analyzed.
Isotopic signatures are characteristic of each phase
and, for a given phase, the isotopic signature is identical
for both outcrops A and B. Data from both outcrops are
plotted together and not distinguished in Figure 11. The
data cluster in three elds. The rstclusterischarac-
terized by extremely low d
13
Cvalues(-38.15to -
50.94)andthehighestd
18
O values (0.21to
2.09). It comprises the early diagenetic phases: mi-
crite (N°1), peloid-rich micrite (N°2), micritic rim
(N°3), botryoidal aragonite (N°4), and aragonite
fans (N°5). Carbonate samples partially trans-
formed to gypsum deviate from the cluster of
gypsum-free carbonate but plot in the same general
area of the diagram.
A second cluster has d
13
Candd
18
Ovaluescloseto
marine values (respectively, from 3.64to -3.68
and from 0.99to -0.52). It comprises micro-
sparite (N°7), sparite (N°8), dyke cement (N°9), and
the earliest cements of the septarian cracks (N°10
N°12). The laminated micrite (N°6) and a few samples
of microsparite (N°7) are scattered in between the
above-cited clusters.
A third cluster is composed of the late phases of
inll of the septarian cracks (N°13, N°15, N°16, and
N°17), characterized by slightly depleted d
13
C values
(-4.99to -7.65) and low, but variable, d
18
O
values (-2.19to -6.06).
INTERPRETATION
The very low d
13
C values of the early carbonate
cements (N°1N°6) characterize methane-derived
authigenic carbonate (MDAC) precipitated from
the archeal AOM combined with bacterial sulfate
reduction in the SMTZ (Boetius et al., 2000;
Peckmann and Thiel, 2004).
The fauna from outcrop B, tube worms, lucinids,
and certain mytilids are typical chemosynthetic ani-
mals (Kiel, 2010). They harbor chemoautotrophic,
sulfur-oxidizing bacteria that allow them to obtain
energy and nutrients in hydrocarbon seep environ-
ment. Moreover, this symbiotic association is pre-
sumed to be essential for all species of the lucinid
family (Taylor and Glover, 2010).
The rst mineral with a clear marine isotopic
signature is the microsparite (N°7); we therefore
consider it to mark the beginning of a second phase
of diagenesis, after deactivation of AOM (Figure 12).
The septarian cracks record a succession of min-
erals from marine to meteoric isotopic signatures.
The microstalagmitic morphology of one of the later
cements, with a meteoric signature, indicates pre-
cipitation in the meteoric vadose zone. The last ce-
ments therefore correspond to a third phase marked
by the inux of meteoric water into the system.
DISCUSSION: SEEPAGE PROCESSES
REVEALED BY THE PARAGENETIC
SEQUENCE OF AUTHIGENIC CARBONATES
Authigenic Carbonate Precipitation
Mechanisms
Micrite (N°1)
The inll of burrows by methane-derived cements
suggests that burrows acted as open pathways for
focused hydrocarbon ow. Their coating by a cortex
of micrite (N°1) is interpreted to result from the
slow diffusion of methane into the immediately sur-
rounding sediment (Wiese et al., 2015). The
presence of detrital grains in the micrite (N°1)
Figure 8. Petrographic features of early diagenetic minerals. (A) Polished slice of a mound sample: gray nodules (micrite N°1) are
embedded within the brown matrix (microsparite N°7). The indicated burrow is coated by micrite (N°1) and lined by cements. (B) View of the
same sample under ultraviolet light. Micrite (N°1) is strongly uorescent, visualizing concentric rims around the burrow. (C) Peloid-rich micrite
(N°2) covered by a micritic rim (N°3) and aragonite fans (N°5). (D) Typical succession of minerals forming the elementary sequence (ES):
micritic rim (N°3), botryoidal aragonite (N°4), and fans of aragonite (N°5). The upper part of the aragonite fans is corroded (underlined by the
red sinuous line). (E) Backscattered electronscanning electron microscope (BSESEM) picture showing the micritic rim (N°3) composed of
brous barite and several carbonate phases. (F) A BSE picture of the micritic rim (N°3) showing brous carbonates. (G) An SEM picture showing
the micritic rim (N°3) with subhedral crystals covered by randomly orientated elongated crystals. The insert shows a representative example of
the elongate crystals at a higher magnication. (H) A BSE picture of the micritic rim (N°3). Detail a shows bright (Sr-rich), aligned, elongated
crystals. Detail b shows dark (Mg- and Mn-rich), dendritic crystals. Note: A color version can be seen in the online version.
BLOUET ET AL. 17
Figure 9. Petrographic features of early diagenetic minerals. (A) Three elementary sequences (ESs) terminating with a layer oflaminated
micrite (N°6). The laminated micrite (N°6) of the last sequence occludes the cavity. The base of each ES is highlighted by a red line. (B)
Several ESs terminating with corroded aragonite fans. The last ES ends with a thick cover of laminated micrite (N°6) hosting a coated grain
(green line underlining the lamination). (C) At least six well-developed ESs. The aragonite fans of the last sequence are well developed.
(D) Corroded aragonite fans at the boundary between two ESs. (E) Same picture under crossed nicols. Note: A color version can be seen in
the online version.
18 Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
indicates that it precipitated within the porous
network of the siliciclastic sea oor sediment. In
addition, the preservation within the micrite (N°1) of
noncompacted bivalves and burrows proves that
carbonate precipitated at shallow burial depth, before
any signicant compaction of the sediment occurred.
The peloids (N°2) present at the bottom of
burrows occur as one episode, before the earliest ES,
suggesting that they are fecal pellets deposited while
the burrow was still in use by the organism.
The Elementary Sequence
The repetition (up to 10 times) of a consistent min-
eral succession in the early diagenetic stage indi-
cates that a cyclic uid ow sequence took place.
Figure 10. Late diagenetic minerals. (A) Burrow coated by micrite (N°1) and lined by early cements (N°2N°6). Both have been crosscut
by a crack lled by microsparite (N°7) and later by a septarian crack. (B) Surface of a well-developed septarian crack covered by hematite
(N°14), sparite (N°16), microsparite (N°17) with a microstalagmitic morphology, and euhedral quartz (N°18).
Figure 11. Stable isotopes of carbonate phases. PDB =Peedee belemnite.
BLOUET ET AL. 19
The growth rings visible under UV light in the
micritic cortex of the tubes (N°1) may result from
the same phenomenon.
Element 1 of the Elementary Sequence: Corrosion Surface
The corrosion surface, which commonly marks the
beginning of the individual ESs (Figure 9D, E), indi-
cates circulation of a uid undersaturated with respect
to carbonate. Observations of corrosion surfaces,
frequentlylinedbypyrite,arecommoninseepcar-
bonates (Campbell et al., 2002). This is explained by the
migration of acidic H
2
SfromtheSMTZfollowedby
a reaction with Fe
2+
(Campbell, 2006). The absence of
pyrite in our particular case may indicate that the
amount of available iron from the sediment was too
low to promote extensive pyrite precipitation, but it
cannot explain why the corrosion is systematically as-
sociated with the rst stage of the ESs. Another ex-
planation for the promotion of carbonate dissolution,
Figure 12. Paragenetic sequence observed in outcrops A and B. In this example, three elementary sequences (ES) are represented. The
average stable isotope signature of each analyzed carbonate phase is indicated. The succession of three types of diagenetic uids can be
distinguished: methane, marine, and meteoric waters. Beginning of late diagenesis is marked by the rst marine cement; telogenesis
corresponds to the precipitation of gypsum. PDB =Peedee belemnite.
20 Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
without pyrite precipitation, is the increase in acidity as-
sociated with the reaction of aerobic oxidation of meth-
ane (Reeburgh, 2007) according to the following equation:
CH4+2O2CO2+2H2O
The following model is proposed in support of the
aerobic oxidation of methane. At an initial steady state,
without external input of methane, the SMTZ was
probably localized at a certain depth within the
sediment (passive SMTZof Regnier et al., 2011)
while the upper part of the sediment was normally
oxygenated. In a second stage, a burst of methane
from below rose up into the subseaoor oxic zone.
Aerobic oxidation of methane then took place until
all available oxygen was consumed. Meanwhile, the
diagenetic horizon stabilized in a new equilibrium
position, above the previous one. In the case where
the methane ux was strong enough to consume all
the oxygen in the sediment, the SMTZ may even
have risen up to the sea oor.
Element 2 of the Elementary Sequence: Micritic Rim (N°3)
The minerals constituting the micritic rim (N°3) are
the rst authigenic precipitates found within the
burrows. The great diversity of mineralogies and
mineral fabrics may reect either frequent varia-
tions in the physicochemical parameters of the
uid (temporal variability) or small-scale parti-
tioning into several microenvironments (spatial
variability). Similar variations in carbonate fabrics
induced by very local conditions are observed in
modern bacterial mats (Riding, 2000).
The presence of barite in association with seep
carbonates is not uncommon (e.g., Greinert et al.,
2002; Aloisi et al., 2004; Feng and Roberts, 2011).
Bariumcontainedincoldseepuids is often thought to
originate from the dissolution of barite contained
in deeper buried sediment, below the sulfate
depletion level (SMTZ) of marine sediments
(Torres et al., 2003; Arning et al., 2015). Sediments
rich in biogenic barite are associated with high
biologically productive oceans (Stroobants et al.,
1991), as was the case during deposition of the
Moreno Formation (Fonseca-Rivera, 1997). The
Moreno Formation was most probably the source
of barium.
The dendritic morphology of some crystals
(Figure 8H) is typical of a very rapid growth rate in
auid highly oversaturated with respect to carbon-
ates. This indicates an intense AOM activity probably
fed by a high hydrocarbon supply. This situation
corresponds to the active SMTZdescribed by
Regnier et al. (2011). In comparison with the pre-
vious stage of corrosion, all the oxygen would have
been consumed, and the SMTZ would have risen up
to a shallower depth in the sediment.
Element 3 of the Elementary Sequence: Botryoidal Aragonite
(N°4) and Aragonite Fans (N°5)
The next precipitates consist of aragonite, rst
botryoidal (N°4) and then in fans of needles (N°5).
From the micritic rim (N°3) to the aragonitic bo-
tryoids and fans (N°4 and N°5), we observed that
the mineralogical diversity decreased drastically,
with disappearance of the dendritic morphology and
increase in crystal size by two orders of magnitude.
This suggests that crystals grew more slowly during
a relatively long period of time. Therefore, the degree
of oversaturation of the uid decreased, as well as the
rate of AOM. Moreover, in seep carbonate envi-
ronments, aragonite is often associated with near
sea oor conditions(Burton, 1993; Aloisi et al.,
2000; Bayon et al., 2007). Thus, this may reect
a deepening of the SMTZ associated with the de-
crease of hydrocarbon ux. As a whole, it suggests
a relatively long period of low uid ux and
a deepening of the SMTZ during the growth of-
botryoidal aragonite (N°4) and aragonite
fans (N°5).
Element 4 of the Elementary Sequence: Laminated Micrite
(N°6)
The laminated micrite (N°6) is not geopetal; it cannot
have been deposited as detrital input and is therefore
authigenic. It lines pore rims, as can be expected for
biolms (Reitner et al., 2005), and so the lamination
could constitute a stromatolitic texture (e.g., Greinert
et al., 2002; Agirrezabala, 2009). Such microbial
deposits within cavities have been described as au-
toendoliths by Marlow et al. (2015).
No general established relationship exists be-
tween ux rate and stromatolitic texture. However,
the decreasing trend of ux within the ES would tend
to demonstrate that these stromatolites may corre-
spond to a moribund stage of seepage activity.
BLOUET ET AL. 21
Seepage Cycles
To conclude, each ES is interpreted as a record of one
seepage cycle corresponding to the evolution from
a sudden strong hydrocarbon pulse that pushed the
SMTZ upward to a decrease of ux intensity and
a deepening of the SMTZ over a longer period of
time. The repetition of ESs precipitation is there-
fore inferred to correspond to the repetition of
methane pulses. The overall evolution observed in
successive ESs, as described in the Early Diagenesis
section, suggests that the energy of methane pulses
decreased at the scale of the seepages life span.
Late Diagenesis
Microsparite (N°7) likely formed as an aggrading
in situ replacement of an earlier micrite (Bathurst,
1971; Campbell et al., 2002), as also suggested by the
presence of shrinkage cracks lled with calcite (N °8).
This micrite precipitated around the MDAC con-
cretions, cementing the mound; it is conceivable that
the MDAC acted as a substrate, thus favoring the
heterogeneous nucleation of micrite (De Yoreo and
Vekilov, 2003).
Laminated micrite N°6 locally recrystallized
to microsparite. This phenomenon is associated
with a shift in isotopic signatures between the
MDAC and the marine values. On the contrary,
the microsparite (N°8) has a clear marine isotope
signature, thus we favor the hypothesis that its
initial signature was marine. The local breccia
texture composed of MDAC clasts embedded
within the microsparite (N°7) indicates hydraulic
fracturing under isotropic stress conditions
(Cosgrove, 1997). This implies a closed or semi-
closed uid system.
The isotopic trend observed in the cements of
septarian cracks, from marine to meteoric uids,
probably results from the progressive inll of the
Great Valley Basin up to its current exhumation.
Presumably, the Franciscan subduction complex
that continually rose up and began to emerge in
the Paleogene (McGuire, 1988; Fonseca-Rivera,
1997) favored lateral uid ow from west to east
through permeable sandy intervals such as the Cima
Sandstone Lentil.
Gypsum precipitation is attributed to epige-
netic oxidation of pyrite contained in shales
(Presser and Swain, 1990). It locally and partially
neomorphosed MDAC, thereby leading to
a complete loss of UV uorescence and a shift in
isotopic signature toward higher d
13
Cvaluesand
randomly scattered d
18
Ovalues.
Figure 13. One-dimensional (1-D) thermal model of the Moreno Formation at the time of seep carbonate precipitation. Three cases have
been calculated using the minimum, average, and maximum values of sea-oor temperature (T
min
T
max
) and heat ow taken from literature
(see references in text). The curve of bacterial activity as a function of temperature is from Haeseler et al. (2015). The three upper limits of the
optimal biogenic gas window were calculated at 200, 400, and 600 m below sea oor. T =temperature; TOC =total organic carbon.
22 Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
PETROLEUM SYSTEM LINKED TO GAS
SEEPAGE
Origin of the Hydrocarbons That Fed the
Methane-Derived Authigenic Carbonate
Measurements in modern seeps have shown that the
d
13
C value of seep carbonates is almost always higher
than the hydrocarbonsd
13
C value from which they
derive (Peckmann and Thiel, 2004). In our case, the
measured d
13
C values of MDAC, as low as -50.94,
indicate an origin from the oxidation of biogenic
methane (cf. Peckmann and Thiel, 2004). As men-
tioned in the Geological Setting section, the only
source rock deposited before the Cima Sandstone
Lentil in the studied region is the Moreno Formation;
its 3% TOC content is largely sufcient to promote
biogenic gas generation (Clayton, 1992). Therefore, it
is relevant to consider whether the Moreno Forma-
tion was under the right conditions to generate bio-
genic gas when the MDACs formed. The following
paragraphs aim at solving this issue.
Figure 14. Relation between evolution of pressure in the injectite network, ux of methane, and precipitation of seep carbonates. Here
s
min
is the minimal stress, the critical value necessary to open a fracture; P
Moreno
is the pore pressure in the Moreno Formation; and P
hydro
is
the hydrostatic pressure. SMTZ =sulfatemethane transition zone.
BLOUET ET AL. 23
24 Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
One key parameter that controls biogenic gas
generation is temperature (Rice and Claypool, 1981):
the optimum interval ranges from 30°C to 50°C
(86°F to 122°F), and sterilization occurs above 70°C
to 80°C (158°F to 176°F) (Wilhelms et al., 2001;
Clayton, 2010; Sandu and Bissada, 2012; Stolper
et al., 2014; Haeseler et al., 2015).
Vigorito and Hurst (2010) determined that the
decompacted depth of the base of the Moreno For-
mation was approximately 1000 m (0.6 mi) during
deposition of the Cima Sandstone Lentil. Paleo-
temperature at this depth was a function of geo-
thermal heat ux, sea-oor temperature, and the
thermal conductivity of the Moreno Formation (see
Goto and Matsubayashi, 2009, for the thermal con-
ductivity of an uncompacted shale: the overall li-
thology of the Moreno Formation).
Dickinson and Seely (1979) indicated that the
geodynamic and structural context of the Great
Valley during the Cenozoic was similar to that of the
modern forearc basin of the Aleutians (ridged forearc
basins). The average geothermal heat ow measured
in the Aleutian forearc basin ranges between 40 and
45 mW/m
2
(430 and 485 mW/ft
2
)(Blackwelland
Richards, 2004). Based on vitrinite reectance data,
He et al. (2014) independently estimated a ux of
48.5 mW/m
2
(522 mW/ft
2
) for the earliest Paleocene
in the Vallecitos Syncline, less than 30 km (19 mi)
south of our study area. It thus seems reasonable to
assess a range of 40 to 48.5 mW/m
2
(430 to
522 mW/ft
2
) for the geothermal ux in the study
area during the Paleocene.
Based on foraminiferal analysis, Martin (1964)
ascribed a shoreface environment to the Dos Palos
Shale Member, just below the Cima Sandstone
Lentil. Given that the Paleocene was a time of
greenhouse climate (Quill´
ev´
er´
e et al., 2008), we may
assume that the sea-oor temperature lay between
12°C (T
min
) and 20°C (T
max
) (53°F and 68°F).
Paleotemperature in the sediment column was as-
sessed from a calculation of heat diffusion using Fouriers
Law. Taking into account paleotemperature error
margins, a signicant part of the organic-rich section of
the Moreno Formation was in the optimum tempera-
ture range for biogenic gas generation (Figure 13).
Bac (1990) provided further indication that
microorganisms are involved in methane produc-
tion. The author reported the presence of biomarkers
typical of Archaea in the Moreno Formation (C
14
to C
30
saturated and unsaturated acyclic isoprenoid
hydrocarbons) and attributed them to methanogens,
the presence of other types of archaea being consid-
ered as unlikely in the given context. The lower part of
the Moreno Formation is thus considered the most
likely source of biogenic methane involved in the
precipitation of the seep carbonates.
Expulsion Mechanisms and Gas Migration
Pathway
The clear geometric association between seep car-
bonates and underlying dykes at both outcrops A and
B (Figure 4) suggests that dykes acted as hydrocarbon
migration pathways. This is in line with the hy-
pothesis of Minisini and Schwartz (2007), who ob-
served a good overall statistical correlation over the
20 km (12 mi) of outcrop between the abundance of
dykes and that of seep carbonates.
We propose the following hypothese for the
mechanisms of expulsion and migration of methane
(Figures 14, 15).
V´
etel and Cartwright (2010) described the mech-
anism of depressurization of an injectite network
Figure 15. Evolution of methane seepage ux and seep carbonate precipitation through time from outcrop (meters) to thin-section scale
(millimeters). Time 1: Initial state of the sediment, before injectite propagation. Burrows are absent. Time 2: After the rapid injectite propagation
and drop of pressure of the injectite network to the hydrostatic regime (below the pressure of the Moreno shale), methane is expelled out of the
shale and rises through the network up to the surface. The high methane ux pushes the sulfatemethane transition zone (SMTZ) upward.
Simultaneously, chemosynthetic organisms settle and thrive in this methane-rich environment, burrowing and producing pellets. The intense
anaerobic oxidation of methane triggers the precipitation of dendritic minerals while methane diffusion from the burrows to the surrounding
sediment initiates the precipitation of the cortex. Time 3: The Moreno shale is progressively depleted in gas, and the ux of methane in the injectites
reduces. Consequently, the SMTZ moves back downward, and the elementary sequence (ES) precipitates. Time 4: Once the gas-charged shale
surrounding the injectite network is signicantly depleted, the upward ux of methane deactivates, and diagenetic interfaces move back close to
their original position. Time 5: Reactivation of the injectite network in undepleted sections of the Moreno Formation (Fm.) triggers a new phase of
gas migration. Because of the sudden and intense rising ux of methane, gas propagates up to oxic sediment and aerobic oxidation of methane
takes place, thus promoting carbonate dissolution. Time 6: A new cycle begins: a new ES precipitates while the shale of the Moreno Fm. depletes.
BLOUET ET AL. 25
after its emplacement as follows: from an initial
pressure above the minimum stress (s
min
), the critical
value necessary for maintaining an open fracture,
pressure rapidly decreases by pervasive uid inltration
into the pore space of the host sediment or by con-
nection to another permeable sand body or to the
surface. In outcrops A and B, the tips of the dykes
reached a depth very close to the sea oor and may
even have erupted locally as sand volcanoes (Vigorito
et al., 2008). Thus, after the injectites propagation,
the pressure rapidly dropped and stabilized at the
hydrostatic regime (P
hydro
) both in the injectite
network and in the parent reservoir (Figure 14).
The pressure rise in the Panoche Formation above
fracturing pressure veries that the Moreno shale con-
stituted an efcient seal. As a result, it is likely that the
pore pressure in the Moreno Formation (P
Moreno
), at least
above the overpressured reservoirs, showed a pressure
ramp, progressively reachingthefracturingvalueofthe
Panoche Formation (Swarbrick et al., 2002). Gas gen-
eration within the shale also potentially contributed to
a rise in pressure (Osborne and Swarbrick, 1997).
Therefore, after pressure in the injectites sta-
bilized at the hydrostatic regime (P
hydro
), the
pressure in the shale (P
Moreno
) exceeded the pres-
sure in the injectites and pore uids of the Moreno
Formation, including gas, started migrating into
the depressurized injectites. In a process analogous to
what happens after articial hydraulic fracturing (e.g.,
for hydrocarbon production purposes in tight reser-
voirsor gas shale), gas drainage efciency was the
highest at the beginning and then progressively de-
creased through time as the shale surrounding the in-
jectites lost its overpressure while its permeability
decreased because of compaction.
The seepage pattern deduced from the observa-
tion of each ES, with a very important initial gas
seepage that progressively decreased over time, would
be explained by this mechanism.
V´
etel and Cartwright (2010) extensively docu-
mented crosscutting relationships of the injectites in
the Panoche Hills and used their observations as evi-
dence for multiple episodes of sand injection caused by
explosions of distinct overpressured sand bodies.
Each reactivation and propagation of the injectite
network would have triggered a new cycle of gas ow
into the injectites, thus explaining the repetitive
character of the ES. The overall decrease in intensity
may have resulted from the progressive rarefaction
of undrained, overpressured, domains in the lower
Moreno Formation.
The 50-m (164-ft)-thick chemosynthetic fauna-rich
area in outcrop B suggests that hydrocarbon seepage,
although variable in intensity, lasted long enough for
50 m (164 ft) of siliciclastic sediment to be deposited.
Ecology of Seep Fauna
The distribution of the fauna in a seep environment is
controlled by the local conditions created by uid
seepage (Levin, 2005).
The presence of chemosynthetic bivalves and
tube worms in outcrop B and their complete ab-
sence in outcrop A and the rest of the study area
are related to the large number and close prox-
imity of underlying dykes. The amount, regularity,
and spatial extension of the methane supply were
probably considerably higher in outcrop B than in
A, allowing the establishment of chemosynthetic
animals.
Unlike bivalves and tubeworms, Thalassinoides is
present at both outcrops A and B, in most mounds of
the Panoche Hills that are not directly connected to
dykes, but very rarely in curst and nodules. If Tha-
lassinoides were widespread within the siliciclastic
seabed sediment before its cementation by carbonate
concretions, Thalassinoides should be found equally
in all types of concretions. Consequently, the crus-
taceans creating the burrows probably sought the
specic environment of the methane seeps, whose
ux was higher around mounds than around nodules
and crusts.
On sea-oor maps, Gay et al. (2006) noticed
that mussels, clams, and tube worms were restricted
to the central area of pockmarks with relatively high
uid ow, whereas burrows extended farther on the
edge of the pockmarks. All known modern lucinids
and Vestimentifera are chemosynthetic and strictly
restricted to reduced environments (Kiel, 2010),
whereas most crustaceans found in seeps are not
endemics but opportunistic species that can take
advantage of this environment, feeding directly or
indirectly on chemosynthetic bacteria (Martin and
Haney, 2005). This may explain the relative wide-
spread distribution of the Thalassinoides in com-
parison with bivalves and tube worms.
In contrast with the scarce distribution of tube
worms and mussels in outcrop B, these species are
26 Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
often regrouped in dense bush and pavement struc-
tures on modern sea-oor observations (Levin, 2005;
Judd and Hovland, 2007) and in the fossil records
(Campbell et al., 2008). Lucinids in association
with tube worms have been reported (Hashimoto
et al., 1995); however, little is known about the
distribution of lucinids around seeps because of their
endobentic habitat. Taylor and Glover (2010) suspect
that they are overlooked by remotely operated ve-
hicle surveys.
Fluids used Thalassinoides burrowsaspref-
erential migration paths (e.g., Wiese et al., 2015).
This process would thus favor crustaceansaccess
to hydrocarbons. A possible positive feedback
could then be promoted. With more burrows
present, the gas ow would become more focused,
and more crustaceans would be attracted to this
area, once again increasing gas ow focus. This
may explain the observed local concentrations of
burrows and the localized precipitation of mounds.
CONCLUSION
The combined studies of two outcrops of seep car-
bonates in the Panoche Hills together with 1-D basin
modeling allow us to propose the following model for
methane migration and associated seep carbonate
precipitation: Sandstone dykes sourced from the
Upper Cretaceous Panoche Formation were intruded
into the overlying biogenic-methanegenerating
Maastrichtian to Danian Moreno Formation. In-
jectites eventually reached the seabed and released the
overpressure of the sand injectite system. From this
point on, gas migrated from the Moreno Formation
into the injectite network and converged upward to
the sea oor. This lead to precipitation of seep car-
bonates. Consistent paragenetic sequences of the seep
carbonates record the progressive decrease of venting
after the initial burst. This is attributed to the pro-
gressive gas depletion of the naturally fracked shale.
Multiple reactivations of the injectite network ulti-
mately led to the reactivation of methane seepage and
repetition of the mineral paragenetic sequences. The
outcrop of the Panoche Hills can thus be viewed as one
of the rst recognized examples of gas-shale pro-
duction by hydraulic fracturing, approximately
60 m.y. before the techniquewas adopted by mankind
for industrial purposes.
APPENDIX: STABLE CARBON AND OXYGEN
ISOTOPE VALUES FOR CARBONATES OF
OUTCROPS A AND B
Table A1.
(continued)
BLOUET ET AL. 27
REFERENCES CITED
Agirrezabala, L. M., 2009, Mid-Cretaceous hydrothermal
vents and authigenic carbonates in a transform margin,
Basque-Cantabrian Basin (western Pyrenees): A multi-
disciplinary study: Sedimentology, v. 56, p. 969996, doi:
10.1111/j.1365-3091.2008.01013.x.
Aloisi, G., C. Pierre, J.-M. Rouchy, J.-P. Foucher, and
J. Woodside, 2000, Methane-related authigenic car-
bonates of eastern Mediterranean Sea mud volcanoes and
their possible relation togas hydrate destabilisation: Earth
and Planetary Science Letters, v. 184, p. 321338, doi:
10.1016/S0012-821X(00)00322-8.
Aloisi, G., K. Wallmann, S. M. Bollwerk, A. Derkachev,
G. Bohrmann, and E. Suess, 2004, The effect of dissolved
barium on biogeochemical processes at cold seeps: Geo-
chimica et Cosmochimica Acta, v. 68, p. 17351748, doi:
10.1016/j.gca.2003.10.010.
Anderson, R., and R. W. Pack, 1915, Geology and oil resources
of the west border of the San Joaquin Valley north of
Coalinga, California: US Geological Survey Bulletin 603,
220 p., doi:10.1086/622554.
Arning, E. T., E. C. Gaucher, W. van Berk, and H.-M. Schulz,
2015, Hydrogeochemical models locating sulfate-
methane transition zone in marine sediments overlying
black shales: A new tool to locate biogenic methane?:
Marine and Petroleum Geology, v. 59, p. 563574, doi:
10.1016/j.marpetgeo.2014.10.004.
Atwater, T., and P. Molnar, 1973, Relative motion of the Pa-
cific and North American plates deduced from sea-oor
spreading in the Atlantic, Indian, and South Pacic
Oceans, in R. L. Kovach and A. Nur, eds., Proceedings of
the Conference on Tectonic Problems of the San Andreas
Fault System: Stanford, California, Stanford University,
p. 136148.
Bac, M. G., 1990, Origin, stratigraphic variability, and sig-
nicance of hydrocarbon biomarkers in the Moreno
formation, San Joaquin Basin, California, Masters thesis,
Stanford University, Stanford, California, 133 p.
Bailey, E. H., W. P. Irwin, and D. L. Jones, 1964, Franciscan
and related rocks and their signicance in the geology of
western California: San Francisco, California, California
Division of Mines and Geology Bulletin 183, 177 p.
Bartow, J. A., 1991, The Cenozoic evolution of the San
Joaquin Valley, California: US Geological Survey Pro-
fessional Paper 1501, 40 p.
Bartow, J. A., and T. H. Nilsen, 1990, Review of the Great
Valley sequence, eastern Diablo Range and northern San
Joaquin Valley, central California: US Geological Survey
Report 90-226, p. 12.
Bathurst, R. G. C., ed., 1971, Carbonate sediments and
their diagenesis: Amsterdam, Elsevier, Developments
in Sedimentology 12, 658 p.
Bayon, G., C. Pierre, J. Etoubleau, M. Voisset, E. Cauquil,
T. Marsset, N. Sultan, E. Le Drezen, and Y. Fouquet,
2007, Sr/Ca and Mg/Ca ratios in Niger Delta sediments:
Implications for authigenic carbonate genesis in cold seep
environments: Marine Geology, v. 241, p. 93109, doi:
10.1016/j.margeo.2007.03.007.
Blackwell, D., and M. C. Richards, 2004, Geothermal map of
North America: AAPG, scale 1:6,500,000, 1 sheet.
Boetius, A., K. Ravenschlag, C. J. Schubert, D. Rickert,
F. Widdel, A. Gieseke, R. Amann, B. B. Jørgensen,
U. Witte, and O. Pfannkuche, 2000, A marine microbial
consortium apparently mediating anaerobic oxidation
of methane: Nature, v. 407, p. 623626, doi:10.1038
/35036572.
Boetius, A., and F. Wenzh¨
ofer, 2013, Seaoor oxygen con-
sumption fuelled by methane from cold seeps: Nature
Geoscience, v. 6, p. 725734, doi:10.1038/ngeo1926.
Table A1. Continued
28 Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
Burton, E. A., 1993, Controls on marine carbonate cement
mineralogy: review and reassessment: Chemical Geology,
v. 105, p. 163179, doi:10.1016/0009-2541(93)90124-2.
Campbell, K. A., 2006, Hydrocarbon seep and hydrothermal
vent paleoenvironments and paleontology: Past develop-
ments and future research directions: Palaeogeography,
Palaeoclimatology, Palaeoecology, v. 232, p. 362407,
doi:10.1016/j.palaeo.2005.06.018.
Campbell, K. A., J. D. Farmer, and D. Des Marais, 2002,
Ancient hydrocarbon seeps from the Mesozoic conver-
gent margin of California: Carbonate geochemistry, uids
and palaeoenvironments: Geouids, v. 2, p. 6394, doi:
10.1046/j.1468-8123.2002.00022.x.
Campbell,K.A.,D.A.Francis,M.Collins,M.R.Gregory,
C. S. Nelson, J. Greinert, and P. Aharon, 2008, Hy-
drocarbon seep-carbonates of a Miocene forearc (East
Coast Basin), North Island, New Zealand: Sedimen-
tary Geology, v. 204, p. 83105, doi:10.1016/j.sedgeo
.2008.01.002.
Clayton, C., 1992, Source volumetrics of biogenic gas gen-
eration, in R. Vially, ed., Bacterial gas: Paris, Technip,
p. 191204.
Clayton, C., 2010, Incorporation of biogenic gas generation
into petroleum system models: Geological Society,
London, Modelling Sedimentary Basins and Their Pe-
troleum Systems Conference, London, June 34, 2010,
28 p.
Cosgrove, J., 1997, Hydraulic fractures and their implications
regarding the state of stress in a sedimentary sequence
during burial, in S. Sengupta, ed., Evolution of geological
structures in micro- to macro-scales: London, Chapman
& Hall, p. 1125, doi:10.1007/978-94-011-5870-1_2.
De Yoreo, J. J., and P. G. Vekilov, 2003, Principles of crystal
nucleation and growth: Reviews in Mineralogy and
Geochemistry, v. 54, p. 5793, doi:10.2113/0540057.
Dickinson, W., and D. Seely, 1979, Structure and stratigraphy
of forearc regions: AAPG Bulletin, v. 63, no. 1, p. 231,
doi:10.1306/c1ea55ad-16c9-11d7-8645000102c1865d.
Duranti, D., and A. Mazzini, 2005, Large-scale hydrocarbon-
driven sand injection in the Paleogene of the North Sea:
Earth and Planetary Science Letters, v. 239, p. 327335,
doi:10.1016/j.epsl.2005.09.003.
Feng, D., and H. H. Roberts, 2011, Geochemical character-
istics of the barite deposits at cold seeps from the
northern Gulf of Mexico continental slope: Earth and
Planetary Science Letters, v. 309, p. 8999, doi:10
.1016/j.epsl.2011.06.017.
Fonseca-Rivera, C., 1997, Late Cretaceous-Early Tertiary
paleoceanography and cyclic sedimentation along the
California margin: Evidence from the Moreno Formation,
Ph.D. thesis, Stanford University, Stanford, California,
449 p.
Gay, A., M. Lopez, H. Ondreas, J.-L. Charlou,
G. Sermondadaz, and P. Cochonat, 2006, Seaoor facies
related to upward methane ux within a Giant Pockmark
of the Lower Congo Basin: Marine Geology, v. 226,
p. 8195, doi:10.1016/j.margeo.2005.09.011.
Goto, S., and O. Matsubayashi, 2009, Relations between the
thermal properties and porosity of sediments in the
eastern ank of the Juan de Fuca Ridge: Earth, Planets,
and Space, v. 61, p. 863870, doi:10.1186/BF03353197.
Graham, S., C. McCloy, M. Hitzman, R. Ward, and R. Turner,
1984, Basin evolution during change from convergent to
transform continental margin in central California:
AAPG Bulletin, v. 68, no. 3, p. 233249, doi:10.1306
/ad460a03-16f7-11d7-8645000102c1865d.
Greinert, J., S. M. Bollwerk, A. Derkachev, G. Bohrmann, and
E. Suess, 2002, Massive barite deposits and carbonate
mineralization in the Derugin Basin, Sea of Okhotsk:
Precipitation processes at cold seep sites: Earth and
Planetary Science Letters, v. 203, p. 165180, doi:10
.1016/S0012-821X(02)00830-0.
Haeseler, F., D. Levache, and B. Lamirand, 2015, Res-
ervoir lling paths revealed by kinetic modeling of
biodegradation at basin scale: International Petroleum
Technology Conference, Doha, Qatar, December 69,
2015, 8 p., doi:10.2523/IPTC-18402-MS.
Hashimoto, J., K. Fujikura, Y. Fujiwara, M. Tanishima,
S. Ohta, S. Kojima, and S. M. Yieh, 1995, Observations
of a deep-sea biological community co-dominated by lu-
cinid bivalve, Lucinoma spectabilis (Yokoyama, 1920) and
vestimentiferans at the Kanesu-no-Se bank, Enshu-Nada,
central Japan: Japan Agency for Marine-Earth Science and
Technology Journal of Deep Sea Research, v. 11,
p. 211217, doi:10.2517/prpsj.7.297.
He, M., S. Graham, A. H. Scheirer, and K. E. Peters, 2014, A
basin modeling and organic geochemistry study in the
Vallecitos syncline, San Joaquin Basin, California: Marine
and Petroleum Geology, v. 49, p. 1534, doi:10.1016/j
.marpetgeo.2013.09.001.
Ho, S., D. Carruthers, and P. Imbert, 2016, Insights into the
permeability of polygonal faults from their intersection
geometries with Linear Chimneys: A case study from the
Lower Congo Basin: Carnets de G´
eologie, v. 16, no. 2,
p. 1726, doi:10.4267/2042/58718.
Ho, S., J. A. Cartwright, and P. Imbert, 2012, Vertical evo-
lution of uid venting structures in relation to gas ux, in
the Neogene-Quaternary of the Lower Congo Basin,
Offshore Angola: Marine Geology, v. 332334, p. 4055,
doi:10.1016/j.margeo.2012.08.011.
Hovland, M., M. Talbot, S. Olaussen, and L. Aasberg, 1985.
Recently formed methane-derived carbonates from the
North Sea oor, in B. M. Thomas, ed., Petroleum geo-
chemistry in exploration of the Norwegian shelf: Salis-
bury, United Kingdom, Graham & Trotman for the
Norwegian Petroleum Society, p. 263266, doi:10.1007
/978-94-009-4199-1_22.
Hurst, A., and J. A. Cartwright, eds., 2007, Sand injectites:
Implications for hydrocarbon exploration and pro-
duction: AAPG Memoir 87, 274 p.
Hurst, A., A. Scott, and M. Vigorito, 2011, Physical charac-
teristics of sand injectites: Earth-Science Reviews, v. 106,
p. 215246, doi:10.1016/j.earscirev.2011.02.004.
Ingersoll, R. V., 1979, Evolution of the Late Cretaceous
forearc basin, northern and central California: Geologi-
cal Society of America Bulletin, v. 90, p. 813826,
doi:10.1130/0016-7606(1979)90<813:EOTLCF>2.0
.CO;2.
BLOUET ET AL. 29
Ingersoll, R. V., 2008, Subduction-related sedimentary basins
of the USA Cordillera, in A. D. Miall, ed., The sedi-
mentary basins of the United States and Canada: Sedi-
mentary Basins of the World 5, p. 395428, doi:10.1016
/S1874-5997(08)00011-7.
Jonk, R., J. Parnell, and A. Hurst, 2005, Aqueous and pe-
troleum uid ow associated with sand injectites: Basin
Research, v. 17, p. 241257, doi:10.1111/j.1365-2117
.2005.00262.x.
Judd, A., and M. Hovland, 2007, Seabed uid ow: The
impact on geology, biology and the marine environment:
Cambridge, United Kingdom, Cambridge University
Press, 492 p., doi:10.1017/CBO9780511535918.
Kiel, S., ed., 2010, The vent and seep biota: Aspects from
microbes to ecosystems: Topics in Geobiology 33, 490 p.,
doi:10.1007/978-90-481-9572-5.
Kiel, S., 2013, Lucinid bivalves from ancient methane seeps:
Journal of Molluscan Studies, v. 79, p. 346363, doi:
10.1093/mollus/eyt035.
Levin, L., 2005, Ecology of cold seep sediments: Interactions
of fauna with ow, chemistry and microbes, in R. N. Gibson,
R.J.A.Atkinsonad,andJ.D.M.Gordon,eds,Oceanog-
raphy and Marine BiologyAn Annual Review, v. 43: Boca
Raton, Florida, CRC Press, p. 146.
Magoon, L. B, and W. G. Dow, eds., 1994, The petroleum
systemFrom source to trap: AAPG Memoir 60, 665 p.
Marlow, J., J. Peckmann, and V. Orphan, 2015, Autoendo-
liths: A distinct type of rock-hosted microbial life: Ge-
obiology, v. 13, p. 303307, doi:10.1111/gbi.12131.
Martin, L., 1964, Upper Cretaceous and Lower Tertiary fora-
minifera from Fresno County, California: Vienna, Austria,
Geologische Bundesanstalt, 128 p., 10.2307/1484100.
Martin, J. W., and T. A. Haney, 2005, Decapod crustaceans
from hydrothermal vents and cold seeps: A review through
2005: Zoological Journal of the Linnean Society, v. 145,
p. 445522, doi:10.1111/j.1096-3642.2005.00178.x.
Mazzini, A., R. Jonk, D. Duranti, J. Parnell, B. Cronin, and
A. Hurst, 2003, Fluidescape from reservoirs: Implications
from cold seeps, fractures and injected sands Part I. The
uid ow system: Journal of Geochemical Exploration,
v. 7879, p. 293296, doi:10.1016/S0375-6742(03)
00046-3.
McGuire, D. J., 1988, Stratigraphy, depositional history, and
hydrocarbon source-rock potential of the Upper
Cretaceous-Lower Tertiary Moreno Formation, central
San Joaquin basin, California, Ph.D. thesis, Stanford
University, Stanford, California, 231 p.
Minisini, D., and H. Schwartz, 2007, An early Paleocene cold
seep system in the Panoche and Tumey Hills, central
California (United States), in A. Hurst and J. Cartwright,
eds., Sand injectites: Implication for hydrocarbon pro-
duction: AAPG Memoir 87, p. 185197.
Naudts, L., J. Greinert, J. Poort, J. Belza, E. Vangampelaere,
D. Boone, P. Linke, J.-P. Henriet, and M. De Batist, 2010,
Active venting sites on the gas-hydrate-bearing Hikurangi
Margin, off New Zealand: Diffusive- versus bubble-
released methane: Marine Geology, v. 272, p. 233250,
doi:10.1016/j.margeo.2009.08.002.
Norris, R. M., and R. W. Webb, 1990, Geology of California:
New York, John Wiley & Sons, 571 p.
Osborne, M. J., and R. E. Swarbrick, 1997, Mechanisms for
generating overpressure in sedimentary basins: A reeval-
uation: AAPG Bulletin, v. 81, no. 6, p. 10231041, doi:
10.1306/8626D36F-173B-11D7-8645000102C1865D.
Paull, C., B. Hecker, R. Commeau, R. Freeman-Lynde,
C. Neumann,W. Corso, S. Golubic, J. Hook, E. Sikes, and
J. Curray, 1984, Biological communities at the Florida
Escarpment resemble hydrothermal vent taxa: Science,
v. 226, p. 965967, doi:10.1126/science.226.4677.965.
Paull, C. K., and W. Ussler III, 2008, Re-evaluating the
signicance of the seaoor accumulations of methane-
derived carbonates: Seepage or erosion indicators?: 6th
International Conference on Gas Hydrates, Vancou-
ver, Canada, July 620, 2008, p. 112, doi:10.14288
/1.0041014.
Payne, M. B., 1951, Type Moreno Formation and overlying
Eocene strata on the west side of the San Joaquin Valley,
Fresno and Merced counties: San Francisco, State of
California Department of Natural Resources, Division of
Mines, 29 p.
Peckmann, J., and V. Thiel, 2004, Carbon cycling at ancient
methaneseeps: Chemical Geology, v. 205, p. 443467,
doi:10.1016/j.chemgeo.2003.12.025.
Peters,K.E.,L.B.Magoon,Z.C.Valin,andP.G.Lillis,2007,
Source-rock geochemistry of the San Joaquin Basin Prov-
ince, California, in A.H.Scheirer,ed.,Petroleumsystems
and geologic assessment of oil and gas in the San Joaquin
Basin Province, California: US Geological Survey Pro-
fessional Paper 1713, p. 1102, doi:10.3133/pp1713.ch11.
Presser, T. S., and W. C. Swain, 1990, Geochemical evidence
for Se mobilization by the weathering of pyritic shale,
San Joaquin Valley, California, U.S.A: Applied Geo-
chemistry, v. 5, p. 703717, doi:10.1016/0883-2927(90)
90066-E.
Quill´
ev´
er´
e, F., R. D. Norris, D. Kroon, and P. A. Wilson, 2008,
Transient ocean warming and shifts in carbon reser-
voirs during the early Danian: Earth and Planetary
Science Letters, v. 265, p. 600615, doi:10.1016/j
.epsl.2007.10.040.
Reeburgh, W. S., 2007, Oceanic methane biogeochemistry:
Chemical Reviews, v. 107, p. 486513, doi:10.1021
/cr050362v.
Regnier, P., A. W. Dale,S. Arndt, D. LaRowe, J.Mogoll ´
on, and
P. Van Cappellen, 2011, Quantitative analysis of anaer-
obic oxidation of methane (AOM) in marine sediments: A
modeling perspective: Earth-Science Reviews, v. 106,
p. 105130, doi:10.1016/j.earscirev.2011.01.002.
Reitner, J., J. Peckmann, M. Blumenberg, W. Michaelis,
A. Reimer, and V. Thiel, 2005, Concretionary methane-
seep carbonates and associated microbial communities in
Black Sea sediments: Palaeogeography, Palaeoclimatology,
Palaeoecology, v. 227, p. 1830, doi:10.1016/j.palaeo
.2005.04.033.
Rice, D. D., and G. E. Claypool, 1981, Generation, accu-
mulation, and resource potential of biogenic gas: AAPG
Bulletin, v. 65, no. 1, p. 525.
30 Gas Migration Mechanisms Revealed by Seep Carbonate Paragenesis
Riding, R., 2000, Microbial carbonates: The geological record of
calcied bacterialalgal mats and biolms: Sedimentology,
v. 47, p. 179214, doi:10.1046/j.1365-3091.2000.00003.x.
Ritger, S., B. Carson, and E. Suess, 1987, Methane-derived
authigenic carbonates formed by subduction-induced
pore-water expulsion along the Oregon/Washington
margin: Geological Society of America Bulletin, v. 98,
no.2,p.147156, doi:10.1130/0016-7606(1987)98<147:
MACFBS>2.0.CO;2.
Roberts, H. H., 2001, Fluid and gas expulsion on the northern
Gulf of Mexico continental slope: Mud-prone to mineral-
prone responses, in C. K. Paull and W. P. Dillon, eds.,
Natural gas hydrates: Occurrence, distribution, and de-
tection: AGU Geophysical Monograph 124, p. 145161,
doi:10.1029/GM124p0145.
Sandu, C., and A. Bissada, 2012, Thermal modeling of mi-
crobial methane generation constrained by laboratory
experiments: AAPG Search and Discovery article 41053,
accessed January 15, 2016, http://www.searchand
discovery.com/pdfz/documents/2012/41053sandu
/ndx_sandu.pdf.html.
Schwartz, H., J. Sample, K. D. Weberling, D. Minisini, and
J. C. Moore, 2003, An ancient linked uid migration
system: Cold-seep deposits and sandstone intrusions in
the Panoche Hills, California, USA: Geo-Marine Letters,
v. 23, p. 340350, doi:10.1007/s00367-003-0142-1.
Scott, A., A. Hurst, and M. Vigorito, 2013, Outcrop-based
reservoir characterization of a kilometer-scale sand-
injectite complex: AAPG Bulletin, v. 97, no. 2, p. 309
343, doi:10.1306/05141211184.
Stolper, D., M. Lawson, C. Davis, A. Ferreira, E. S. Neto,
G. Ellis, M. Lewan, A. Martini, Y. Tang, and M. Schoell,
2014, Formation temperatures of thermogenic and bio-
genic methane: Science, v. 344, p. 15001503, doi:
10.1126/science.1254509.
Stroobants, N., F. Dehairs, L. Goeyens, N. Vanderheijden, and
R. Van Grieken, 1991, Barite formation in the Southern
Ocean water column: Marine Chemistry, v. 35, p. 411
421, doi:10.1016/S0304-4203(09)90033-0.
Swarbrick, R. E., M. J. Osborne, and G. S. Yardley, 2002,
Comparison of overpressure magnitude resulting from
the main generating mechanisms, in A. R. Huffman and
G. L. Bowers, eds., Pressure regimes in sedimentary
basins and their prediction: AAPG Memoir 76, p. 112.
Taylor, D., and E. A. Glover, 2010, Chemosymbioticbivalves,
in S. Kiel, ed., The vent and seep biota: Aspects from
microbes to ecosystems: Topics in Geobiology 33,
p. 107135, doi:10.1007/978-90-481-9572-5_5.
Torres, M. E., G. Bohrmann, T. E. Dub´
e, and F. G. Poole,
2003, Formation of modern and Paleozoic stratiform
barite at cold methane seeps on continental margins:
Geology, v. 31, p. 897900, doi:10.1130/G19652.1.
US Geological Survey, 1956, Geologic map of the Chounet
Ranch quadrangle, Fresno County, California (revised
1971): 7.5 Minute Series (Topographic), scale 1:24,000,
1 sheet, accessed November 17, 2016, https://prd-tnm
.s3.amazonaws.com/StagedProducts/Maps/HistoricalTopo
/2/13256/4763906.pdf.
V´
etel, W., and J. Cartwright, 2010, Emplacement mechanics
of sandstone intrusions: Insights from the Panoche Giant
Injection Complex, California: Basin Research, v. 22,
p. 783807, doi:10.1111/j.1365-2117.2009.00439.x.
Vigorito, M., and A. Hurst, 2010, Regional sand injectite
architecture as a record of pore- pressure evolution and
sand redistribution in the shallow crust: Insights from the
Panoche Giant Injection Complex, California: Journal of
the Geological Society, v. 167, p. 889904, doi:10.1144
/0016-76492010-004.
Vigorito, M., A. Hurst, J. Cartwright, and A. Scott, 2008,
Regional-scale subsurface sand remobilization: Geometry
and architecture: Journal of the Geological Society,
v. 165, p. 609612, doi:10.1144/0016-76492007-096.
Weberling, K. D., 2002, Clastic intrusion and cold seeps in the
Late Cretaceousearly Tertiary Great Valley forearc
basin, Panoche Hills, CA: Structural context of a linked
uid system, Masters thesis, University of California,
Santa Cruz, California, 48 p.
Wiese, F., S. Kiel, A. Pack, E. O. Walliser, and L. M. Agirrezabala,
2015, The beast burrowed, the uid followedCrustacean
burrows as methane conduits: Marine and Petroleum
Geology, v. 66, no. 3, p. 631640, doi:10.1016/j.
marpetgeo.2015.03.004.
Wilhelms, A., S. R. Larter, I. Head, P. Farrimond, R. di-Primio,
and C. Zwach, 2001, Biodegradation of oil in uplifted
basins prevented by deep-burial sterilization: Nature,
v. 411, p. 10341037, doi:10.1038/35082535.
BLOUET ET AL. 31
... Such conduits are called chimneys, pipes and columns (Capozzi et al., 2015). A number of those carbonate conduits and marine cold seep systems have been identified for over 25 years all around the world from Mesozoic up to present (Suess, 2014), lot of them belonging to convergent settings (Lewis and Marshall, 1996;Orpin, 1997;Aiello et al., 2001;Suess et al., 2001;Torres et al., 2002Torres et al., , 2003Díaz del-río et al., 2003;Ledésert et al., 2003;Aiello, 2005;De boever et al., 2006aDe boever et al., , 2006bDe boever et al., , 2009aDe boever et al., , 2009bDe boever et al., , 2011Nyman et al., 2006Nyman et al., , 2010Campbell et al., 2008Campbell et al., , 2010Sahling et al., 2008;Sellanes et al., 2008;Aggirezabala, 2009;Hoareau et al., 2009;DelaPierre et al., 2010;Feng et al., 2010;Pearson et al., 2010;Aggirezabala et al., 2013;Han et al., 2013;Zapatahernandez et al., 2013;Suess, 2014;Blouet et al., 2017;Malié et al., 2017;Wang et al., 2017;Watson et al., 2020). ...
... Most of the previous works dedicated to tubular carbonate concretions have demonstrated that they correspond to the shallow subsurface plumbing systems of conveying mainly biogenic (e.g., Aiello et al., 2001;De boever et al., 2009b;Blouet et al., 2017)but also thermogenic gas (e. g., Díaz del-río et al., 2003, for the Gulf of Cádiz (Spain); Nyman et al., 2010 andPearson et al., 2010, for the East Coast Basin of New Zealand) toward the seafloor. This study presents new results from the analysis of samples coming from source rocks, fault gouges, tubular carbonate concretions and their host rocks that are exposed within the emerged part of the Hikurangi subduction wedge in the East Coast Basin petroleum province of New Zealand. ...
Article
In the Hikurangi subduction wedge (New Zealand), a strong relationship exists between tectonic structures and fluid migrations. In the study area, outcropping tubular carbonate concretions, corresponding to the shallow subsurface plumbing systems of paleo-cold seeps, are hosted by Miocene syn-subduction mudstones. New observations demonstrate the presence of migrated solid bitumen within the tubular concretions and in the fault gouge of a major fault zone. A multi-proxy approach was performed to determine the organic matter thermal maturity in the study area (organic matter petrography and solid bitumen reflectance (BR) Rr% (Rr: random reflectance)). We also used Rock-Eval pyrolysis, vitrinite reflectance (VR) Rr%, and clay mineral reaction progress (illite Kübler-Index and clay mineral paragenesis) to determine the diagenesis grade of the rocks. Low Tmax values and clay minerals indicate a thermally immature sedimentary cover. The main source rock of the region, the Waipawa Formation is locally thermally mature (VR = 0.86 Rr%) suggesting that tectonic thrust-sheet stacking isresponsible for a structural thickening causing local organic maturation. The seaward propagation of out-of-sequence thrusts at base of intra-slope basins could be responsible for the inititation of biogenic fluid flows sourced in the shallow sedimentary cover that is subjected to deformation above the blind thrusts, leading to the earlier generation of the first carbonate tubular concretions. With the continuation of blind thrusting, deep thermogenic fluids then migrated laterally through fault planes (primary migration) and finally vertically through the intrabasinal pre-existing tubular concretions (secondary migration). In this paper, solid bitumen is used for the identification of a fossil thermogenic fluid migration from the source rock, along faults and through tubular carbonate conduits within a subduction thrust-wedge. The study evidences a multi-genetic tubular concretion formation, related with the timing and style of the deformation, being therefore a potential reliable indicator for the evolution of tectonic activity.
... Excluding the effects of silicification, the cement infill of Phase-3 burrows and their host sediment is quite simple: AOM-related carbonate precipitation in the SMTZ is expressed by diagenetic micrite within the pores of homogenized "depositional" micrite, by microspar crystals in the pores of pellet grainstone and in the first 0.1-0.5 mm off the burrow walls (BMSpar-CMSpar), and by spar-sized crystals (Spar-1) in the remaining free space of the lumen. The radiaxial sets of gray carbonate crystals that occasionally terminate the sequence of cements (RAx) may represent recrystallized equivalents to the aragonite fans described by Blouet et al. (2017) in the Panoche Hills (CA, USA) or by Peckmann et al. (1999) from Beauvoisin. Calcite crystals continuing across the BMSpar-CMSpar contacts indicate that the brown color of the former simply reflects the inclusion of the filamentous bushes into the growing microspar. ...
... Which parameters controlled the vertical growth of the pseudobioherm between the marker beds? The role of Thalassinoides in focusing migration of methane(charged fluids) has been identified by Wetzel (2013) and later by Wiese et al. (2015), Zwicker et al. (2015), and Blouet et al. (2017). Evidently, in spite of likely overprinting by shallower-tier burrowers, individual Phase-3 burrows remained open until burial reached at least 1 km and provided obvious fluid migration pathways as deep as the burrowing organisms penetrated -likely in the range of 1-3 m, as known for present-day decapod crustaceans (see above). ...
Article
Full-text available
The mechanisms that govern the vertical growth of seep carbonates were deciphered by studying the sedimentary architecture of a 15 m thick, 8 m wide column of limestone encased in deep-water marl in the middle Callovian interval of the Terres Noires Formation in the SE France Basin. The limestone body, also called “pseudobioherm”, records intense bioturbation, with predominant traces of the Thalassinoides/Spongeliomorpha suite, excavated by decapod crustaceans. Bioturbation was organized in four tiers. The uppermost tier, tier 1, corresponds to shallow homogenization of rather soft sediment. Tier 2 corresponds to pervasive burrows dominated by large Thalassinoides that were later passively filled by pellets. Both homogenized micrite and burrow-filling pellets are depleted in 13C in the range from −5 ‰ to −10 ‰. Tier 3 is characterized by small Thalassinoides that have walls locally bored by Trypanites; the latter represent tier 4. The diagenetic cements filling the tier-3 Thalassinoides are arranged in two phases. The first cement generation constitutes a continuous rim that coats the burrow wall and has consistent δ13C values of approximately −8 ‰ to −12 ‰, indicative of bicarbonate originating from the anaerobic oxidation of methane. In contrast, the second cement generation is dominated by saddle dolomite precipitated at temperatures >80 ∘C, at a time when the pseudobioherm was deeply buried. The fact that the tubes remained open until deep burial means that vertical fluid communication was possible over the whole vertical extent of the pseudobioherm up to the seafloor during its active development. Therefore, vertical growth was fostered by this open burrow network, providing a high density of localized conduits through the zone of carbonate precipitation, in particular across the sulfate–methane transition zone. Burrows prevented self-sealing from blocking upward methane migration and laterally deflecting fluid flow. One key aspect is the geometric complexity of the burrows with numerous subhorizontal segments that could trap sediment shed from above and, hence, prevent their passive fill.
... Figure 5.10b whitened prior to photography. For localities, see Table 5.1 mid-Cretaceous (Albian) of Spain (Wiese et al. 2015), the latest Cretaceous (Maastrichtian) of the James Ross Basin in Antarctica (Little et al. 2015), the earliest Paleocene (Danian) of California (Blouet et al. 2017), the early Oligocene of Washington State (Zwicker et al. 2015), and the Miocene of New Zealand (Campbell et al. 2008;Troup 2010). Burrows attributed to crustaceans, including shrimps, have recently been reported from Late Cretaceous seeps in the Western Interior Seaway, USA (Landman et al., this volume). ...
Chapter
Full-text available
Numerous crustaceans such as ostracods, decapod crustaceans, and some barnacles inhabit modern cold seep ecosystems, but little is known about their fossil record in these chemosymbiotic-based ecosystems. Consequently, their importance in structuring faunas at these biodiversity hotspots on the sea floor is poorly known, including to what extent seeps acted as refuges from extinction, the timing of occupancy of cold seeps, the degree of endemism, depth preferences, and the longevity of crustacean lineages. We provide the first synthesis of crustaceans in ancient seeps and show that they have been found in each continent due to a rapid increase in research since the 1990s. Ostracods and barnacles are known from body fossils alone. Conversely, decapods are represented by two types of fossils: body fossils primarily attributed to true crabs and ghost shrimps and their traces such as coprolites, repair scars, and burrows. The last ~150 million years saw a remarkable rise in the number of localities and occurrences of seep crustaceans, mostly caused by the diversification of decapods in a variety of environments including seeps. Although considerable progress was made in 30 years, the relatively unexplored fossil record of seep crustaceans provides ample opportunities for further taxonomic, macroevolutionary, and paleoecological research.KeywordsArthropodaAxiideaBarnacleBrachyuraBurrowCirripediaCoproliteCrustaceaDecapodaMethane seepOstracodaRepair scarTrace fossil
... These units dip approximately 40⁰ eastward toward the San Joaquin Valley. These form part of the West-side fold belt (Vétel and Cartwright, 2010;Martin, 2016;Blouet et al., 2017). ...
Article
In this study, we examine the errors and uncertainties associated with orientation measurements collected from digital outcrop models using the geometrical property, collinearity. Collinearity is expressed as the characteristics of a set of points lying on a single straight line, and is beneficial because a trace far from collinear is required for obtaining accurate orientation measurements of planar geological bodies from digital outcrop models. We, thus, demonstrate this relationship, as well as assess the impact any associated errors have on orientation distribution forms using orientation measurement traces collected from sandstone intrusions with a digital outcrop model of the Panoche Giant Injection Complex acquired through light detection and ranging and photogrammetry techniques. Our experiments highlight how in addition to sampling bias and sample size, unreliable orientation estimates can negatively affect interpretation. We show that the distribution of orientation for a network of geological structures (e.g., fractures, sandstone intrusions) in a particular region can be altered with the inclusion of erroneous measurements. From our case study, it was noted that the high proportion of unreliable orientation measurements obtained when using a digital outcrop model of the study area resulted in inconsistencies in the structural analysis that could have been attributed to other factors. Thus, without putting into consideration the reliability of the samples, a sampling issue will still be faced, regardless of using the appropriate sample size and technique.
... These depressions tend to be circular and measure a few to hundreds of meters in diameter and between 1 m and 20 m in depth (Judd and Hovland, 2007). Gas seepage influences the seafloor topography by creating pockmarks and sea mounds, which are seafloor depression features on seabed caused by sediment removal and dome-shaped features caused by precipitation of seep carbonates or methane-derived authigenic carbonates (Davis, 1992;Judd and Hovland, 1992;Hovland et al., 2002;Ho et al., 2012;Blouet et al., 2017). Another class of acoustic anomalies associated with shallow gas on seismic data is 'bright spots' characterized by high amplitude reverse polarity reflectors (i.e., Sheriff and Geldart, 1995). ...
Article
High resolution multi-channel seismic reflection data (∼1000 km) and multibeam echosounder bathymetry from the southeastern Korean continental shelf of the East Sea (Japan Sea) reveal numerous shallow gas indicators and seepage-related features, such as bright spots, enhanced reflections, seismic chimneys, acoustic blanking, pockmarks, and bathymetric mounds. Bright spots, indicating gas-charged layers, appear as local negative-polarity reflection anomalies (up to 5 km wide) and occur at various stratigraphic levels within a subsurface depth of ∼320 m. Bright spots covering an area of ∼60 km² are clustered at the tip of NE-SW–trending reverse faults in the northeastern and southeastern part of the investigated region, suggesting gas entrapment. Enhanced reflections (ca. 20-km-long) are developed along erosional unconformities and tilted sedimentary layers below them. This suggests that unconformities formed during sea-level low stands in the study area are potential reservoirs and may have acted as potential conduits for lateral migration of gas-rich fluids due to their permeable nature. Some enhanced reflections are formed along interfluves of channels where channel walls cut them, and thus they may potentially act as fluid reservoirs. Seismic chimneys, expressed as vertical disturbances in seismic data, are interpreted as the upward movement of fluids (i.e., either in liquid or gaseous form). Lack of faulting in some seismic chimneys suggests higher permeability in the sedimentary interval, which would allow the migration of deeper-sourced fluids. Pockmarks (up to 500 m in diameter) are typically associated with seismic chimneys in the sub-seabed, suggesting that they were formed by the explosive emission of gas or gas fluids. Some exhibit mound-like features near their crests that are interpreted as carbonate mounds. The locations of mounds above uplifted fault blocks in the central part suggest a structural control on the formation of these seabed features.
Chapter
This chapter summarizes information about ancient hydrocarbon seeps from around the world. The information is organized into two tables, one comprising both Americas and Antarctica, the other Africa, Arctic, Asia, Europe, and New Zealand. Within each table, entries are organized by continent, and within the continent, by country, region, and state. Each entry contains the following information: the site, including locality information and geological formation, the age, the geologic context, the inferred water depth at which the seeps developed, a description of the deposits, the minimum reported value of δ13C of the seep carbonates if they have been analyzed, whether biomarkers have been reported, a description of the fauna, and relevant references.KeywordsBathymetryCarbonateChemosynthesisDeep seaEvolutionHydrocarbonsPhanerozoicSedimentary basin
Article
Giant sand injection complexes form, intricate, basin-scale fluid plumbing systems and document the remobilisation and intrusion of several tens of cubic kilometres of sand within the shallow crust in stratigraphic units 100's metres thick. This is the first detailed and extensive account of the Panoche Giant Injection Complex (PGIG), a regionally significant outcrop (>300 km2) and part of a larger subsurface development (>4000 km2) identified in boreholes and on seismic reflection data. Magnificent exposure of the PGIC occurs along the north western margin of the San Joaquin Valley and presents the opportunity to examine the regional geological significance of a giant sand injection complex and its origin in the context of a late Cretaceous – early Paleocene forearc basin. Between 25 and 49 km3 of sand were remobilised and injected, at least 0.35 km3 of which extruded onto the paleo-seafloor. Large sandstone intrusions often >10 m thick and laterally extensive on a kilometer scale formed saucer-shaped intrusions, wing-like intrusions and a variety of sill geometries along with volumetrically smaller randomly oriented dikes in a 200–300 m thick interval. Dikes prevail below and above this interval, some reaching the paleo seafloor and extruding sand. Networks of propagating hydrofractures form intensely brecciated host strata, some of which were intruded by sand. All intrusions formed in a single pulsed event in which the most intense hydrofracturing caused by supra-lithostatic fluid pressure occurred approximately 600 to 800 m below the paleo seafloor. A crudely orthogonal arrangement of dikes is preserved with most oriented normal, and less commonly oriented parallel to the oceanic trench associated with the late Mesozoic to early Tertiary North Pacific subduction. Dikes orthogonal to the trench opened against the minimum horizontal stress, which was parallel to the trench. Dikes parallel to the trench opened against the regional maximum horizontal stress along minor faults formed in extension caused by shallow crustal deformation. There is no evidence that compressional tectonics influenced the onset of elevated pore fluid pressure necessary to promote sand injection. However, tectonic compression was responsible for creating the basin physiography that locally increased subsidence and accelerated chemical diagenesis in the basin centre. PGIC outcrop, located along the basin margins, was unlikely to have experienced heating above 70 °C, equivalent about 2 km burial, so the effects of chemical diagenesis in the host strata of the injection complex had negligible potential to evolve significant pore water volume. In a deeper part of the basin approximately 150 km to the south, lateral equivalents of the host strata were subjected to heating >100 °C and would expel significant volumes of water displaced by quartz cementation and clay dehydration that caused lateral pressure transfer to the north and western margin of the basin where the PGIC formed. Estimates of the total volume of water expelled from the deep basin suggest that a fluid volume equivalent to a gross rock volume reduction <1% would have provided a fluid budget sufficient to fluidise and inject the sand that forms the PGIC. In terms of areal and vertical extent, volume and architecture the PGIC shares strong similarity with the regionally developed giant injectite systems of Tertiary age in the North Sea basin. In both cases regional sand injection is genetically linked to pressure transfer toward the basin margin from more rapidly subsiding basin centres. Aqueous fluid is derived from thermally driven chemical diagenesis of thick deep water clastic sandstone and smectitic mudstone or from deeper, stratigraphically older, aquifers.
Article
Full-text available
Natural surface gas seeps provide a significant input of greenhouse gas emissions into the Earth’s atmosphere and hydrosphere. The gas flux is controlled by the properties of underlying fluid‐escape conduits, which are present within sedimentary basins globally. These conduits permit pressure‐driven fluid flow, hydraulically connecting deeper strata with the Earth’s surface; however they can only be fully resolved at sub‐seismic scale. Here, a novel ‘minus cement and matrix permeability’ method using three‐dimensional X‐ray micro‐computed tomography imaging enables the improved petrophysical linkage of outcrop and sub‐surface data. The methodology is applied to the largest known outcrop of an inactive fluid‐escape system, the Panoche Giant Intrusion Complex in Central California, where samples were collected along transects of the 600 to 800 m stratigraphic depth range to constrain porosity and permeability spatial heterogeneity. The presence of silica cement and clay matrix within the intergranular pores of sand intrusions are the primary control of porosity (17 to 27%) and permeability (≤1 to ca 500 mD) spatial heterogeneity within the outcrop analogue system. Following the digital removal of clay matrix and silica (opal‐CT and quartz) cement derived from the mudstone host strata, the sand intrusions have porosity‐permeability ranges of ca 30 to 40% and 103 to 104 mD. These calculations are closely comparable to active sub‐surface systems in sedimentary basins. Field observations revealed at decreasing depth, the connected sand intrusion network reduces in thickness and becomes carbonate cemented, terminating at carbonate mounds formed from methane escape at the seafloor. A new conceptual model integrates the pore‐scale calculations and field‐scale observations to highlight the key processes that control sand intrusion permeability, spatially and temporally. The study demonstrates the control of matrix and cement addition on the physical properties of fluid‐escape conduits, which has significance for hydrocarbon reservoir characterization and modelling, as well as subsurface CO2 and energy storage containment assessment.
Article
Natural fractures are widespread in shales in sedimentary basins and can significantly increase the bulk permeability and enhance fluid flow, which are thereby considered to be a key factor in shale gas development, seal integrity analysis, site selection for CO2 sequestration and nuclear waste disposal. Although many fractures in shale owe their origin to tectonism, a substantial proportion of fractures are induced by shale diagenesis, hydrocarbon generation and expulsion during different stages of burial. The conditions under which non-tectonic fractures are generated during shale diagenesis generally include (1) fluid overpressure due to clay mineral dehydration (e.g., smectite to illite transition); (2) differential compaction caused by abrupt porosity reduction during silica diagenesis (e.g., opal-A dissolution); and (3) pressure solution in the rock matrix and neomorphic calcite. With an increasing thermal maturity of organic matter, natural hydraulic fractures can be produced in organic-rich shale during (1) early generation of biogenic gas; (2) oil generation and primary migration; and (3) cracking of oil to gas. At the early stage of the oil window, contraction fractures may also be formed in organic matter due to thermal shrinkage. A better understanding of the origins of non-tectonic fractures induced during shale burial may yield important implications for the interplay between fracture mechanics, inorganic/organic matter evolution and fluid flow in shale.
Article
Full-text available
Layer-bound arrays of polygonal compaction faults have long been considered as important migration routes for hydrocarbon fluids leaking to the surface across thick shale sequences. A classic example is the deep offshore of the Lower Congo Basin where numerous fluid-venting structures are present above a Pliocene polygonal fault system. In this paper we present a detailed seismic analysis of a newly recognised system of Quaternary-aged Linear Chimneys and their intersection geometries with pre-existing Pliocene-aged polygonal faults (PF). Most (73%) of the 209 chimneys analysed intersect the lower portions of polygonal faults and almost half of these are rooted in strata below the PF interval. This indicates that fluid (in this case gas) migrated vertically, cross-cutting polygonal faults as it ascended through the tier. This is a strong indicator that PFs did not provide viable migration pathways otherwise chimneys would terminate at the upper tip of the fault, which would be the most likely migration exit point. Only twice in the whole system of Linear Venting Systems did this occur. A sub-set of chimneys stems from or above PF planes but these are restricted to either the lower footwall or from the apex area of hanging wall. At best they are evidence of fluids migrating up the lower part of polygonal faults and exiting deep within the tier, then migrating through most of the tier in their own vertical leakage vents. These results provide strong indicators that at least within this part of the Lower Congo Basin polygonal faults were the least effective/favoured migration pathway and that it was more energy-efficient for migrating gas to hydrofracture its fine-grained overburden than to re-open polygonal faults.
Article
Full-text available
Seabed fluid flow is a widespread and important natural process. It has important consequences for sub-seabed and seabed geological features, and also for marine biological processes, and the composition of the oceans. Seabed fluid flow provides both hazards and benefits for human activities, and it is recognised that some sites are precious and need protection. Spectacular discoveries made during investigations of ocean spreading centres like the East Pacific Rise and the Mid-Atlantic Ridge have turned upside-down concepts of how our planet works. On the continental shelves, and more recently in the deeper waters of the slope and rise, the oil industry has not only driven technological advances, but has also been responsible for an increasing awareness of the fundamental role of fluids in sedimentary processes. Tryon, et al., 2001) pointed out that: "Subsurface fluid flow is a key area of earth science research, because fluids affect almost every physical, chemical, mechanical, and thermal property of the upper crust." They went on by saying that research in the deep biosphere, gas hydrates, subduction zone fluxes, seismogenic zone processes, and hydrothermal systems all are “directly impacted by the transport of mass, heat, nutrients, and other chemical species in hydrogeological systems." Mankind’s activities, particularly during the last century, have resulted in increasingly serious pollution of the marine environment. Some of the principal causes relate to the petroleum industry, yet natural processes have been responsible for petroleum ‘pollution’ for a far greater period of time. In the Bible God instructed Noah to make an ark and “coat it inside and out with pitch” (Genesis 6:14). Indigenous populations from parts of the world where seeps occur have made good use of the special properties of natural petroleum products; Native Americans in California used 'asphaltum' to caulk their canoes, hold together hunting weapons and baskets, for face paints, and even chewing gum (USGS, 2000). The 'eternal flames' of natural gas seeps in Azerbaijan are central to the Zoroastrian faith.
Article
The methane, which is dominantly biogenic, is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement. Carbonate precipitation occurs below the oxic layer. Cementation may be induced by three factors. The convergent margin setting engenders precipitation of authigenic carbonates in several ways.-from Authors
Book
Oases of life around black smokers and hydrocarbon seeps in the deep-sea were among the most surprising scientific discoveries of the past three decades. These ecosystems are dominated by animals having symbiotic relationships with chemoautotrophic bacteria. Their study developed into an international, interdisciplinary venture where scientists develop new technologies to work in some of the most extreme places on Earth. This book highlights discoveries, developments, and advances made during the past 10 years, including remarkable cases of host-symbiont coevolution, worms living on frozen methane, and a fossil record providing insights into the dynamic history of these ecosystems since the Paleozoic.
Conference Paper
This paper presents a methodology contributing to describe the filing of petroleum reservoirs based on the quantification of fluid alteration processes. Biodegradation of hydrocarbons in oil reservoirs is one of the important alteration processes of petroleum. Described by Bastin in 1926, it is a widespread mechanism. Hydrocarbon degrading microorganisms have been found until 4 km depths and temperatures ranging up to 80 °C . The alteration of oil leads to an increase of sulphur and metal compounds, and also to the increase of acidity, viscosity and density. The level of biodegradation is classically described by the presence of biomarkers presenting specific resistivity to biodegradation (Peters and Moldowan, 1993). Haeseler et al. proposed in 2010 a quantitative numerical model based on stoichiometric equations, describing, step by step, the evolution of hydrocarbon composition and physical properties in biodegraded oils. This tool was applied successfully to different African Basins (Lower Congo Basin, Niger Delta Basin and Albert Lake Basin) showing excellent correlation between the computed and the analysed composition of oils presenting increasing biodegraded grades. The biodegradability coefficients of the different hydrocarbon classes were shown to follow always the same rules. In the present paper, we applied the dynamic version of this model. In addition to the compositional data, parameters provided by Basin simulators like expulsion history from the source rock, reservoir temperature curves, reservoir volumes… were integrated. Applied to a biodegraded play located offshore Africa, this biodegradation model allowed various parameters describing the filling history of the Oligocene and Miocene reservoirs to be determined: total quantities of hydrocarbons lost by biodegradation, total quantity of methane generated. This allowed to calculate the methane losses over geological times by taking into account presently measured GOR and GOR expected from the numerical simulation, gas loss fluxes: providing a geological description of reservoir sealing efficiency, the kinetic parameters of the biodegradation for each compound class, oil filling path in the reservoirs. The oils from the Miocene reservoirs must have previously been stored (and altered) in the Oligocene reservoirs. A direct arrival in the Miocene reservoirs from the source rock was excluded, the storage time of the oils in Oligocene and Miocene reservoirs. This multidisciplinary work allows to be optimistic about the possible existence of a general law governing the kinetics of the biodegradation and applicable for sedimentary Basins. It also showed that a qualitative and quantitative description of an oil alteration process contributes to a better understanding of fluid paths and the nature of fluids in place. It builds a strong scientific base for the description of fluid differences at reservoir scales that can be applied for various geochemical applications.
Article
Active continental margins and the active flanks of island arcs lie in the forearc regions of arc-trench systems generated by plate consumption. Arc-trench systems are initiated by contractional activation of previously rifted continental margins, by reversal of subduction polarity following arc collisions, and as island arcs within oceanic regions. The varied configurations of shelved, sloped, terraced, and ridged forearcs arise partly from differences in initial geologic setting, but mainly from differences in structural evolution during subduction. In regions where large quantities of sediment are delivered, forearc terranes enlarge during subduction through linked tectonic and sedimentary accretion of deformed ocean-floor sediments and igneous oceanic crust, uplifted rench-floor and trench-slope sediments, and the depositional fills of subsiding forearc basins. Where sediment delivery is small, enlargement is subdued or absent, and shortening of the arc-trench gap may be possible. Trench inner slopes typically are underlain by growing subduction complexes composed of imbricate underthrust packets of ocean-basin, trench-floor, and trench-slope sediments in thrust sheets, isoclines, and melanges. The structure of subduction complexes is governed by the thickness and nature of oceanic layers rafted into the subduction zone, variable thicknesses of trench and slope sediments, and the rate and obliquity of plate convergence. Forearc basins between the magmatic arc and the trench axis include (a) intramassif basins lying within and on basement terranes of the arc massif, (b) residual basins lying on oceanic or transitional crust trapped between the arc massif and the site of initial subduction, (c) accretionary basins lying on accreted elements of the growing subduction complex, (d) constructed basins lying on the arc massif and accreted subduction complex, and (e) a composite of these basins. Strata deposited in forearc basins are typically immature clastic sediments composed of unstable clasts derived from rapid erosion of volcanic mountains or uplands of plutonic and metamorphic rocks within the arc massif. In equatorial regions reef-carbonate associations are also common. Facies patterns of turbidites, shelf sequences, and fluviodeltaic complexes within forearc basins are governed by the elevation of the basin thresholds, the rate of sediment delivery, and the rate of subsidence of the substratum. Petroleum prospects in forearc regions typically are limited by the prevalence of small, obscure structures within the subduction complex, the scarcity of good reservoirs in the forearc basin, the limited occurrence of source beds, and low geothermal gradients except within the arc massif where heat flux is commonly excessive.