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

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 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.

Full text

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
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
Caslow
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.
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
Total’s headquarters from 2012 to 2014. He
is currently researching the petroleum
significance of seep carbonates in an industry-
funded Ph.D. project with Total at the
University of Fribourg (Switzerland).
Patrice Imbert ~Centre Scientifique 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 first in
turbidite sedimentology and seismic
interpretation in Total’s headquarters and
occupied various positions overseas in
exploration and reservoir geology. His
current research interests are fluid flow 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 financing 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; final acceptance October 17, 2016.
DOI:10.1306/10171616021
AAPG Bulletin, v. nn, no. nn (nn 2016), pp. 1–31 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 sulfate–methane transition zone (SMTZ; Boetius et al.,
2000). The first direct observations of seep carbonates pre-
cipitating on the sea floor originated from Hovland et al. (1985)
and Ritger et al. (1987). Later, Roberts (2001) demonstrated that
seep carbonate facies were controlled by the hydrocarbon flux 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 flux were sufficient to
exceed the rate of sulfate diffusion into the sediment or within
the subseafloor 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
world’s 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, fluidization, 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
identified, 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 field 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 field 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 beneficial.
EDITOR’SNOTE
Color versions of Figures 1–15 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 defined 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, identification of a recurrent paragenetic
sequence, and distribution of a chemosynthetic fossil
fauna, this study evaluates the paleofluid flux. 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 floor 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,
anorthwest–southeast 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 filled 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 infill 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
fine-grained units divided into five 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 defined (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 floor 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 floor.
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 floor
(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-
fied geological map of California.
(C) Schematic cross section of the
western American plate during
the early Paleocene. (D) Recent
geological profile (modified 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 identification of all minerals was performed
Figure 2. General stratigraphy
and lithology of the Panoche Hills
in the Escarpado–Right 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 –0.05‰for carbon and –0.14‰for
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 A”and “outcrop B”hereafter.
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
five 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, filled 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 identified 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 first
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 first. 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 flute 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 fine-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 filled 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 identified 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 define a specificstratigraphic
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;
peloids’diameters 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
fibrous, subhedral,
dendritic
Maximal fiber
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 fluid 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
fluorescence,
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
fluorescence,
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 fluorescence,
“dirty”white
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 fibrous fans with undulose
extinction; extremely rich in
micron-sized, monophased
liquid fluid 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.;
fibrous 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 fluo-
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 infill 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
fibrous (Figure 8F), subhedral (Figure 8G), and
dendritic (Figure 8G, H). Two different morphol-
ogies of dendrites have been identified: 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 fills 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 fluoresce 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, filled 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 fields. The firstclusterischarac-
terized by extremely low d
13
Cvalues(-38.15‰to -
50.94‰)andthehighestd
18
O values (0.21‰to
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 3.64‰to -3.68‰
and from 0.99‰to -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
infill of the septarian cracks (N°13, N°15, N°16, and
N°17), characterized by slightly depleted d
13
C values
(-4.99‰to -7.65‰) and low, but variable, d
18
O
values (-2.19‰to -6.06‰).
INTERPRETATION
The very low d
13
C values of the early carbonate
cements (N°1–N°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 first 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 influx 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 infill of burrows by methane-derived cements
suggests that burrows acted as open pathways for
focused hydrocarbon flow. 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 fluorescent, 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 electron–scanning electron microscope (BSE–SEM) picture showing the micritic rim (N°3) composed of
fibrous barite and several carbonate phases. (F) A BSE picture of the micritic rim (N°3) showing fibrous 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 magnification. (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 floor 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 significant 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 fluid flow sequence took place.
Figure 10. Late diagenetic minerals. (A) Burrow coated by micrite (N°1) and lined by early cements (N°2–N°6). Both have been crosscut
by a crack filled 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 fluid 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 first 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 fluids can be
distinguished: methane, marine, and meteoric waters. Beginning of late diagenesis is marked by the first 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+2O2↔CO2+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 SMTZ”of 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 subseafloor 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 flux was strong enough to consume all
the oxygen in the sediment, the SMTZ may even
have risen up to the sea floor.
Element 2 of the Elementary Sequence: Micritic Rim (N°3)
The minerals constituting the micritic rim (N°3) are
the first authigenic precipitates found within the
burrows. The great diversity of mineralogies and
mineral fabrics may reflect either frequent varia-
tions in the physicochemical parameters of the
fluid (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).
Bariumcontainedincoldseepfluids 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
afluid 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 SMTZ”described 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, first
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 fluid decreased, as well as the
rate of AOM. Moreover, in seep carbonate envi-
ronments, aragonite is often associated with “near
sea floor conditions”(Burton, 1993; Aloisi et al.,
2000; Bayon et al., 2007). Thus, this may reflect
a deepening of the SMTZ associated with the de-
crease of hydrocarbon flux. As a whole, it suggests
a relatively long period of low fluid flux 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
biofilms (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 flux rate and stromatolitic texture. However,
the decreasing trend of flux 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 flux 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 seepage’s 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 filled 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 fluid system.
The isotopic trend observed in the cements of
septarian cracks, from marine to meteoric fluids,
probably results from the progressive infill 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 fluid flow 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 fluorescence 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-floor temperature (T
min
–T
max
) and heat flow 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 floor. 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 hydrocarbons’d
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 sufficient 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, flux 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 =sulfate–methane 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 flux, sea-floor 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 flow 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 reflectance data,
He et al. (2014) independently estimated a flux 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 flux 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-floor 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 Fourier’s
Law. Taking into account paleotemperature error
margins, a significant 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 flux 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 flux pushes the sulfate–methane 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 flux 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 significantly depleted, the upward flux 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 flux 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 fluid infiltration
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 floor and may
even have erupted locally as sand volcanoes (Vigorito
et al., 2008). Thus, after the injectite’s 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 verifies that the Moreno shale con-
stituted an efficient 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 fluids of the Moreno
Formation, including gas, started migrating into
the depressurized injectites. In a process analogous to
what happens after artificial hydraulic fracturing (e.g.,
for hydrocarbon production purposes in “tight reser-
voirs”or gas shale), gas drainage efficiency 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 flow
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 fluid
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
specific environment of the methane seeps, whose
flux was higher around mounds than around nodules
and crusts.
On sea-floor maps, Gay et al. (2006) noticed
that mussels, clams, and tube worms were restricted
to the central area of pockmarks with relatively high
fluid flow, 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-floor 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 crustaceans’access
to hydrocarbons. A possible positive feedback
could then be promoted. With more burrows
present, the gas flow would become more focused,
and more crustaceans would be attracted to this
area, once again increasing gas flow 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-methane–generating
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 floor. 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 first 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

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