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Thermal Maturity and Kerogen Type of Badenian Dispersed Organic Matter from the Getic Depression, Romania

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The aim of this study is to evaluate the thermal maturity of Upper Badenian (Middle Miocene) petroleum source rocks of the Getic Depression, Romania, and to characterize the dispersed organic matter using organic petrography associated with Rock-Eval pyrolysis. A total of 33 core samples of Upper Badenian source rocks from the central–southern part of Getic Depression was studied. The results show that most samples with values of total organic carbon (TOC) < 1% wt.% have a limited potential of hydrocarbons (HC) generation, and 30% of samples with TOC < 1.82 wt.% and kerogen type III, presenting particularly gas generation potential. In three samples from the Bibești, Grădiște and Socu structures the kerogen type III-II was identified, indicating the capability of oil and gas generation. The Badenian source rocks are thermally immature, as few samples are in the pre-oil window, with values of vitrinite reflectance (VRo%) ranging between 0.41% and 0.55%, and the values of Tmax between 409 °C and 443 °C. Optical microscopy with reflected white light and fluorescence blue light was used for identification of terrigenous macerals (vitrinite, liptinite as, resinite, cutinite, sporinite, and inertinite) associated with marine liptinite macerals (telalginite and lamalginite) showing yellow and bright–yellow epifluorescence.
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Citation: Ghiran, M.D.; Popa, M.E.;
Maris
,, I.; Predeanu, G.; Gheorghe, S
,.;
al˘anescu, N.M. Thermal Maturity
and Kerogen Type of Badenian
Dispersed Organic Matter from the
Getic Depression, Romania. Minerals
2023,13, 202. https://doi.org/
10.3390/min13020202
Academic Editor: Thomas Gentzis
Received: 31 December 2022
Revised: 26 January 2023
Accepted: 27 January 2023
Published: 30 January 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
minerals
Article
Thermal Maturity and Kerogen Type of Badenian Dispersed
Organic Matter from the Getic Depression, Romania
Maria Doina Ghiran 1,2, Mihai Emilian Popa 2,3,4,* , Izabela Maris
,5, Georgeta Predeanu 6, S
,tefania Gheorghe 1
and Niculina Mihaela Bălănescu 6
1OMV PETROM S.A.-I.C.P.T. Câmpina, 29 Culturii Ave., 105600 Câmpina, Romania
2Doctoral School, Faculty of Geology and Geophysics, University of Bucharest, 6 Traian Vuia Str.,
020956 Bucharest, Romania
3Laboratory of Palaeontology, Department of Geology, Faculty of Geology and Geophysics, University of
Bucharest, 1 N. Bălcescu Ave., 011401 Bucharest, Romania
4School of Geosciences and Technology, Southwest Petroleum University, 8, Xindu Ave.,
Chengdu 610500, China
5Department of Mineralogy, Faculty of Geology and Geophysics, University of Bucharest, 1 N. Bălcescu Ave.,
011401 Bucharest, Romania
6Research Center for Environmental Protection and Ecofriendly Technologies, University Politehnica of
Bucharest, 1-7 Gheorghe Polizu Str., 011061 Bucharest, Romania
*Correspondence: mihai@mepopa.com
Abstract:
The aim of this study is to evaluate the thermal maturity of Upper Badenian (Middle
Miocene) petroleum source rocks of the Getic Depression, Romania, and to characterize the dispersed
organic matter using organic petrography associated with Rock-Eval pyrolysis. A total of 33 core
samples of Upper Badenian source rocks from the central–southern part of Getic Depression was
studied. The results show that most samples with values of total organic carbon (TOC) < 1% wt.%
have a limited potential of hydrocarbons (HC) generation, and 30% of samples with TOC < 1.82 wt.%
and kerogen type III, presenting particularly gas generation potential. In three samples from the
Bibe
s
,
ti, Grădi
s
,
te and Socu structures the kerogen type III-II was identified, indicating the capability
of oil and gas generation. The Badenian source rocks are thermally immature, as few samples are in
the pre-oil window, with values of vitrinite reflectance (VR
o
%) ranging between 0.41% and 0.55%,
and the values of T
max
between 409
C and 443
C. Optical microscopy with reflected white light
and fluorescence blue light was used for identification of terrigenous macerals (vitrinite, liptinite as,
resinite, cutinite, sporinite, and inertinite) associated with marine liptinite macerals (telalginite and
lamalginite) showing yellow and bright–yellow epifluorescence.
Keywords:
organic matter; macerals; kerogen; Upper Badenian; thermal maturity source rocks;
Southern Carpathians; Romania
1. Introduction
The Getic Depression occurs in the foreland of the Southern Carpathians, an important
geological unit of Romania with numerous oil fields between the Danube and Dâmbovi
t
,
a
Valley. A limited number of studies on the use of organic petrology for the thermal maturity
evaluation of the Getic Depression was published. This work addresses the maturity
of Badenian source rocks of the Getic Depression (central-southern part) following the
previous research of the immature Oligocene source rocks of Getic Depression (central-
western part) done by [
1
]. The current research continues and completes previous geological
assessments dealing with the basin with new organic petrology data.
In the area of the Getic Depression investigated by the authors, the generation and
expulsion of hydrocarbons took place in two separate areas: a central-southern area and an
eastern area [
2
9
]. In the central - southern area, where the maximal depths were reached,
Minerals 2023,13, 202. https://doi.org/10.3390/min13020202 https://www.mdpi.com/journal/minerals
Minerals 2023,13, 202 2 of 22
the oil generation occurred from Eocene and Lower Oligocene (Rupelian) formations during
the late Oligocene (Chattian) in pelitic rocks, while the oil expulsion began in the early
Burdigalian. In the eastern area, where the Oligocene formations occur at shallower depths,
the generation and expulsion began in the early Miocene from Burdigalian formations and
continues today [4,6,7,10].
For several decades the distribution and type of petroleum prospects based on the
evaluation of petroleum source rocks, hydrocarbon generation potential estimation and oil
& gas fields identification were undertaken [220].
The ways of evaluating the characters of the source rocks are well established world-
wide, such as:
1.
assessment of the level of thermal maturity and the amount of organic matter and
type of kerogen [2131].
2.
microscopic identification of hydrocarbons source rocks, using thermal alteration
index and vitrinite reflectance [21,25,2730,3237].
The colors differences of organic matter induced by thermal changes can be observed
in transmitted and fluorescent light, aiding the identification of the phytoclast alteration
rank on a 1 to 5 scale of the thermal alteration index (TAI, [21,37]).
The thermal alteration provides the organic matter color changing [
21
,
37
]. Total
organic matter (TOC) content commonly assesses the amount of organic matter in a rock
sample.
The maturation parameters such as temperature (T
max
) and production index (PI),
resulted from the Rock—Eval pyrolysis associated with vitrinite reflectance (VR
o
%), allow
to assess the stage of maturation [23,32,3845].
The thermal maturity of organic matter increases with temperature, as it converts and
further expels hydrocarbons during diagenesis, catagenesis, and metagenesis [
21
,
32
,
37
,
46
57
].
With the aim to express thermal maturity and determination of kerogen type from
the perspective of organic petrology, the correlation between dispersed organic matter
microscopy and geochemical analysis is detailed, combining geological and petrographical
approaches with statistical approaches applied to Badenian source rocks of the Getic
Depression. The vitrinite reflectance was measured and the types of macerals in reflected
and fluorescent light were identified. Fluorescent properties of the liptinite macerals were
proven to be an important distinguishing feature leading to the identification of the kerogen
types and hydrocarbons generation potential of the selected samples.
Therefore, it is possible to outline the Getic Depression and to assess the organic facies
in this area. The results obtained on the core samples from the Institute of Research and
Technological Design I.C.P.T. Câmpina—OMV PETROM S.A. are going to be useful both
nationally and internationally, due to the significance of the organic matter identified in
the studied source rocks of Romania, in the frame of their wider stratigraphic, European
context.
2. Geological Setting
The Getic Depression is developed as a narrow sedimentary basin, elongated from west
to east, between the South Carpathians Orogen in the north and the Moesian Platform in the
south, the Dâmbovi
t
,
a Valley in the east and the Danube River in the west (Figure 1), [
58
60
].
The tectonic evolution of the Getic Depression was marked by a northward continuous
subduction movement of the Moesian Platform beneath the South Carpathians Orogen.
Since the moment of its formation as a sedimentary basin during the Eocene, the Getic De-
pression recorded three significant orogenetic phases: the Saavic (at the end of Oligocene),
Styrian (extension/transpression during the Palaeogene until the Lower Miocene) and
Moldavian (deformations during the middle to late Miocene) [5862].
Previous authors [
63
72
] detailed the strike-slip evolution of the basin during the
Palaeogene to the early Miocene. These strike-slip deformations were mainly generated
during the early Miocene, related to the movement and rotation of the Inner Carpathians
over the Moesian Platform [62,66,71,72].
Minerals 2023,13, 202 3 of 22
Figure 1.
(
a
) Occurrence of the studied area (Google Earth source); (
b
) Simplified tectonic map of the
Getic Depression, and of the South and East Carpathians, with the studied area marked as rectangular
figure. Modified from [58,66,69].
The Getic Depression accumulated sediments which were transported southwards
from the Carpathians chain, showing a fining upward general trend [
58
60
]. The almost
entirely clastic succession was interrupted by evaporites (as local interlayers of anhydrite
and salt in Aquitanian, Burdigalian and Badenian) during the early Burdigalian and the
Badenian, while the mountain belt in the north was continuously uplifted during the
geological evolution of the Getic Depression, beginning with the Eocene, and ending
during the Pliocene (Romanian, Figures 2and 3).
Since the Eocene, marine conditions occurred throughout the entire Getic Depression.
The Eocene sediments lay unconformably and transgressively over the Cretaceous and
Jurassic formations, including conglomerates, coarse and fine sandstones in the west and
with interlayers of marls in the rest of the region (Figure 2, [59,60]).
During the Oligocene, the marine environment was anoxic, with deep sea episodes,
the basin acquiring an asymmetrical and narrow shape, elongated along a west—east
trend. A strong subsidence occurred in the central part of the basin, concurrently with a
strong uplift of its northern margin, while the erosion process of the Mesozoic crystalline
formations and of the Eocene deposits was activated. During the late Oligocene, the sea
extended to the north, beyond the outcrops along the Getic Depression’s margin [59,60].
The Miocene formations lay conformably over the Oligocene. During the Burdi-
galian, two distinct sedimentation cycles were recorded, the early Burdigalian and the
late Burdigalian sequences as the sea covered most of the Getic Depression. The Lower
Burdigalian sediments include marly sandstones, locally conglomeratic, with marls and
shales rarely associated with thin anhydrite interlayers and a salt bearing sequence. The
climate was arid and the accumulation of evaporites was possible. In the eastern part
of the Getic Depression, the Lower Burdigalian occurs as local salt and anhydrites se-
quences. The Upper Burdigalian deposits occur transgressively overlaying older forma-
tions, starting with a sequence with coarse sandstone, conglomerate, and marl interlayers
(100–1000 m), followed by a marly-sandstone sequences (400–600 m), and ending with
sandstone (500–600 m). The Upper Burdigalian sequence was influenced by a marine phase,
with a strong freshwater phase to its top. The water freshening was related to the water
supply from the continent, inducing a brackish fauna [
60
]. In the eastern part of Getic
Depression, the thickness of Upper Burdigalian is reduced due to the partial non-deposition
of the lower sandy complex and to strong erosion [60].
Minerals 2023,13, 202 4 of 22
Figure 2. Stratigraphic log of the Getic Depression, Romania, simplified after [60].
Figure 3.
N–S Geological cross-section of the Getic Depression through Coliba
s
,
i–Vladimir–Bulbuceni
structures. Modified from [61].
The Badenian sequence lays unconformably over the Burdigalian (Figures 2and 3),
with the same spreading area during the Sarmatian. The Badenian stratigraphy of the Getic
Depression is marked by four horizons corresponding to a sedimentary transition from
deep marine to epicontinental conditions:
1. the Globigerina-bearing tuffs and marls (Lower Badenian).
2. the salt breccia (Upper Badenian), with gypsum interlayers.
3. the radiolaria-bearing shales.
4. the Spiratella-bearing marls (ending the Badenian).
The depositional and geochemical evolution of the Getic Depression during the Bade-
nian was not standard, with lots of geochemical changes, with sediment-starved, remanent
basins controlled tectonically. The depositional shift was frequent, therefore in the sediment-
starved basins fine sediments (as the shales with radiolarians) were deposited, although
the depths were getting shallower continuously.
All these levels correspond to a deep marine—epicontinental environment [
60
], marked
by the Orbulina (Lower Badenian) and Valapertina (including the assemblage with Sphaeroid-
Minerals 2023,13, 202 5 of 22
ina bulloides, Upper Badenian) foraminiferal biozones. To these foraminiferal assemblages
are added the nannoplanktonic assemblages with Discoaster exilis (Lower Badenian) and
Helicosphaera minuta (Upper Badenian) and the palynological assemblages with Nemath-
osphaeropsis and Svalbardella (entire Badenian). The tuffs and marls with Globigerina were
locally identified in wells, although their thickness varies strongly due to non-deposition
or erosion.
The salt breccia crossed by wells occurs in structurally lower areas (Vladimir and
Grădi
s
,
te), and these horizons yields locally gypsum interlayers. The thickest Badenian
deposits occur in the southern and western parts of the Getic Depression, while they
disappear eastwards over large areas. The transition from marine to brackish environments
occurred during the Sarmatian.
The tectonic movements of the Moldavian (Attic) paroxysmal phase generated the
southward thrust of the Getic Depression over the Moesian Platform. These movements
occurred during the whole Sarmatian with structural uplifts, especially in the western part
of the Getic Depression.
The western part recorded continuous downward movements, generating thick se-
quences of Sarmatian and Badenian deposits, while in the eastern part, due to the uplift
movements, these deposits are missing on large areas.
The Sarmatian deposits include sandstones, sand, and grayish marl layers, variable in
thickness. The Maeotian deposits are predominantly pelitic, with marls, thin sands, and
sandstones, influenced by proximal shelf conditions with brackish, mesohaline, dynamic
waters. The Pontian deposits conformably overlay the Maeotian sequences, with marls and
sandy marls, while the Pontian-Dacian-Romanian sequences include sandstones and marls,
with coal seams [60] (Figures 2and 3).
3. Materials and Methods
The samples were selected from borehole cores having their repository with the Insti-
tute of Research and Technological Design I.C.P.T. Câmpina—OMV PETROM S.A. A total
of 33 Badenian core samples from 21 wells belonging to 14 structures were studied for the
hydrocarbon’s generation potential and for their organic geochemical and petrographical
features (Table 1). Organic-rich, black shales and marls samples occurred on the following
structures: Socu, Totea, Hurezani, Vladimir, Piscu Stejarului, Bibe
s
,
ti, Bibe
s
,
ti-Bulbuceni,
Bulbuceni, Logres
,ti, Drăganu, Grădis
,te, Colibas
,i, Rădines
,ti and Budieni (Figure 4).
A series of wells yield single core samples from Upper Badenian sequences
(A-1 Grădi
s
,
te, A-1 Drăganu, A-1 Budieni, A-1 Coliba
s
,
i, and A-1 Rădine
s
,
ti). They were
relevant for the Upper Badenian from depths between 2200–2900 m. The Upper Badenian
is thin, therefore, supplementary samples could not be probed.
Samples preparation was performed in accordance with SR ISO 5069-2: 1994 [73].
The microscopic studies on dispersed organic matter were performed on polished
blocks to identify petrographic composition, and vitrinite reflectance. For preparation
of pellets for the microscopical study, samples with various lithologies, from clays with
poorly consolidated siltstone, claystone to marly siltstone were embedded in epoxy resin
and polished with different grain sizes of carborundum paper and alumina according to
ISO 7404-2: 2009 [
74
]. The vitrinite reflectance analysis was performed in accordance with
ASTM D7708-11: 2014 [
75
] methodologies. The highest number of vitrinite reflectance
measured points was 30. The standard deviation is between 0.03 and 0.08.
The pellets were analyzed using an Olympus BX50 optical microscope, equipped
with a 50
×
oil immersion objective and a Tidas photometer MSP 200 Vers. 3.47. Ro,
calibrated against the Sapphire reflectance standard (0.594% Ro) for vitrinite reflectance
measurements. For the Rock-Eval analysis (conducted on duplicate samples), about 70 mg
of each selected core sample was crushed, sieved, and weighed into a steel crucible [
76
].
A Rock-Eval 6 equipment was used to investigate the type of kerogen, thermal maturity,
source rock potential, and generated hydrocarbons. The guidelines provided by [
25
27
,
41
]
were followed in interpreting the results.
Minerals 2023,13, 202 6 of 22
Table 1.
Geochemical results of Rock-Eval analysis and vitrinite reflectance of Badenian Source Rock from Central-Southern part of the Getic Depression.
Abbreviations and Acronyms: VRo%—measured vitrinite reflectance; TOC—total organic carbon of the rock sample, automatically calculated and recorded in
weight ratio (wt.%); S
1
—amount of free hydrocarbons from the rock samples (mg HC/g rock); S
2
—remaining hydrocarbons generation potential of the source rock
(mg HC/g rock); S
3
—oxygen compounds, amount of CO
2
from organic source (mg CO
2
/g rock); Tmax-temperature of maximum hydrocarbon generation from a
rock sample during pyrolysis analysis (
C); HI—Hydrogen Index (HI = S
2
/TOC)—the amount of hydrogen relative to the amount of organic carbon present in a
sample (mg HC/g TOC); OI—Oxygen Index (OI = S
3
/TOC)-the amount of oxygen relative to the amount of organic carbon present in a sample (mg CO
2
/g TOC);
PI—Production Index = S
1
/S
1
+ S
2
; S
2
/S
3
ratio-oil or gas generation potential. Petrographyc composition: H—huminite; V—vitrinite; L—liptinite; S—sporinite;
C—cutinite; R—resinite; Lag—lamalginite; Tag—telalginite; Ts—Tasmanites; Pr—other Prasinophyceae; Ls—Leiosphaeridia; Ld—liptodetrinite; I—inertinite;
F—fusinite; In—inertodetrinite; Py—pyrite; Sd—siderite; B—bioclasts.
No. Well Depth, m TOC wt.% S1, mg
HC/g rock
S2, mg
HC/g rock
S3, mg
CO2/g
rock
Tmax,
C
HI, mg
HC/g
TOC
OI, mg
CO2/g
TOC
PI
(S1/
(S1+S2)
S2/S3VRo, % Standard
Dev.
Petrographic
Composition
Kerogen
Type
Generation
Potential
1 A-1 Grădis
,te
2200–2300
1.82 0.23 3.74 1.25 409 206 69 0.06 3.00 0.47 0.05 V, Lag, Tag, R,
In, Py, Sd, B III gas
2 A-1 Drăganu 0.41 0.01 0.13 0.74 430 32 180 0.07 0.18 0.45 0.04 V, I, Sd, B IV poor
3 A-1 Colibas
,i 2600–2700 0.07 0.01 0.01 0.93 433 14 211 0.05 0.01 0.49 0.03 V, Tag (Pr), Py IV poor
4 A-1 Rădines
,ti 2700–2800 1.35 0.14 2.01 0.73 419 150 55 0.07 2.75 0.41 0.07 V, C, S, Tag (Pr,
Ls), Py III gas
5 A-1 Budieni 2800–2900 0.59 0.07 0.6 0.26 423 102 44 0.11 2.31 0.42 0.05 V, Lag, Tag (Pr,
Ls), I, Py III gas
6 A-1 Hurezani 3000–3100 0.89 0.06 1.05 0.45 434 118 51 0.05 2.33 0.42 0.05
V, C, R, S, Tag
(Ts, Pr), F, Py,
Sd
III gas
7 B-1 Hurezani
3100–3200
0.99 0.03 1.02 0.87 430 103 88 0.03 1.17 0.46 0.03
V, S, C, Tag (Ts,
Ls, Pr), Ld, I,
Py, Sd
III gas
8 B-1 Socu 0.68 0.02 0.66 0.64 430 97 94 0.03 1.03 0.44 0.03 V, Lag, Tag (Ts,
Pr), R, S, Py, Sd III gas
9 A-2 Hurezani 1.20 0.06 1.61 0.52 433 134 43 0.04 3.10 0.48 0.04 V, Ld, Py, Sd III gas
10 A-1 Totea 0.81 0.04 1.23 0.75 433 152 92 0.03 1.64 0.52 0.03 V, S, Tag (Pr, Ls,
Ts), I, Py III gas
11 C-1 Socu 0.54 0.02 0.55 0.81 435 103 150 0.03 0.68 0.49 0.04 H/V, Tag (Ts,
Pr, Ls), R, S, Py IV poor
12 D-1 Socu 0.5 0.03 1.25 0.96 433 272 195 0.03 1.30 0.47 0.04 V, Tag (Ts, Ls,
Pr), R, I, Sd, B III gas
13 D-2 Socu 0.39 0.01 0.4 1.06 435 103 278 0.02 0.38 0.46 0.04 V, Tag (Ts, Ls,
Pr), S, I, Py IV poor
Minerals 2023,13, 202 7 of 22
Table 1. Cont.
No. Well Depth, m TOC wt.% S1, mg
HC/g rock
S2, mg
HC/g rock
S3, mg
CO2/g
rock
Tmax,
C
HI, mg
HC/g
TOC
OI, mg
CO2/g
TOC
PI
(S1/
(S1+S2)
S2/S3VRo, % Standard
Dev.
Petrographic
Composition
Kerogen
Type
Generation
Potential
14 A-2 Colibas
,i
3200–3300
0.48 0.02 0.24 1.08 429 50 255 0.08 0.22 0.52 0.04 V, Tag (Ls, Pr),
Ld, S, Py IV poor
15 C-2 Socu 1.04 0.07 1.15 0.83 434 111 80 0.05 1.39 0.44 0.06 V, S, Tag (Ts,
Pr), Py, Sd III gas
16 A-1 Vladimir 1.08 0.05 1.09 0.93 432 101 86 0.04 1.17 0.48 0.03 V, Tag (Pr, Ls),
S, R, Py III gas
17 A-1 Socu 0.55 0.03 0.43 0.55 437 78 100 0.07 0.78 0.54 0.04
H/V, Tag (Ts,
Pr, Ls), S, R, Ld,
I, Py, Sd, B
IV poor
18 D-3 Socu 0.52 0.02 0.8 0.87 435 154 168 0.02 0.34 0.52 0.05 V, Tag (Ts, Ls,
Pr), R, I, Py IV poor
19 A-3 Colibas
,i
3400–3500
0.31 0.03 0.2 0.32 435 65 103 0.13 0.63 0.52 0.04 V, Tag (Ls, Pr),
Sd, B IV poor
20 B-1 Totea 1.43 0.07 2.07 1.07 433 145 75 0.03 1.93 0.48 0.04 V, Tag (Ts), Py III gas
21 A-1 Logres
,ti 3500–3600 0.49 0.02 0.4 0.96 430 82 196 0.05 0.42 0.46 0.04 V, Lag, Tag
(Pr), S, Py, Sd IV poor
22 A-1 Piscu
Stejarului 3600–3700 0.61 0.03 0.54 1.04 437 88 171 0.05 0.52 0.52 0.05
V, Tag (Ts, Pr),
Lag, R, F, Py,
Sd
IV poor
23 A-2 Piscu
Stejarului 3800–3900 0.83 0.12 1.41 0.95 442 177 115 0.08 1.48 0.55 0.04
V, Tag (Ts, Pr),
Lag, Lp, S, R, I,
Py, Sd
III gas
24 A-1 Bibes
,ti
3900–4000
0.51 0.05 0.46 0.21 440 90 41 0.1 2.19 0.44 0.04
V, Tag(Ts, Ls,
Pr), L(S, R), I,
Py, Sd
III gas
25 A-1 Bulbuceni 0.43 0.04 0.28 0.27 437 65 63 0.13 1.04 0.5 0.04
V, Lag, L(S),
Tag (Pr, Ts, Ls),
Py, Sd
III gas
26 A-1
Bib-Bulbuceni
4000–4100
0.51 0.05 0.35 0.26 440 69 51 0.13 1.35 0.48 0.03 V, Tag (Ls),
L(S), Py III gas
27 A-2
Bib-Bulbuceni 0.59 0.06 0.52 0.21 443 88 36 0.01 2.48 0.48 0.04
V, Tag (Ls, Pr,
Ts), Lp, S, R, I,
Py
III gas
28 A-3
Bib-Bulbuceni 0.47 0.04 0.35 0.19 441 74 40 0.01 1.84 0.49 0.04
V, Tag (Ls, Ts),
Lp, L(S, R), I,
Py
III gas
29 A-2 Bibes
,ti 1.04 0.11 2.13 0.41 436 205 39 0.05 5.19 0.45 0.03 V, Tag (Ts, Pr,
Ls), R, S, Py III/II gas and oil
Minerals 2023,13, 202 8 of 22
Table 1. Cont.
No. Well Depth, m TOC wt.% S1, mg
HC/g rock
S2, mg
HC/g rock
S3, mg
CO2/g
rock
Tmax,
C
HI, mg
HC/g
TOC
OI, mg
CO2/g
TOC
PI
(S1/
(S1+S2)
S2/S3VRo, % Standard
Dev.
Petrographic
Composition
Kerogen
Type
Generation
Potential
30 A-3 Bibes
,ti
4100–4200
0.62 0.06 0.56 0.2 443 90 32 0.1 2.80 0.48 0.06
V, Tag (Ts), Lag,
Lp, R, S, I, Py,
Sd
III gas
31 B-1 Bulbuceni 1.01 0.22 1.86 0.45 442 184 45 0.11 4.13 0.44 0.06
V, Tag (Ts, Ls),
Lag, Lp, L(S,
C), Py, B
III gas
32 B-2 Bulbuceni
4300–4400
0.57 0.04 0.51 0.24 439 89 42 0.07 2.13 0.5 0.08 V, Tag (Pr), Lag,
Ld, I, Py, Sd III gas
33 C-1 Bulbuceni 0.55 0.08 0.93 0.28 440 169 51 0.08 3.32 0.51 0.05 V, Tag (Ts), Ld,
Py III gas
Minerals 2023,13, 202 9 of 22
Figure 4.
Map with Miocene oil and gas fields. Selected perimeter with structures and cross-section
(Figure 3) of the center-southern part from the Getic Depression. (OMV PETROM S.A.-I.C.P.T. Câmpina).
The classifications developed by the International Committee for Coal and Organic
Petrology for vitrinite [77], liptinite [78]), huminite [79]) and inertinite [80] were used.
4. Results and Discussions
4.1. Petrology of the Dispersed Organic Matter
The vitrinite reflectance measurements and Rock-Eval results are given in Table 1.
In the Upper Badenian samples, macerals such as vitrinite, liptinite (sporinite, resinite,
cutinite), and inertinite are integrated with marine liptinite macerals (acritarchs) such as:
telalginite, lamalginite and liptodetrinite. Qualitative petrographic composition was carried
out, and it is presented as macerals and minerals in Table 1. Vitrinite is frequent in all
samples, and it is associated with a variety of liptinite macerals (Figures 57). Cutinite
is accompanied by sporinite, which is yellow in fluorescent light (A-1 Socu, A-3 Coliba
s
,
i,
A-1, B-1 Hurezani, B-1 Bulbuceni, B-2, 3 Bibe
s
,
ti) (Figure 7). Resinite occurs mostly as
globular bodies, golden-yellow to brownish—yellow in epifluorescence (D-1 Socu, A-1,
2, 3 Bibe
s
,
ti). Inertinite is rare and was identified in the following wells: A-1 Budieni,
A-1 Hurezani—fusinite; B-2 Bulbuceni—funginite.
Alginite as telalginite is related to algae such as Tasmanites,Leiosphaeridia and other
Prasinophyceae, and it was identified using optical microscopy following the ICCP classi-
fications [
78
], [
81
83
]. Alginite shows a variable fluorescence emission intensity ranging
from green-yellow to bright yellow (A-1 Socu, B-1 Socu, C-2 Socu, A-1, 2 Piscu Stejarului,
A-1 Totea, B-1 Totea, A-1 Bulbuceni, B-2 Bibe
s
,
ti—Bulbuceni, A-1 Rădine
s
,
ti, A-1 Budieni)
(Figure 5).
Alginite as lamalginite was identified only in A-2 Piscu Stejarului, C-2 Socu,
A-1 Logre
s
,
ti, A-1 Grădi
s
,
te, and A-1 Budieni wells, A-1 Bulbuceni, A-3 Bibe
s
,
ti and B-1
and B -2 Bulbuceni with irregular shapes, with a lack of internal structure and having a
lower fluorescence intensity than the telalginite.
Spores were identified in most of wells, accompanied by resinite and cutinite: Socu,
Totea, Hurezani, Vladimir, Piscul Stejarului, Bibe
s
,
ti, Bibe
s
,
ti-Bulbuceni, Bulbuceni, Logre
s
,
ti,
and Rădines
,ti (Figures 68), with TAI between 1+and 3.
Minerals 2023,13, 202 10 of 22
Figure 5.
Photomicrographs of C-1 Socu Upper Badenian source rocks of the Getic Depression
showing vitrinite associated with mixed, continental, and marine origins liptinite in a mineral
groundmass with pyrite. Reflected light (RL: figures
A
,
C
,
E
) and fluorescence (FL: figures
B
,
D
,
F
), oil
immersion, 500×. Py: pyrite, MC: mineral carbonate.
Figure 6.
Photomicrographs of C-2 Socu Upper Badenian source rocks of the Getic Depression
showing continental liptinite origin in a mineral carbonate groundmass with pyrite. Reflected light
(RL: figures A,C,E) and fluorescent light (FL: figures B,D,F), oil immersion, 500×.
Minerals 2023,13, 202 11 of 22
Figure 7.
Photomicrographs of A-1 Hurezani Upper Badenian source rocks of the Getic Depression
showing the continental origins of macerals in a mineral carbonate (MC) groundmass with quartz
(Qz), and pyrite (Py). Reflected light (RL: figures
A
,
C
,
E
) and fluorescent light (FL: figures
B
,
D
,
F
), oil
immersion, 500×.
Liptodetrinite with irregular shape shows increased fluorescence intensity in B-1
Bulbuceni, B-1 Hurezani, A-1, 3 Bibe
s
,
ti, B-2, 3 Bibe
s
,
ti and A-1 Rădine
s
,
ti samples. Although
pyrite occurrence alone is not enough for demonstrating the anoxic conditions, it is an useful
marker to assess depositional conditions. Thus, pyrite occurs generally as framboidal and
dispersed, associated with iron carbonates (siderite) in Vladimir-Totea, Rădine
s
,
ti, Bibe
s
,
ti,
Bulbuceni showing anoxic conditions, and in Coliba
s
,
i, Socu, Logre
s
,
ti, Piscu Stejarului,
Drăganu, structures showing oxic conditions (Figure 5C, Figure 6A,E and Figure 8E,F).
Bioclasts occur in Upper Badenian samples (A-1 Socu, D-1 Socu, A-1 Piscu Stejarului,
A-3 Coliba
s
,
i, B-1 Bulbuceni, A-1 Drăganu, A-1 Grădi
s
,
te), but occasionally, bioclasts show
the frequent pyrite inclusions (A-1 Vladimir).
The random vitrinite reflectance (VR
o
%) ranges between 0.41% and 0.55%, with T
max
values between 409
C and 443
C (Figures 9and 10), both parameters suggesting that the
organic matter maturity varies from thermal immaturity to very early oil maturity.
The values of measured vitrinite reflectance (VR
o
%), thermal alteration index (TAI)
and T
max
show that 23 analyzed rock samples are immature, and the other 10 samples
occur in the early maturity part of the oil window (Figures 9and 10).
Minerals 2023,13, 202 12 of 22
Figure 8.
Photomicrographs of phytoclasts, altered spores (A-1, 2, 3 Bibe
s
,
ti, A-1, 2, 3 Bibe
s
,
ti—Bulbuceni,
B-1 Bulbuceni) and pyrite nests (A-1 Vladimir, A-1,2 Totea, B-1 Totea and A-1 Rădine
s
,
ti), FL
(AD) and RL, (E,F), oil immersion, 500×, scale bar: 20 µm.
The Badenian samples show values of T
max
and VR
o
(%) (Table 1and Figure 9) that
help to group them as following:
a. T
max
< 435
C, VR
o
< 0.50%, indicating immaturity (B-1, C-2, D-1 Socu, B-1 Totea,
A-1, 2, B-1 Hurezani; A-1 Vladimir, A-1 Logre
s
,
ti, A-1 Drăganu, A-1 Grădi
s
,
te, A-1 Coliba
s
,
i,
A-1 Rădines
,ti and A-1 Budieni).
b. T
max
435
C, VR
o
% < 0.50%, indicating vitrinite values lower than expected
(C-1 Socu, D-2 Socu, A-1, 2, 3 Bibes
,ti, B-1, 2, 3 Bibes
,ti—Bulbuceni and B-1 Bulbuceni).
c. T
max
435
C, VR
o
%
0,50%, indicating thermal maturity, in the very early part
of the oil generation zone (A-1, D-3 Socu, A-1, 2 Piscu Stejarului, A-1, B-2, C-1 Bulbuceni
and A-3 Colibas
,i).
d. T
max
< 435
C, VR
o
% > 0.50%, indicating T
max
values lower than expected
(A-1 Totea and A-2 Colibas
,i).
The T
max
parameter is influenced not only by the shape of S
2
, but also by the type and
quantity of organic matter, the lithological matrix, the lithology of the rock and by other
factors [
36
,
52
,
81
]. The diagram of VR
o
(%) vs. T
max
(
C) (Figure 9), shows that the T
max
values are very low, especially in samples from A-1 Grădi
s
,
te, A-1 Budieni and A-1 Rădine
s
,
ti
Minerals 2023,13, 202 13 of 22
wells. The extent of T
max
reduction tends to be higher in the early stage of hydrocarbon
generation window (435 C–440 C) [84,85].
Figure 9.
Vitrinite reflectance VR
o
(%) vs. T
max
(
C), showing the thermal maturity of the Upper
Badenian source rocks from the central-southern part of the Getic Depression.
The low VR
o
% values (Figure 10) first suggested vitrinite suppression. This process is
linked to impregnation of vitrinite with hydrocarbons generated during kerogen maturation
(not this case, as the thermal maturity is low), increased hydrogen content of vitrinite either
as a result of a Hydrogen-rich matrix (lamalginite, bituminite) or of hydrogen-rich plant
precursors. Both processes would result in high HI values (>400 mg HC/g TOC), and again,
this is not the case in our dataset. Moreover, vitrinite reflectance variation may be linked
to chemical differences in organic matter, possibly induced by diagenesis through syn-
or post depositional degradation, occurring in sediments with clay minerals as catalysts.
Differences in vitrinite reflectance are related to time-temperature history. The selected
samples, deeper than 3500 m, from Bibe
s
,
ti, Bibe
s
,
ti-Bulbuceni, Bulbuceni, Piscu-Stejarului
structures, have the most pronounced decrease of VR
o
(%) and the highest values of the T
max
(
C). As there is not enough evidence to argue the suppression of VR
o
(%), the measured
vitrinite reflectance values can be explained by retardation, as a possible mechanism for
this group of samples, in the lack of overpressured sections and additional data required
for such an interpretation.
The samples from A-1 Drăganu, A-1 Grădi
s
,
te and A-1 Coliba
s
,
i wells show a normal
variation of VR
o
vs. depth. The possible retardation process in the Getic Depression was
influenced by tectonic factors (extensions and transpressions) mentioned in Section 2.
T
max
values are correlated with depth (Figure 11), although a group of samples have a
slightly upward deviation from the theoretical variation, as the structures Socu, Coliba
s
,
i,
Hurezani, Totea, and Vladimir have suffered uplift movements.
Minerals 2023,13, 202 14 of 22
Figure 10.
Vitrinite reflectance VR
o
(%) vs. depth (m), showing the thermal maturity of the Upper
Badenian source rocks from the central-southern part of the Getic Depression.
Figure 11.
T
max
(
C) vs. depth (m) diagram, showing the thermal maturity of the Upper Badenian
source rocks from the central-southern part of the Getic Depression.
Minerals 2023,13, 202 15 of 22
4.2. Geochemical Assessment
Depending on their TOC values and on their petroleum generation potential, the
analyzed rock samples are split in three main groups (Figure 12, Table 1):
Figure 12.
Hydrocarbon generation potential of Upper Badenian samples of the Getic Depression
evaluated from TOC (wt.%) and S2(mg HC/g rock) parameters.
a. poor generation potential group, with TOC values between 0.07 wt.%. and 0.49 wt.%
in D-2 Socu, B-3 Bibe
s
,
ti—Bulbuceni, A-1 Bulbuceni, A-1 Logre
s
,
ti, A-1 Drăganu, and
A-1, 2 and 3 Colibas
,i wells.
b. fair generation potential group, with TOC values between 0.5 wt.% and 0.99 wt.%
in A-1, B-1, C-1, D-1, D-3 Socu, A-1 Totea, A-1, B-1 Hurezani, A-1, 2 Piscu Stejarului,
A-1, 3 Bibes
,ti, B-1, 2 Bibes
,ti Bulbuceni, B-2, C-1 Bulbuceni and A-1 Budieni wells;
c. good generation potential group, with TOC values between 1.01 wt.% and 1.82 wt.%
in C-2 Socu, B-1 Totea, A-2 Hurezani, A-1 Vladimir, A-2 Bibe
s
,
ti, B-1 Bulbuceni,
A-1 Grădis
,te and A-1 Rădines
,ti wells.
Based on the S
2
parameter, with values lower than 2.5 mg HC/g rock, the hydrocar-
bons generation potential is poor for 32 samples (Figure 13). Only a single sample from A-1
Grădis
,te well has S2of 3.74 mg HC/g rock and a fair hydrocarbons generation potential.
The values of Hydrogen Index (HI) in Upper Badenian samples range between
14 and 272 mg HC/g TOC, indicating a potential source for gas generating, type III kerogen
(Table 1). In three samples from A-2 Bibe
s
,
ti, D-1 Socu and A-1 Grădi
s
,
te, HI values higher
than 200 mg HC/g TOC were recorded only as these samples contain increased alginite
and resinite macerals, type III/II, with a predominance of type III, generating mixed gas
and oil, but mainly gas (Table 1, Figure 13).
Minerals 2023,13, 202 16 of 22
Figure 13.
Oxygen Index (OI) (mg CO
2
/g TOC) vs. Hydrogen Index (HI) (mg HC/g TOC), in a
modified Pseudo Van Krevelen diagram, showing the kerogen types of Upper Badenian samples
from the Getic Depression.
In C-1, D-1, 2, 3 Socu, A-1 Piscu Stejarului, A-1 Logre
s
,
ti, A-1 Drăganu and A-1, 2
Coliba
s
,
i wells, the Oxygen Index (OI) values range from 150 to 278 mg CO
2
/g TOC. OI
values higher than 150 mg CO
2
/g TOC are related to TOC values lower than 0.5 wt.% due
to mineral matrix effects or to mineral decomposition during the pyrolysis procedure and
they are an indicator for the terrestrial organic matter occurrence or for immature organic
matter from all sources [
25
]. The A-1, 2 Coliba
s
,
i and D-2 Socu samples have high values
of OI because of their low values of S
1
, S
2
, TOC, the sample showing oxidizing effects.
The values of S
2
/S
3
ratio lower than 5 indicate the type III kerogen, suitable for gas and
condensate, and only one sample has a S2/S3ratio higher than 5 which indicates a type II
kerogen, suitable for oil (A-2 Bibes
,ti).
Another important pyrolysis parameter is the Production Index (PI, Table 1), influ-
enced by the conversion of kerogen into free hydrocarbons, and by the ratio of S
1
to the
sum of S
1
+ S
2
, [
30
]. This ratio is significant when it reaches values between 0.05 and
0.5 (oil window). The Upper Badenian samples have an average PI of 0.08, with a maximal
value of 0.13, the analyzed samples being in the immaturity area or in the early maturity
area from the oil generation window (Table 1, Figure 14).
Studies of [
29
,
30
] indicated a correlation between the pyrolysis data and the hydro-
carbon generation potential values. Some authors [
27
] showed that T
max
values less than
435
C, measured vitrinite reflectance (VR
o
%) less than 0.5%, together with thermal alter-
ation index (TAI) between 1–2
+
show immature stages of organic matter. From the analyzed
rock samples, 23 samples are immature, and other 10 samples occur in the early maturity
state of the oil window (Table 1). The modified Pseudo Van Krevelen diagram, (Figure 13)
confirms that the analyzed rock samples present the Type III of kerogen, indicating a
gas-generation potential.
Minerals 2023,13, 202 17 of 22
Figure 14.
Production Index (PI) vs. T
max
(
C) diagram, showing the hydrocarbon generation zone.
4.3. Statistical Analysis
For the identification of the potential correlation between the parameters, a preliminary
statistical analysis of the datasets was performed (Table 2).
Table 2. Statistical parameters for all characters.
Statistical
Parameter TOC S1S2S3Tmax HI OI PI VRoS2/S3Depth
Mean 0.723 0.058 0.925 0.645 434 113.939 101.181 0.080 0.478 1.673
3439.388
Standard
Deviation 0.365 0.052 0.775 0.330 7.017 54.483 68.543 0.082 0.035 1.217 556.980
Minimum 0.070 0.010 0.010 0.190 409 14 32 0.020 0.410 0.010 2218
Maximum 1.820 0.230 3.740 1.250 443 272 278 0.500 0.550 5.190 4346.5
Confidence Level
for mean (95.0%) ±0.129 ±
0.018
±
0.275
±
0.117
±2.488 ±
19.318
±
24.304
±
0.029
±
0.012
±
0.431
±
197.496
The correlation coefficients (Table 3) indicate high correlation values between six
parameters such as S2and TOC (0.918), S1and S2(0.820), Tmax and Depth (0.807).
Table 3. The values of the correlation coefficients.
TOC S1S2S3Tmax HI OI PI VRoS2/S3Depth
TOC 1
S10.717 1
S20.918 0.820 1
S30.301 0.013 0.327 1
Tmax 0.503 0.285 0.502 0.513 1
HI 0.554 0.581 0.760 0.206 0.163 1
OI 0.430 0.474 0.335 0.690 0.150 0.182 1
PI 0.411 0.065 0.305 0.134 0.091 0.437 0.095 1
VRo 0.304 0.257 0.234 0.121 0.362 0.102 0.244 0.076 1
S2/S30.550 0.719 0.626 0.435 0.045 0.566 0.731 0.150 0.356 1
Depth 0.228 0.047 0.205 0.610 0.807 0.057 0.418 0.045 0.259 0.381 1
Background colors indicate the following variation ranges of correlation coefficients: orange: 0.6–0.7, light green:
0.7–0.8, green: 0.8–0.9, dark green: >0.9.
T
max
and depth parameters dependency is described by a linear model (Figure 15)
with a high adjusted R-squared value (0.651).
Minerals 2023,13, 202 18 of 22
Figure 15. Tmax (C) variation vs. depth (m).
Considering the strong correlation between three parameters (S
1
, S
2
, TOC) and accord-
ing to the linear models presented in Figure 16, a multiple linear regression model was
assessed. The equation obtained is as follows:
TOC = 0.328 0.753 ×S1+ 0.475 ×S2(1)
Figure 16. S2(mg HC/g rock) variation vs. TOC (wt%, left) and vs. S1(mg HC/g rock, right).
The model shows an extremely high adjusted R—square coefficient (R2= 0.846).
The model can be used for other values from the variation intervals of S
1
and S
2
variables for the estimation of TOC.
5. Conclusions
In the Getic Depression, the Upper Badenian organic matter is immature at depths
between 2200–3200 m, with values of reflectance of 0.42%–0.49% and early mature at depths
higher than 3200 m, with values of reflectance between 0.5%–0.55%. The types of organic
matter are terrigenous and marine.
The type of terrigenous organic matter has a high diversity of macerals: vitrinite,
liptinite (sporinite, resinite, cutinite) and inertinite. The marine organic matter has algae,
Minerals 2023,13, 202 19 of 22
as telalginite, lamalginite, and liptodetrinite, with Tasmanites, and Leiosphaeridia and other
Prasinophyceae, in the following wells: A-1 Budieni, A-1 Rădine
s
,
ti, C-1,2 Socu, D-1, 3 Socu,
B-1 Hurezani, B-1, 2 Bulbuceni, A-1 Logre
s
,
ti, A-1, 2, 3 Bibe
s
,
ti, B-2, 3 Bibe
s
,
ti, A-2 Piscu
Stejarului. The Hydrogen Index of the Upper Badenian samples is less than 200 (mg HC/g
TOC), as the shales contain gas prone, type III kerogen, in the frame of a C-type oganic
facies.
Although pyrite occurrence alone is not enough for demonstrating the anoxic condi-
tions, it is a useful marker to assess depositional conditions. Thus, pyrite occurs generally
as framboidal and dispersed, associated with iron carbonates (siderite) in Vladimir-Totea,
Rădine
s
,
ti, Bibe
s
,
ti, Bulbuceni (showing anoxic conditions). In Coliba
s
,
i, Socu, Logre
s
,
ti, Piscu
Stejarului, Drăganu, structures (showing oxic conditions), anoxic-oxic variable depositional
conditions occur.
Most of samples with values of total organic carbon (TOC) < 1% wt.% have a limited
potential of hydrocarbons (HC) generation, and 30% of samples with TOC < 1.82 wt.%
and kerogen type III, present particularly gas generation potential. The samples have
poor generation potential, depending on S
2
(mg HC/g rock) values, which are lower than
2.5 mg/g, only a single sample having S
2
value of 3.74 mg/g (A-1 Grădi
s
,
te). The dominant
kerogen is type III.
The statistical assessment highlighted the existence of some interesting relationships
between the investigated parameters, among which the model between S
1
, S
2
, TOC can be
used for other values from the variation intervals of these variables.
Author Contributions:
Conceptualization, M.D.G.; methodology, M.D.G. and
S
,
.G.; software, M.D.G.,
S
,
.G., N.M.B. and I.M., validation, M.D.G. and
S
,
.G.; formal analysis, M.D.G.,
S
,
.G. and N.M.B.; investi-
gation, M.D.G., M.E.P., G.P.,
S
,
.G. and N.M.B.; data curation: M.D.G. and
S
,
.G.; writing—original draft
preparation, M.D.G., M.E.P. and G.P.; writing—review and editing, M.D.G., M.E.P., G.P., I.M. and
S
,
.G.; visualization, M.D.G. and I.M.,
S
,
.G.; supervision, M.E.P. and G.P. All authors have read and
agreed to the published version of the manuscript.
Funding: This research received no external funding.
Data Availability Statement:
OMV-PETROM S.A. I.C.P.T. Câmpina is the repository of the data
supporting the results.
Acknowledgments:
The authors gratefully acknowledge OMV-PETROM S.A. I.C.P.T. Câmpina
company for allowing to publish geological, petrographical and geochemical data and results. The
authors also acknowledge the constructive contributions of several anonymous peer-reviewers who
improved the quality of the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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... From a geological point of view, the site containing Corbii de Piatrǎ Monastery is located in the Getic Depression (Figure 2), a narrow sedimentary basin in the Southern Carpathian foreland with sediments from the Cretaceous to Miocene ages [14]. The monastery was excavated in massive sandstones of the Oligocene age, which belong to the Corbi Formation [9,15]. ...
... From a geological point of view, the site containing Corbii de Piatrǎ Monastery is located in the Getic Depression ( Figure 2), a narrow sedimentary basin in the Southern Carpathian foreland with sediments from the Cretaceous to Miocene ages [14]. The monastery was excavated in massive sandstones of the Oligocene age, which belong to the Corbi Formation [9,15]. ...
... Before AMS analysis, the samples were converted into graphite by combustion using the AGE 3 graphitization installation [38], which works in conjunction with the elemental analyzer (VarioMicroCube, Elementar, Hanau, Germany)™. The experimental data obtained for all the samples were normalized against NIST SRM 4990C-Oxalic Acid II (NIST SRM 4990C International Standard Reference Material for Contemporary Carbon- 14,1983) to the modern radiocarbon level, while to estimate the blank level, a fossil coal of Romanian origin was used. By measuring the 13C/12C ratio, the results were also corrected by δ13C parameters determined by AMS data processing software version 4.06 (according to A-4-35-501-7621 Rev. A Operator Manual 1.0 MV Tandetron for AMS B7621 IFIN-HH Magurele, Romania, page 73), representing the cumulative isotopic fractionation of all physico-chemical processes on the analysis chain. ...
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Thermal maturity of six organic rich samples from Jurassic continental successions cropping out in the Holy Cross Mountains in Central Poland, has been characterized by classic thermal maturity indicators, micro-Raman spectroscopy and Palynomorph Darkness Index, in order to create a multi-method workflow for complex palynofacies thermal maturity assessment. Transmitted light observations on dispersed organic matter define a Hettangian lacustrine depositional environment, characterized by periods of reducing/oxidizing conditions and variable sedimentation rates. Thermal maturity detected by classical maturity indicators and PDI indicates an early maturation stage of hydrocarbon generation and is in agreement with spectroscopic analyses performed on phytoclast groups. Moreover, Raman parameters in the sporomorph group indicate a systematic shift toward a lower degree of aromatization compared to the phytoclast group. Finally, the multivariate statistical analysis performed on Raman spectra is found to be a promising tool to define and predict the heterogeneity of dispersed organic matter in sediments.
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Kerogen concentrates obtained from Lopingian (Late Permian) to Upper Triassic mudrock lithologies of seven coal exploration boreholes, drilled in the Moatize – Minjova Coal Basin (N'Condédzi sub-basin, Mozambique), were studied by means of vitrinite reflectance (VR), spore fluorescence and spore colour, in order to constrain the thermal history and basin evolution by organic maturation levels. VR increases with depth, indicating organic maturation related to sediment burial for most of the boreholes. Modelled VR data indicate a regional palaeogeothermal gradient between 35 and 40 °C/km. Lower Jurassic doleritic intrusions observed in three boreholes had only local thermal effects without affecting the regional palaeogeothermal gradient. Two boreholes located near the basin margin show high palaeogeothermal gradients suggesting thermal processes other than heating due to burial were involved. These processes may have involved hot diagenetic fluids circulating through fault zones and/or permeable lithologies, locally elevating geothermal gradients. Circulation of these fluids was induced by lithostatic pressure due to rapid rates of sedimentation. These high sedimentation rates lead to the accumulation of a thick succession (over 2000 m) of Lopingian (Late Permian) to Upper Triassic siliciclastic sediments. All the organic maturation indices measured and the age of the successions indicate that organic maturation occurred during or after Late Triassic times. However, the presence of reworked Permian palynomorphs into Upper Triassic sediments and the absence of Middle Triassic sediments indicate an exhumation and erosion of Permian strata in Middle Triassic times. The organic maturation levels of the reworked palynomorph population are considerably higher than the indigenous Upper Triassic population, indicating that they attained higher burial temperatures prior to being reworked.
Article
The aims of this study include the evaluation and interpretation of Oligocene source rocks from the Central-Western part of the Getic Depression, Romania, and the identification of their maturation and their hydrocarbon potential. 24 argillite samples belonging to the source rocks were studied using organic petrography and geochemistry methods, including Rock-Eval pyrolysis, and the following sets of results were obtained: maceral diversity, values of vitrinite reflectance, thermal alteration index, and organic geochemical values. The Oligocene disperse organic matter contains both, humic and terrestrial organic matter, as well as sapropelic, marine, algal matter, represented by telalginite and lamalginite. Several anoxic deep marine water intervals were recorded in the Getic Depression. The samples are immature, with Tmax values ranging between 415 °C and 434.5 °C, and random vitrinite reflectance of 0.32% and 0.53%. The samples yield a total organic content between 0.55% and 5.71%, with a hydrogen index between 9 and 379 mg/g, showing two types of kerogen: a. mixed type II-III kerogen, indicating a generative potential for hydrocarbon and gas, and b. type III kerogen, indicating a gas generative potential.
Article
This study focuses on the thermal maturity assessment of Silurian-Devonian sediments from the Ghadamis Basin, North Africa, comparing optical and geochemical analyses of palynomorphs. In southern Tunisia, the investigated subsurface cored section comprises the Argiles Principales Formation of Silurian age. In Libya, the succession studied covers the Awaynat Wanin III and IV formations, assigned to the Late Devonian (Frasnian-Famennian). Geochemical approaches used to reconstruct thermal alteration of sediments necessitate advanced, relatively expensive analytical techniques. In this study, the effectiveness of the less costly, relatively simple approaches of visually assessing palynomorph colour to determine thermal alteration (i.e., SCI: Spore Colour Index, TAI: Thermal Alteration Index and PDI: Palynomorph Darkness Index) was evaluated. SCI and TAI are qualitative methods, strictly related to the operator's perception, which use ten and five point scales respectively, to characterize colour in terms of illustrated specimens and/or descriptions. In contrast, PDI is obtained from the measurement of the red, green and blue (RGB) intensities of light transmitted through palynomorphs, using standard optical microscopes and digital cameras. The palynomorph-based thermal alteration estimates were compared to Rock-Eval pyrolysis data from the same samples. This calibration showed a linear relationship between these quantitative parameters and PDI. These results show that PDI is more reliable than the SCI and TAI methods.