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Article

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

by
Maria Doina Ghiran
1,2,
Mihai Emilian Popa
2,3,4,*,
Izabela Mariș
5,
Georgeta Predeanu
6,
Ștefania Gheorghe
1 and
Niculina Mihaela Bălănescu
6
1
OMV PETROM S.A.-I.C.P.T. Câmpina, 29 Culturii Ave., 105600 Câmpina, Romania
2
Doctoral School, Faculty of Geology and Geophysics, University of Bucharest, 6 Traian Vuia Str., 020956 Bucharest, Romania
3
Laboratory of Palaeontology, Department of Geology, Faculty of Geology and Geophysics, University of Bucharest, 1 N. Bălcescu Ave., 011401 Bucharest, Romania
4
School of Geosciences and Technology, Southwest Petroleum University, 8, Xindu Ave., Chengdu 610500, China
5
Department of Mineralogy, Faculty of Geology and Geophysics, University of Bucharest, 1 N. Bălcescu Ave., 011401 Bucharest, Romania
6
Research Center for Environmental Protection and Ecofriendly Technologies, University Politehnica of Bucharest, 1-7 Gheorghe Polizu Str., 011061 Bucharest, Romania
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(2), 202; https://doi.org/10.3390/min13020202
Submission received: 31 December 2022 / Revised: 26 January 2023 / Accepted: 27 January 2023 / Published: 30 January 2023
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Versions Notes

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 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.

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 Dâmbovița 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 [2,3,4,5,6,7,8,9]. In the central - southern area, where the maximal depths were reached, 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 [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20].
The ways of evaluating the characters of the source rocks are well established worldwide, such as:
  • assessment of the level of thermal maturity and the amount of organic matter and type of kerogen [21,22,23,24,25,26,27,28,29,30,31].
  • microscopic identification of hydrocarbons source rocks, using thermal alteration index and vitrinite reflectance [21,25,27,28,29,30,32,33,34,35,36,37].
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 (Tmax) and production index (PI), resulted from the Rock—Eval pyrolysis associated with vitrinite reflectance (VRo%), allow to assess the stage of maturation [23,32,38,39,40,41,42,43,44,45].
The thermal maturity of organic matter increases with temperature, as it converts and further expels hydrocarbons during diagenesis, catagenesis, and metagenesis [21,32,37,46,47,48,49,50,51,52,53,54,55,56,57].
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 Dâmbovița Valley in the east and the Danube River in the west (Figure 1), [58,59,60].
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 Depression 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) [58,59,60,61,62].
Previous authors [63,64,65,66,67,68,69,70,71,72] 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].
The Getic Depression accumulated sediments which were transported southwards from the Carpathians chain, showing a fining upward general trend [58,59,60]. 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, Figure 2 and Figure 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 Burdigalian, 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 sequences. The Upper Burdigalian deposits occur transgressively overlaying older formations, 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].
The Badenian sequence lays unconformably over the Burdigalian (Figure 2 and Figure 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:
  • the Globigerina-bearing tuffs and marls (Lower Badenian).
  • the salt breccia (Upper Badenian), with gypsum interlayers.
  • the radiolaria-bearing shales.
  • the Spiratella-bearing marls (ending the Badenian).
The depositional and geochemical evolution of the Getic Depression during the Badenian 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 Sphaeroidina 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 Nemathosphaeropsis 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 Grădiște), 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 sequences 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] (Figure 2 and Figure 3).

3. Materials and Methods

The samples were selected from borehole cores having their repository with the Institute 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, Bibești, Bibești-Bulbuceni, Bulbuceni, Logrești, Drăganu, Grădiște, Colibași, Rădinești and Budieni (Figure 4).
A series of wells yield single core samples from Upper Badenian sequences (A-1 Grădiște, A-1 Drăganu, A-1 Budieni, A-1 Colibași, and A-1 Rădinești). 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 [25,26,27,41] were followed in interpreting the results.
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 (Figure 5, Figure 6 and Figure 7). Cutinite is accompanied by sporinite, which is yellow in fluorescent light (A-1 Socu, A-3 Colibași, A-1, B-1 Hurezani, B-1 Bulbuceni, B-2, 3 Bibești) (Figure 7). Resinite occurs mostly as globular bodies, golden-yellow to brownish—yellow in epifluorescence (D-1 Socu, A-1, 2, 3 Bibești). 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 classifications [78], [81,82,83]. 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 Bibești—Bulbuceni, A-1 Rădinești, A-1 Budieni) (Figure 5).
Alginite as lamalginite was identified only in A-2 Piscu Stejarului, C-2 Socu, A-1 Logrești, A-1 Grădiște, and A-1 Budieni wells, A-1 Bulbuceni, A-3 Bibești 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, Bibești, Bibești-Bulbuceni, Bulbuceni, Logrești, and Rădinești (Figure 6, Figure 7 and Figure 8), with TAI between 1+ and 3−.
Liptodetrinite with irregular shape shows increased fluorescence intensity in B-1 Bulbuceni, B-1 Hurezani, A-1, 3 Bibești, B-2, 3 Bibești and A-1 Rădinești 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, Rădinești, Bibești, Bulbuceni showing anoxic conditions, and in Colibași, Socu, Logrești, 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 Colibași, B-1 Bulbuceni, A-1 Drăganu, A-1 Grădiște), but occasionally, bioclasts show the frequent pyrite inclusions (A-1 Vladimir).
The random vitrinite reflectance (VRo%) ranges between 0.41% and 0.55%, with Tmax values between 409 °C and 443 °C (Figure 9 and Figure 10), both parameters suggesting that the organic matter maturity varies from thermal immaturity to very early oil maturity.
The values of measured vitrinite reflectance (VRo%), thermal alteration index (TAI) and Tmax show that 23 analyzed rock samples are immature, and the other 10 samples occur in the early maturity part of the oil window (Figure 9 and Figure 10).
The Badenian samples show values of Tmax and VRo (%) (Table 1 and Figure 9) that help to group them as following:
a. Tmax < 435 °C, VRo < 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 Logrești, A-1 Drăganu, A-1 Grădiște, A-1 Colibași, A-1 Rădinești and A-1 Budieni).
b. Tmax ≥ 435 °C, VRo% < 0.50%, indicating vitrinite values lower than expected (C-1 Socu, D-2 Socu, A-1, 2, 3 Bibești, B-1, 2, 3 Bibești—Bulbuceni and B-1 Bulbuceni).
c. Tmax ≥ 435 °C, VRo% ≥ 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 Colibași).
d. Tmax < 435 °C, VRo% > 0.50%, indicating Tmax values lower than expected (A-1 Totea and A-2 Colibași).
The Tmax parameter is influenced not only by the shape of S2, 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 VRo (%) vs. Tmax (°C) (Figure 9), shows that the Tmax values are very low, especially in samples from A-1 Grădiște, A-1 Budieni and A-1 Rădinești wells. The extent of Tmax reduction tends to be higher in the early stage of hydrocarbon generation window (435 °C–440 °C) [84,85].
The low VRo% 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 Bibești, Bibești-Bulbuceni, Bulbuceni, Piscu-Stejarului structures, have the most pronounced decrease of VRo (%) and the highest values of the Tmax (°C). As there is not enough evidence to argue the suppression of VRo (%), 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 Grădiște and A-1 Colibași wells show a normal variation of VRo vs. depth. The possible retardation process in the Getic Depression was influenced by tectonic factors (extensions and transpressions) mentioned in Section 2.
Tmax 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, Colibași, Hurezani, Totea, and Vladimir have suffered uplift movements.

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):
a. poor generation potential group, with TOC values between 0.07 wt.%. and 0.49 wt.% in D-2 Socu, B-3 Bibești—Bulbuceni, A-1 Bulbuceni, A-1 Logrești, A-1 Drăganu, and A-1, 2 and 3 Colibași 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 Bibești, B-1, 2 Bibești 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 Bibești, B-1 Bulbuceni, A-1 Grădiște and A-1 Rădinești wells.
Based on the S2 parameter, with values lower than 2.5 mg HC/g rock, the hydrocarbons generation potential is poor for 32 samples (Figure 13). Only a single sample from A-1 Grădiște well has S2 of 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 Bibești, D-1 Socu and A-1 Grădiște, 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).
In C-1, D-1, 2, 3 Socu, A-1 Piscu Stejarului, A-1 Logrești, A-1 Drăganu and A-1, 2 Colibași wells, the Oxygen Index (OI) values range from 150 to 278 mg CO2/g TOC. OI values higher than 150 mg CO2/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 Colibași and D-2 Socu samples have high values of OI because of their low values of S1, S2, TOC, the sample showing oxidizing effects. The values of S2/S3 ratio lower than 5 indicate the type III kerogen, suitable for gas and condensate, and only one sample has a S2/S3 ratio higher than 5 which indicates a type II kerogen, suitable for oil (A-2 Bibești).
Another important pyrolysis parameter is the Production Index (PI, Table 1), influenced by the conversion of kerogen into free hydrocarbons, and by the ratio of S1 to the sum of S1 + S2, [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 hydrocarbon generation potential values. Some authors [27] showed that Tmax values less than 435 °C, measured vitrinite reflectance (VRo%) less than 0.5%, together with thermal alteration 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.

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).
The correlation coefficients (Table 3) indicate high correlation values between six parameters such as S2 and TOC (0.918), S1 and S2 (0.820), Tmax and Depth (0.807).
Tmax and depth parameters dependency is described by a linear model (Figure 15) with a high adjusted R-squared value (0.651).
Considering the strong correlation between three parameters (S1, S2, TOC) and according 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
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 S1 and S2 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, as telalginite, lamalginite, and liptodetrinite, with Tasmanites, and Leiosphaeridia and other Prasinophyceae, in the following wells: A-1 Budieni, A-1 Rădinești, C-1,2 Socu, D-1, 3 Socu, B-1 Hurezani, B-1, 2 Bulbuceni, A-1 Logrești, A-1, 2, 3 Bibești, B-2, 3 Bibești, 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 conditions, 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, Rădinești, Bibești, Bulbuceni (showing anoxic conditions). In Colibași, Socu, Logrești, 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 S2 (mg HC/g rock) values, which are lower than 2.5 mg/g, only a single sample having S2 value of 3.74 mg/g (A-1 Grădiște). 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 S1, S2, 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 Ș.G.; software, M.D.G., Ș.G., N.M.B. and I.M., validation, M.D.G. and Ș.G.; formal analysis, M.D.G., Ș.G. and N.M.B.; investigation, M.D.G., M.E.P., G.P., Ș.G. and N.M.B.; data curation: M.D.G. and Ș.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 Ș.G.; visualization, M.D.G. and I.M., Ș.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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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].
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].
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Figure 2. Stratigraphic log of the Getic Depression, Romania, simplified after [60].
Figure 2. Stratigraphic log of the Getic Depression, Romania, simplified after [60].
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Figure 3. N–S Geological cross-section of the Getic Depression through Colibași–Vladimir–Bulbuceni structures. Modified from [61].
Figure 3. N–S Geological cross-section of the Getic Depression through Colibași–Vladimir–Bulbuceni structures. Modified from [61].
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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).
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).
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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 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.
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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×.
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×.
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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×.
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×.
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Figure 8. Photomicrographs of phytoclasts, altered spores (A-1, 2, 3 Bibești, A-1, 2, 3 Bibești—Bulbuceni, B-1 Bulbuceni) and pyrite nests (A-1 Vladimir, A-1,2 Totea, B-1 Totea and A-1 Rădinești), FL (A–D) and RL, (E,F), oil immersion, 500×, scale bar: 20 μm.
Figure 8. Photomicrographs of phytoclasts, altered spores (A-1, 2, 3 Bibești, A-1, 2, 3 Bibești—Bulbuceni, B-1 Bulbuceni) and pyrite nests (A-1 Vladimir, A-1,2 Totea, B-1 Totea and A-1 Rădinești), FL (A–D) and RL, (E,F), oil immersion, 500×, scale bar: 20 μm.
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Figure 9. Vitrinite reflectance VRo (%) vs. Tmax (°C), showing the thermal maturity of the Upper Badenian source rocks from the central-southern part of the Getic Depression.
Figure 9. Vitrinite reflectance VRo (%) vs. Tmax (°C), showing the thermal maturity of the Upper Badenian source rocks from the central-southern part of the Getic Depression.
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Figure 10. Vitrinite reflectance VRo (%) vs. depth (m), showing the thermal maturity of the Upper Badenian source rocks from the central-southern part of the Getic Depression.
Figure 10. Vitrinite reflectance VRo (%) vs. depth (m), showing the thermal maturity of the Upper Badenian source rocks from the central-southern part of the Getic Depression.
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Figure 11. Tmax (°C) vs. depth (m) diagram, showing the thermal maturity of the Upper Badenian source rocks from the central-southern part of the Getic Depression.
Figure 11. Tmax (°C) vs. depth (m) diagram, showing the thermal maturity of the Upper Badenian source rocks from the central-southern part of the Getic Depression.
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Figure 12. Hydrocarbon generation potential of Upper Badenian samples of the Getic Depression evaluated from TOC (wt.%) and S2 (mg HC/g rock) parameters.
Figure 12. Hydrocarbon generation potential of Upper Badenian samples of the Getic Depression evaluated from TOC (wt.%) and S2 (mg HC/g rock) parameters.
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Figure 13. Oxygen Index (OI) (mg CO2/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.
Figure 13. Oxygen Index (OI) (mg CO2/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.
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Figure 14. Production Index (PI) vs. Tmax (°C) diagram, showing the hydrocarbon generation zone.
Figure 14. Production Index (PI) vs. Tmax (°C) diagram, showing the hydrocarbon generation zone.
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Figure 15. Tmax (°C) variation vs. depth (m).
Figure 15. Tmax (°C) variation vs. depth (m).
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Figure 16. S2 (mg HC/g rock) variation vs. TOC (wt%, left) and vs. S1 (mg HC/g rock, right).
Figure 16. S2 (mg HC/g rock) variation vs. TOC (wt%, left) and vs. S1 (mg HC/g rock, right).
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Table
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.%); S1—amount of free hydrocarbons from the rock samples (mg HC/g rock); S2—remaining hydrocarbons generation potential of the source rock (mg HC/g rock); S3—oxygen compounds, amount of CO2 from organic source (mg CO2/g rock); Tmax-temperature of maximum hydrocarbon generation from a rock sample during pyrolysis analysis (°C); HI—Hydrogen Index (HI = S2/TOC)—the amount of hydrogen relative to the amount of organic carbon present in a sample (mg HC/g TOC); OI—Oxygen Index (OI = S3/TOC)-the amount of oxygen relative to the amount of organic carbon present in a sample (mg CO2/g TOC); PI—Production Index = S1/S1 + S2; S2/S3 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.
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.%); S1—amount of free hydrocarbons from the rock samples (mg HC/g rock); S2—remaining hydrocarbons generation potential of the source rock (mg HC/g rock); S3—oxygen compounds, amount of CO2 from organic source (mg CO2/g rock); Tmax-temperature of maximum hydrocarbon generation from a rock sample during pyrolysis analysis (°C); HI—Hydrogen Index (HI = S2/TOC)—the amount of hydrogen relative to the amount of organic carbon present in a sample (mg HC/g TOC); OI—Oxygen Index (OI = S3/TOC)-the amount of oxygen relative to the amount of organic carbon present in a sample (mg CO2/g TOC); PI—Production Index = S1/S1 + S2; S2/S3 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.WellDepth, mTOC 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 CompositionKerogen TypeGeneration Potential
1A-1 Grădiște2200–23001.820.233.741.25409206690.063.000.470.05V, Lag, Tag, R, In, Py, Sd, BIIIgas
2A-1 Drăganu0.410.010.130.74430321800.070.180.450.04V, I, Sd, BIVpoor
3A-1 Colibași2600–27000.070.010.010.93433142110.050.010.490.03V, Tag (Pr), PyIVpoor
4A-1 Rădinești2700–28001.350.142.010.73419150550.072.750.410.07V, C, S, Tag (Pr, Ls), PyIIIgas
5A-1 Budieni2800–29000.590.070.60.26423102440.112.310.420.05V, Lag, Tag (Pr, Ls), I, PyIIIgas
6A-1 Hurezani3000–31000.890.061.050.45434118510.052.330.420.05V, C, R, S, Tag (Ts, Pr), F, Py, SdIIIgas
7B-1 Hurezani3100–32000.990.031.020.87430103880.031.170.460.03V, S, C, Tag (Ts, Ls, Pr), Ld, I, Py, SdIIIgas
8B-1 Socu0.680.020.660.6443097940.031.030.440.03V, Lag, Tag (Ts, Pr), R, S, Py, SdIIIgas
9A-2 Hurezani1.200.061.610.52433134430.043.100.480.04V, Ld, Py, SdIIIgas
10A-1 Totea0.810.041.230.75433152920.031.640.520.03V, S, Tag (Pr, Ls, Ts), I, PyIIIgas
11C-1 Socu0.540.020.550.814351031500.030.680.490.04H/V, Tag (Ts, Pr, Ls), R, S, PyIVpoor
12D-1 Socu0.50.031.250.964332721950.031.300.470.04V, Tag (Ts, Ls, Pr), R, I, Sd, BIIIgas
13D-2 Socu0.390.010.41.064351032780.020.380.460.04V, Tag (Ts, Ls, Pr), S, I, PyIVpoor
14A-2 Colibași3200–33000.480.020.241.08429502550.080.220.520.04V, Tag (Ls, Pr), Ld, S, PyIVpoor
15C-2 Socu1.040.071.150.83434111800.051.390.440.06V, S, Tag (Ts, Pr), Py, SdIIIgas
16A-1 Vladimir1.080.051.090.93432101860.041.170.480.03V, Tag (Pr, Ls), S, R, PyIIIgas
17A-1 Socu0.550.030.430.55437781000.070.780.540.04H/V, Tag (Ts, Pr, Ls), S, R, Ld, I, Py, Sd, BIVpoor
18D-3 Socu0.520.020.80.874351541680.020.340.520.05V, Tag (Ts, Ls, Pr), R, I, PyIVpoor
19A-3 Colibași 3400–35000.310.030.20.32435651030.130.630.520.04V, Tag (Ls, Pr), Sd, BIVpoor
20B-1 Totea1.430.072.071.07433145750.031.930.480.04V, Tag (Ts), PyIIIgas
21A-1 Logrești3500–36000.490.020.40.96430821960.050.420.460.04V, Lag, Tag (Pr), S, Py, SdIVpoor
22A-1 Piscu Stejarului3600–37000.610.030.541.04437881710.050.520.520.05V, Tag (Ts, Pr), Lag, R, F, Py, SdIVpoor
23A-2 Piscu Stejarului3800–39000.830.121.410.954421771150.081.480.550.04V, Tag (Ts, Pr), Lag, Lp, S, R, I, Py, SdIIIgas
24A-1 Bibești3900–40000.510.050.460.2144090410.12.190.440.04V, Tag(Ts, Ls, Pr), L(S, R), I, Py, SdIIIgas
25A-1 Bulbuceni0.430.040.280.2743765630.131.040.50.04V, Lag, L(S), Tag (Pr, Ts, Ls), Py, SdIIIgas
26A-1 Bib-Bulbuceni4000–41000.510.050.350.2644069510.131.350.480.03V, Tag (Ls), L(S), PyIIIgas
27A-2 Bib-Bulbuceni0.590.060.520.2144388360.012.480.480.04V, Tag (Ls, Pr, Ts), Lp, S, R, I, PyIIIgas
28A-3 Bib-Bulbuceni0.470.040.350.1944174400.011.840.490.04V, Tag (Ls, Ts), Lp, L(S, R), I, PyIIIgas
29A-2 Bibești1.040.112.130.41436205390.055.190.450.03V, Tag (Ts, Pr, Ls), R, S, PyIII/IIgas and oil
30A-3 Bibești4100–42000.620.060.560.244390320.12.800.480.06V, Tag (Ts), Lag, Lp, R, S, I, Py, SdIIIgas
31B-1 Bulbuceni1.010.221.860.45442184450.114.130.440.06V, Tag (Ts, Ls), Lag, Lp, L(S, C), Py, BIIIgas
32B-2 Bulbuceni4300–44000.570.040.510.2443989420.072.130.50.08V, Tag (Pr), Lag, Ld, I, Py, SdIIIgas
33C-1 Bulbuceni0.550.080.930.28440169510.083.320.510.05V, Tag (Ts), Ld, PyIIIgas
Table
Table 2. Statistical parameters for all characters.
Table 2. Statistical parameters for all characters.
Statistical ParameterTOCS1S2S3TmaxHI OIPIVRoS2/S3Depth
Mean0.7230.0580.9250.645434113.939101.1810.0800.4781.6733439.388
Standard Deviation0.3650.0520.7750.3307.01754.48368.5430.0820.0351.217556.980
Minimum0.0700.0100.0100.19040914320.0200.4100.0102218
Maximum1.8200.2303.7401.2504432722780.5000.5505.1904346.5
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
Table
Table 3. The values of the correlation coefficients.
Table 3. The values of the correlation coefficients.
TOCS1S2S3TmaxHI OIPIVRoS2/S3Depth
TOC1
S10.7171
S20.9180.8201
S30.3010.0130.3271
Tmax −0.503−0.285−0.502−0.5131
HI 0.5540.5810.7600.206−0.1631
OI−0.430−0.474−0.3350.690−0.150−0.1821
PI−0.411−0.065−0.305−0.1340.091−0.4370.0951
VRo−0.304−0.257−0.2340.1210.362−0.1020.2440.0761
S2/S30.5500.7190.626−0.4350.0450.566−0.731−0.150−0.3561
Depth−0.2280.047−0.205−0.6100.8070.057−0.418−0.0450.2590.3811
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.
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Ghiran, M.D.; Popa, M.E.; Mariș, I.; Predeanu, G.; Gheorghe, Ș.; Bălănescu, 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

AMA Style

Ghiran MD, Popa ME, Mariș I, Predeanu G, Gheorghe Ș, Bălănescu NM. Thermal Maturity and Kerogen Type of Badenian Dispersed Organic Matter from the Getic Depression, Romania. Minerals. 2023; 13(2):202. https://doi.org/10.3390/min13020202

Chicago/Turabian Style

Ghiran, Maria Doina, Mihai Emilian Popa, Izabela Mariș, Georgeta Predeanu, Ștefania Gheorghe, and Niculina Mihaela Bălănescu. 2023. "Thermal Maturity and Kerogen Type of Badenian Dispersed Organic Matter from the Getic Depression, Romania" Minerals 13, no. 2: 202. https://doi.org/10.3390/min13020202

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