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Article

Mineralogical and Maturation Considerations of the Coqueiros Formation (Campos Basin, Brazil): Insights from Multi-Technique Analyses of Source Rocks

by
Gabriel A. Barberes
1,*,
Flávia C. Marques
2,
Dalva A. L. Almeida
2,
Linus Pauling F. Peixoto
2,
Lenize F. Maia
2,
Antonio Carlos Sant’Ana
2,
Gustavo F. S. Andrade
2,
Celly M. S. Izumi
2,
Victor Salgado-Campos
1,
Thiago Feital
3,
Luiz Fernando C. de Oliveira
2 and
Ana Luiza Albuquerque
1
1
Departamento de Geologia e Geofísica, Universidade Federal Fluminense, Av. Gen. Milton Tavares de Souza s/nº—Gragoatá—Campus da Praia Vermelha, Niterói 24210-346, RJ, Brazil
2
Núcleo de Espectroscopia e Estrutura Molecular, Centro de Estudos de Materiais, Departamento de Química, Universidade Federal de Juiz de Fora, R. José Lourenço Kelmer s/n, Martelos, Juiz de Fora 36036-900, MG, Brazil
3
OPTIMATECH Ltd.a., Rio de Janeiro 22410-905, RJ, Brazil
*
Author to whom correspondence should be addressed.
Geosciences 2024, 14(11), 286; https://doi.org/10.3390/geosciences14110286
Submission received: 24 September 2024 / Revised: 18 October 2024 / Accepted: 21 October 2024 / Published: 25 October 2024
(This article belongs to the Section Geochemistry)
Browse Figures
Versions Notes

Abstract

:
The Coqueiros Formation, a strategic stratigraphic unit within the Lagoa Feia Group (LFG) in the Campos Basin offshore Brazil, is known for its lacustrine carbonate deposits, which include both organic-rich shales and economically important “coquina” reservoirs. While coquina facies are widely recognized as reservoirs, the source-rock potential of the intercalated shales remains relatively underexplored. This study aims to characterize the mineralogy and thermal maturity of the Coqueiros Formation to assess its potential as a source rock, using a multi-technique approach integrating X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Raman spectroscopy analyses of shale samples from two wells: 3-BP-11-RJS and 6-DEV-18P-RJS. XRD analyses revealed a heterogeneous mineralogy dominated by carbonates (calcite and dolomite) and quartz, with significant contributions from clay minerals and trace minerals such as pyrite and barite. SEM imaging revealed a heterogeneous fabric with grain size, morphology, and porosity variations, reflecting a dynamic lacustrine depositional setting influenced by storm events and fluctuations in terrigenous input. The presence of authigenic minerals, as reported in other studies, such as saddle dolomite, mega-quartz, and various sulfides, provides evidence for hydrothermal alteration, likely related to Late Cretaceous magmatic activity in the Campos Basin. Raman spectroscopy yielded equivalent vitrinite reflectance (Ro%) values consistently exceeding 1.00, ranging from 1.03 to 1.40, indicating that the organic matter in the Coqueiros Formation shales has attained a high thermal maturity level, surpassing the oil window and reaching the condensate wet gas zone. The mineralogical and equivalent maturation data presented herein provide a valuable foundation for future studies, highlighting the complexity and heterogeneity of the Coqueiros Formation and its potential significance as a source rock within the Campos Basin petroleum system.

1. Introduction

Exploration and exploitation of conventional petroleum resources remain crucial for meeting the global energy demand, particularly in the Brazilian economy. Understanding the geological processes that control hydrocarbons’ formation, migration, and accumulation in these systems is essential for successful exploration and production [1]. A fundamental component of conventional petroleum systems is the presence of organic-rich source rocks, which generate the hydrocarbons that migrate and accumulate in reservoir rocks.
A key aspect of petroleum system analysis involves evaluating the source-rock potential of organic-rich sediments [2]. This evaluation encompasses three critical parameters: the quantity, quality, and thermal maturity of the organic matter present in the rock. Organic richness, often expressed as total organic carbon (TOC) content, indicates the amount of organic matter available for hydrocarbon generation [3,4]. The type of organic matter, categorized into distinct kerogen types, determines the type and volume of hydrocarbons that can be generated [5,6]. Lastly, thermal maturity, reflecting the temperature history of the rock, governs the extent to which the kerogen has been transformed into hydrocarbons [6]. This geochemical characterization is particularly important for shale formations, which can act as both source rocks and reservoirs in conventional and unconventional petroleum systems [7].
This study focuses on characterizing the Coqueiros Formation (Fm.) in the Campos Basin, offshore Brazil (Figure 1), as a potential source rock within a conventional petroleum system, through two different wells: 3-BP-11-RJS (C-M-473 field; Figure 1) and 6-DEV-18P-RJS (C-M-471 field; Figure 1). Both wells were operated by BP Energy, and their core (cutting) samples are under the care of Fluminense Federal University (UFF). For this purpose, our research used a multi-technique approach, integrating X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Raman spectroscopy, to provide a comprehensive understanding of the mineralogical composition and thermal maturity of the Coqueiros Fm. Similar multi-technique approaches have been successfully employed to characterize the source-rock potential of other formations [8]. By elucidating these key aspects, this study aims to contribute to a more thorough evaluation of the source-rock potential of this formation and its role in the conventional petroleum system of the Campos Basin.
The Coqueiros Fm., a stratigraphic unit within the Lagoa Feia Group (LFG), is known for its lacustrine carbonate deposits, which include both organic-rich shales and economically significant “coquina” reservoirs [9,10]. While the coquina facies are widely recognized as reservoirs, the source-rock potential of the intercalated shales remains relatively underexplored.
Characterizing the mineralogical composition of the Coqueiros Fm. can provide valuable insights into the paleoenvironmental conditions and diagenetic processes that influenced organic matter accumulation and preservation. For example, the presence of pyrite, often observed within the formation, strongly suggests deposition under anoxic conditions conducive to the preservation of organic matter. Furthermore, specific clay mineral assemblages can provide additional clues about the depositional environment and the nature of the organic matter present.

2. Geological Setting

The Coqueiros Fm., a key component of the Lagoa Feia Group (LFG) within the Campos Basin, Brazil, comprises a significant succession of lacustrine carbonates deposited during the Early Cretaceous period [9], as can be seen in Figure 2 (the stratigraphic context of the Coqueiros Fm.). This formation hosts prolific hydrocarbon reservoirs, primarily composed of “coquinas”—bioclastic rudstones and grainstones dominated by bivalve shells [10]. However, it is also significant for its potential as a source rock. Figure 3 presents the lithological profiles of wells 3-BP-11-RJS and 6-DEV-18P-RJS, highlighting the stratigraphic distribution of the main lithofacies within the Coqueiros Fm., including the coquinas and the shale intervals targeted in this study. The lower boundary of Coqueiros Fm. is the pre-Jiquiá unconformity, dated at 125.8 Ma, and its upper boundary corresponds to the pre-neo-Alagoas unconformity (occurring between 120 Ma and 123.1 Ma), which is readily identifiable in seismic sections [11].

2.1. Tectonic Setting

The Campos Basin is a passive margin basin formed due to Gondwana’s breakup during the Late Jurassic/Early Cretaceous periods [12]. The Coqueiros Fm. was deposited during the late syn-rift to early post-rift (sag) phase, transitioning from a depositional setting dominated by fault-controlled subsidence to a more regional regime of thermal subsidence [10,13,14]. This tectonic evolution is reflected in the stratigraphic architecture of the formation, transitioning from localized thickening near faults in the basal units to more widespread, continuous layers in the upper units [10]. The thick Aptian deposits of the Coqueiros Fm., exhibiting limited influence from syn-rift faulting, are consistent with deposition during a period of regional thermal subsidence, characteristic of the post-rift phase [10,15].

2.2. Depositional Environment

According to [10], two distinct depositional models have been proposed to explain the genesis of the Coqueiros Fm. The first model posits deposition in a shallow lacustrine setting, characterized by the reworking of bioclasts near their life habitat, primarily driven by storm-generated currents [16,17]. Alternatively, the second model proposes a deeper lacustrine environment, where bioclasts, initially formed in shallower water depths, were subsequently redeposited into deeper settings via debris flows triggered by tectonic activity [18].
The Coqueiros Fm. preserves evidence of a large brackish-water lacustrine system, as indicated by the presence of bivalve, gastropod, and ostracod communities, together with an absence of charophytes and evaporites [19]. This lacustrine depositional history was preceded by extensive volcanic activity during the Neocomian period, resulting in a widespread outpouring of basaltic lavas. These lavas are represented by the Serra Geral Fm. in the Paraná Basin, the Guaratiba Fm. in the Santos Basin, and the Cabiúnas Fm. in the Campos Basin [13].

2.3. Implications for Source-Rock Characterization

According to the Sumário ANP [15], the primary source rock for the Campos Basin comprises shales deposited during the rift phase, corresponding to the Barremian/Aptian age, attributed to local stages Buracica and Jiquiá. These shales are characterized by Type I kerogen, with residual total organic carbon content ranging from 2% to 6%. The Coqueiros Fm. comprises a heterogeneous lithological assemblage, including organic-rich shales and the coquina reservoirs. Mohriak [13] identifies the lacustrine shales of the LFG, encompassing the Coqueiros Fm., as containing intervals that represent the primary source rock for significant hydrocarbon accumulations within the Campos Basin. Therefore, understanding the interplay between facies distribution, mineralogy, and depositional processes is crucial for unraveling the source-rock potential of the Coqueiros Fm.

3. Materials and Methods

This study focuses on the mineralogical characterization and thermal maturity evaluation of the Coqueiros Formation, aiming to assess its potential as a source rock in the Campos Basin. A comprehensive analytical approach was employed to achieve these objectives, integrating XRD, Scanning Electron Microscopy (SEM), and Raman spectroscopy techniques. The following sections detail the materials and methods used in this investigation.

3.1. Sample Collection and Preparation

Thirteen cutting samples from the two different wells were collected for this study. Six of these samples originate from the 6-DEV-18P-RJS well, spanning depths from 5324 to 5444 m. The remaining seven samples were retrieved from the 3-BP-11-RJS well, encompassing depths between 6220 and 6382 m. The stratigraphic locations of the samples collected from both wells are indicated in the lithological profiles presented in Figure 3.
Technical data from the 3-BP-11-RJS and 6-DEV-18P-RJS wells were acquired to enhance the source-rock evaluation. This drilling information comprises chronostratigraphy, lithostratigraphy, lithology, and Scanning Electron Microscopy analyses (exclusively for the 6-DEV-18P-RJS well). All data were provided by the Brazilian National Petroleum Agency (ANP) through their Exploration and Production Database (BDEP).
Two rounds of sample processing were performed: grinding (maceration) followed by decarbonation. A ball mill was employed to grind the samples in 5 min cycles at 40 RPM for the grinding process. The resulting material was subsequently sieved using a 106-micron mesh. These samples, after grinding and sieving, are hereafter referred to as “macerated”.
For the decarbonation process, 10 g of the macerated material was immersed in a buffered solution (4M) of acetic acid (2M) and sodium acetate (2M), which was heated to 80 °C for one hour [20,21]. Following the heating process, the mixtures were filtered and dried in a controlled environment, and these samples are referred to as “decarbonated”. As a result, two types of samples were prepared for X-ray Diffraction analysis: macerated and decarbonated. Additionally, fresh (untreated) samples were included alongside the macerated and decarbonated samples for Raman analysis.

3.2. Analytical Techniques

3.2.1. X-Ray Diffraction (XRD)

XRD analyses were conducted using a Bruker D2 Phaser diffractometer equipped with a copper (Cu-Kα radiation 1.54 Å) X-ray tube operating at 30 kV and 10 A. The following instrumental conditions were set: a scanning 2θ angle range of 4° to 90°, a step size of 0.02°, a counting time of 3 s per step, and a sample rotation of 15 rpm.
XRD spectra obtained from the rock samples were evaluated in qualitative and quantitative steps. Initially, each spectrum was compared to several mineral phase reference patterns by analyzing the proximity of multiple spectrum peaks to reference phase peaks. This comparison aided in identifying the most probable mineral phases present in the sample. Subsequently, these probable phases were utilized in the Rietveld refinement method, employing Profex software® (using the own software database). The Rietveld refinement was then used to determine the concentration of each identified mineral phase, and the results can be seen in the next section.

3.2.2. Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) was employed to investigate the shale samples’ micro-scale morphological, textural, and mineralogical characteristics from well 6-DEV-18P-RJS.
Samples selected for SEM analysis were first broken or split to expose fresh surfaces. They were then mounted on sample holders using conductive carbon paste and coated with palladium gold in a “cool” sputter coater to prevent heat damage to sensitive clay minerals or friable samples. The samples were analyzed using a Topcon Model SM-300 Digital Scanning Electron Microscope operating at 10 kV.

3.2.3. Raman Spectroscopy

Raman spectroscopy is a well-known technique for evaluating structural changes in carbonaceous materials. This study employed Raman spectroscopy to investigate the thermal maturity of the shale samples using equivalent vitrinite reflectance (Ro%) [22,23,24]. Raman measurements were performed using a Bruker SENTERRA spectrometer coupled to an Olympus BX51 microscope with a 50× magnification objective (NA = 0.51). All spectra were collected with a confocal aperture of 50 μm. A 532 nm Nd-YAG solid-state laser was used for excitation, with a diffraction grating of 1200 grooves per millimeter. The laser power was set to 0.2 mW, with an exposure time of 50 s and a spectral range of 1100 to 1750 cm−1. For each sample, at least three different areas were selected, and at least three points were recorded in each area to generate average Raman spectra.
All Raman spectra were processed using spectral smoothing (Savitzky–Golay method with a 21-point cubic polynomial algorithm) and linear baseline correction over the defined spectral range. The spectra were then normalized, setting the maximum value to one and the minimum to zero [24]. Deconvolution of the processed Raman spectra was performed using Origin software. This deconvolution yielded several parameters, including the full width at half maximum (FWHM) of the G and D bands, Raman band separation (RBS), the integrated area of each band, the band centers, and intensities of deconvoluted bands. These parameters were used to predict the equivalent Ro% of the samples by correlation with standard vitrinite data using a linear adjustment.
Based on the gradual changes in organic matter with increasing maturation, a correlation between the spectral profile obtained by Raman analysis and the thermal maturity of the sample was assessed. According to Schmidt [22], there is a strong correlation between the width of the G band in the Raman spectra of organic matter and the maximum reflectance of vitrinite. Thus, through mathematical adjustments of the Raman parameters obtained via deconvolution, thermal maturity was estimated by calculating equivalent vitrinite reflectance from G-band Raman data.

4. Results

4.1. Lithofacies Classification

The lithological classification of the studied cutting samples from the Coqueiros Formation is presented in Table 1, integrating data from both shale mineralogy and petrophysical well logs.
The analysis of the 3-BP-11-RJS well samples revealed a predominance of silica-rich calcareous shale lithofacies (C1), indicating a significant contribution of siliceous material in a predominantly carbonate environment. This lithofacies is associated with various petrophysical classifications, including marl, limestone/shale, and limestone/marl, reflecting the variability within this shale type. Only one sample, at 6301 m, was classified as carbonate-rich siliceous shale lithofacies (S1), representing a shale with a higher proportion of carbonate minerals.
In contrast, the 6-DEV-18P-RJS well samples exhibited a more diverse range of shale mineralogies. The mixed calcareous shale lithofacies (C2), identified at 5324 m, suggests a more balanced contribution of siliceous and carbonate components. The calcareous/siliceous mixed shale (M1), found at 5339 m, represents a transitional lithofacies with a relatively balanced proportion of both components. Interestingly, the carbonate-rich siliceous shale lithofacies (S1), which dominates in the deeper intervals of this well (5372 m and 5405 m), could be associated with the presence of coquina, indicating a close association between these shale types and the bioclastic carbonate facies. The deeper intervals also exhibit a mixture of C1 and M1 lithofacies.

4.2. Equivalent Vitrinite Reflectance (Ro%)

The thermal maturity of the Coqueiros Formation shales was investigated using equivalent vitrinite reflectance (Ro%) values obtained from parameters derived from Raman spectral analysis. Dispersed organic matter measurements were conducted on fresh (untreated), macerated, and decarbonated samples from wells 3-BP-11-RJS and 6-DEV-18P-RJS to assess the impact of sample preparation on the measured thermal maturity. The results are presented in Table 2.
All analyzed samples exhibit Ro% values consistently exceeding 1.00, ranging from 1.03 to 1.40, indicating a high thermal maturity level for the organic matter in the Coqueiros Formation shales, surpassing the conventional threshold for the oil window and reaching the condensate wet gas zone.
The influence of sample preparation methods on Ro% values is evident in both wells, although the magnitude of the variation is relatively small. In general, macerated and decarbonated samples tend to show slightly lower Ro% values than fresh samples. However, certain samples exhibit an opposite trend, such as the one at 6274 m from well 3-BP-11-RJS. These inconsistencies might be related to the heterogeneous distribution of organic matter within the samples or the potential removal of specific organic components during the maceration and decarbonation processes. Both the heterogeneous distribution and the potential removal of organic components influence the characteristic Raman signature of the organic matter and consequently the parameters used to obtain the Ro%, generating these small variations and some inconsistences.

4.3. Well 3-BP-11-RJS

4.3.1. Macerated Samples

The mineralogical composition and equivalent vitrinite reflectance (Ro%) values of the macerated samples from well 3-BP-11-RJS, obtained through XRD and Raman spectroscopy analyses, are presented in Figure 4 and Supplementary Materials (File S1). Calcite is the dominant mineral in most samples, ranging from 17.46% at 6274 m to 36.49% at 6328 m. Quartz is the second most abundant mineral, with concentrations between 12.68% and 23.16%. Dolomite is present in significant quantities in the shallower depth (6274 m). K-feldspar and plagioclase are also consistently present, with minor variations in their relative proportions. Notably, K-feldspar is absent in the 6328 m and 6355 m samples.
Several other minerals occur in trace amounts, including barite, pyrite, anhydrite, and pyroxene. The presence of pyrite, even in small quantities, suggests episodes of anoxic conditions during deposition, which are favorable for organic matter preservation. The variability in mineral composition with depth reflects the heterogeneous nature of the Coqueiros Formation, likely influenced by fluctuations in terrigenous input and changes in water chemistry during deposition.
Clay minerals represent a significant portion of the mineralogy, ranging from 2.06% to 15.83%. The highest concentration is observed in the sample from 6355 m, suggesting a potential interval with enhanced organic matter preservation.

4.3.2. Decarbonated Samples

After decarbonation, the mineralogical composition of the samples from well 3-BP-11-RJS, as determined by XRD analysis, changes significantly, contrary to vitrinite reflectance values (Figure 5 and Supplementary Materials (File S2)). Quartz becomes the dominant mineral in almost all samples, with concentrations ranging from 13.66% to 32.18%. K-feldspar is the second most abundant mineral, followed by barite. Clay minerals also show a significant increase in their relative proportions after decarbonation. The presence of pyrite persists in some decarbonated samples, although in lower concentrations compared to the macerated samples. Trace amounts of other minerals, such as pyroxene, amphibole, dolomite, zeolite, titanite, and cristobalite, are also observed.
The impact of decarbonation on the mineralogical composition of the 3-BP-11-RJS samples is depicted in Figure 5. These seven graphs illustrate the relative abundance of different mineral phases after the removal of carbonate minerals. As said before, quartz and clay minerals become the dominant components, reflecting their presence in the original shale fraction. The variability in their proportions across the studied depths highlights the heterogeneous nature of the Coqueiros Formation shales.

4.4. Well 6-DEV-18P-RJS

4.4.1. Macerated Samples

Figure 6 and Supplementary Materials (File S3) present the mineralogical composition and equivalent vitrinite reflectance (Ro%) values of the macerated samples from well 6-DEV-18P-RJS, obtained from XRD and Raman spectroscopy analyses. Quartz is the dominant mineral in almost all samples, ranging from 12.91% at 5324 m to 35.35% at 5444 m. Calcite is the second most abundant mineral, varying between 21.5% and 47.04%, with a noticeable decrease from the depths 5324 to 5339 m. Ca-dolomite displays a more variable trend, with a higher concentration in the shallower sample (23.33% at 5324 m) and fluctuating presence throughout the remaining depths. K-feldspar and plagioclase are also consistently present, with their relative proportions changing among the samples.
Clay minerals are present in all samples, ranging from 2.2% to 16.58%. The highest concentration is observed in the sample from 5339 m, suggesting a potential zone with good organic matter preservation.
Several other minerals occur in trace amounts, including barite, olivine, halite, and anhydrite. Notably, pyrite is only present in two samples (5372 m and 5414 m), potentially indicating less pervasive anoxic conditions during deposition compared to well 3-BP-11-RJS.

4.4.2. Decarbonated Samples

The removal of carbonate minerals through decarbonation significantly impacts the mineralogical composition of the samples from well 6-DEV-18P-RJS. Figure 7 and File S4 in the Supplementary Materials present the mineralogical composition and equivalent vitrinite reflectance (Ro%) values of the decarbonated samples from well 6-DEV-18P-RJS. Quartz becomes the dominant mineral in all samples, with percentages ranging from 26.55% to 48.29%. Clay minerals also increase considerably, varying between 6.72% and 46.31%, highlighting their significance as a major component of the shale fraction.
K-feldspar remains a notable component, with concentrations ranging from 6.51% to 15.65%. Other minerals, such as Ca-dolomite, barite, pyrite, and various trace minerals (e.g., pyroxene, amphibole, titanite, and whewellite) show variable presence and abundance throughout the sampled depths.

4.4.3. Scanning Electron Microscopy (SEM)

The SEM micrographs of the sample from 5324 m (Figure 8) reveal a heterogeneous texture, characterized by a mixture of angular to subrounded quartz grains (Figure 8C: F6), anhedral calcite crystals of varying sizes (Figure 8B: F8, KL10, L3; Figure 8C: B13, C10, C6.5, J8; Figure 8D: A2–4, J12, L7), and euhedral rhombic dolomite crystals (Figure 8B: D12, G3, J13; Figure 8C: A5–7; Figure 8D: E2, H3.5, KL9). The presence of illite and chlorite (Figure 8B: A6–7, DE7, KL11.8; Figure 8C: J1–4, E9, HJ14; Figure 8D: C1, C9.2), identified as coatings on grains and within the matrix, is consistent with the XRD data, which indicates a significant proportion of clay minerals in this sample. The intergranular porosity observed in the micrographs suggests potential pathways for fluid migration and hydrocarbon expulsion.
The sample from 5339 m (Figure 9) exhibits a more homogeneous texture compared to the previous sample, with a finer grain size dominated by silt- to fine sand-sized quartz (Figure 9B: G3, D2.3, GH5, M3.5; Figure 9C: GH9–11; Figure 9D: HK12–14, KM3–8, C6) and feldspar grains (Figure 9B: C13, C8, G11–12, HK5–7, HK8–10; Figure 9C: A10, H6; Figure 9D: CF12–14). The presence of pyrite (Figure 9D: EF3.9), observed as small euhedral crystals scattered throughout the sample, corroborates the XRD results for other levels in the same well, suggesting episodes of anoxic conditions during diagenesis. Intergranular pores are visible in the higher magnification micrographs (Figure 9C,D), indicating potential spaces for hydrocarbon storage.
The SEM analysis of the 5371.5 m (Figure 10) sample reveals a predominance of anhedral calcite crystals (Figure 10B: AC1–4, AC5–8, M10; Figure 10C: D14, DE6–7, J8.5; Figure 10D: D6–7), ranging in size from approximately 50 µm to 300 µm. The presence of quartz (Figure 10B: BC10–11, DE3–4, HJ12–14; Figure 10C: A11, K2; Figure 10D: C3, B13, K4) is less evident in the micrographs, possibly indicating a smaller grain size or a more dispersed distribution within the matrix. While ca-dolomite is mentioned in the description of XRD results (Figure 10B: EF2–3), it is not identifiable in the images. The porosity appears limited, with few pores visible.
In contrast to the previous sample, the 5372.5 m sample exhibits abundant euhedral rhombic dolomite crystals (Figure 11B: GK13–14, CD9–11, JK3–4; Figure 11C: CJ1–3, DE13–14), with sizes ranging from approximately 20 µm to 100 µm (Figure 11). Calcite (Figure 11B: AB3, CD4–5, GH7–8; Figure 11C: HM4–7) is also present but appears less abundant than dolomite. Illite/smectite (Figure 11B: EF1.3, LM3–4; Figure 11C: B3–4, CD4), as coatings on grains and within the matrix, is consistent with the XRD data (clay minerals). Intergranular porosity is more evident in this sample compared to the previous one, particularly in micrographs Figure 11B,C.
The sample from 5404.5 m is dominated by euhedral rhombic dolomite crystals (Figure 12B: B4, BC7.5, J5.2; Figure 12C: AB2, G6, E7; Figure 12D: BC3, H3, LM7, E12), similar to the 5372.5 m sample, with sizes ranging from approximately 30 µm to 300 µm (Figure 12). The dolomite crystals exhibit well-defined faces and sharp edges, indicating good crystallinity. Authigenic quartz (Figure 12C: GH11, J9) and illite (Figure 12B: GH10–11, D3.8, HJ9.7; Figure 12C: C7–8, F1.3, KL10.6; Figure 12D: B6, DE1.3, HJ7–8), mentioned in the description of XRD results (clay minerals), are not distinguishable in the images. Porosity appears to be low, with few intercrystalline pores visible.
The 5413 m sample is characterized by abundant, well-developed, euhedral rhombic dolomite crystals (Figure 13B: C5, E2, GH9.8, HJ13; Figure 13C: B5, F13.5, H9; Figure 13D: A6, B1, J12, K2), similar to the previous two samples (Figure 13). However, the dolomite crystals in this sample are generally smaller, ranging from 10 µm to 100 µm, which might indicate more rapid crystallization or a more confined growth environment. Authigenic quartz (Figure 13C: E10.8, HJ13.2; Figure 13D: A3, B2.4, C2.8) and illite (Figure 13B: E11.5, FG2.7, M5.8; Figure 13C: A9.5, DE2.3, G7.4) are not readily discernible in the micrographs. Porosity is very low, with few pores visible.
The sample from 5444 m (Figure 14) exhibits a distinct mineralogical composition compared to the other samples, being dominated by anhedral calcite crystals (Figure 14B: AB11–12, B3.5, G9, GH4.5; Figure 14C: A4, DE1.8, HJ6; Figure 14D: A11, F10, H10.5, LM6), with sizes ranging from 20 µm to 300 µm. The calcite crystals display various habits, including prismatic and rhombohedral forms. Quartz grains (Figure 14B: BC1.8, FG10; Figure 14C: A8, A12.5, G13, DE5–6; Figure 14D: B1, AB5.5, DE7–8, FG7) are also present but are generally smaller than the calcite crystals. Intergranular porosity is visible, particularly in micrographs Figure 14B,C.

5. Discussion

The lithological findings, presented in the Section 4, highlight the heterogeneous nature of this formation, with variations in shale mineralogy and petrophysical well log characteristics observed between the two wells and even within the same well. This variability likely reflects fluctuations in the depositional environment, such as changes in terrigenous sediment input and variations in water chemistry, which influenced the relative proportions of carbonate and siliceous components in the shales [10].

5.1. Mineralogical Composition and Depositional Environment

The mineralogical composition of the Coqueiros Formation shales, as revealed by XRD analyses, highlights the heterogeneous nature of these rocks, with variations observed both between and within the studied wells.
Well 3-BP-11-RJS is characterized by a higher abundance of carbonate minerals, particularly calcite and dolomite, indicating a depositional environment with a significant carbonate influence. The presence of silica-rich calcareous shale lithofacies (C1) throughout the section suggests a consistent influx of siliceous material, likely derived from the erosion of continental basement rocks. The observed fluctuations in the relative proportions of calcite, dolomite, and quartz may reflect changes in lake water chemistry, detrital input, or diagenetic processes. These fluctuations in carbonate and siliciclastic input are also reflected in the presence of both marl and limestone intervals in the well log data (Figure 3). The occurrence of dolomite is consistent with the findings of [10,26], who described dolomite as a common component of the Coqueiros Formation, particularly in the lower portion of the formation. The presence of saddle dolomite, as described by Lima et al. [26], and its association with hydrothermal minerals suggest that the dolomitization process was at least partially influenced by hydrothermal fluids circulating within the formation.
Well 6-DEV-18P-RJS, in contrast, exhibits a higher proportion of quartz, indicating a stronger influence of terrigenous sediment input. This is also reflected in the predominance of sandstone intervals observed in the well log data for this well (Figure 3). The presence of calcareous/siliceous mixed shale (M1) and carbonate-rich siliceous shale lithofacies (S1) suggests variations in the balance between carbonate and siliciclastic deposition, possibly related to fluctuations in lake level or proximity to fluvial input sources, as proposed by Muniz and Bosence [19]. This variability in clastic input is consistent with the observations of Figueiredo Jr. et al. [27], who described a distinct terrigenous domain in the inner shelf of the Campos Basin, with coarser siliciclastic sediments derived from the continent.
Vertical variations in mineralogy are observed in both wells, with a notable distinction in the distribution of pyrite. In well 3-BP-11-RJS, pyrite is present in almost all samples, indicating more persistent anoxic conditions throughout the sampled interval. This well also exhibits a trend of decreasing dolomite abundance with depth, which could be attributed to factors such as a deepening of the lake, a decrease in the influence of hydrothermal fluids, or a change in the Mg/Ca ratio of the lake water during deposition. In contrast, pyrite is only observed in two macerated samples from well 6-DEV-18P-RJS, suggesting more localized and ephemeral anoxic conditions. Interestingly, the 3-BP-11-RJS samples are significantly deeper than those from 6-DEV-18P-RJS, raising the possibility that burial depth played a role in creating more persistent anoxic conditions conducive to pyrite formation and, consequently, enhancing organic matter preservation. Ransom et al. [28] suggested that organic matter preservation in continental margin sediments is often associated with localized zones of pyrite enrichment.
The presence of barite in some of the analyzed samples, as revealed by XRD, requires careful consideration. Barite [BaSO4], a dense sulfate mineral, is commonly used as a weighting agent in drilling fluids to control downhole pressure [29]. This practice is common in the Campos Basin, as documented by Rezende et al. [30], who reported elevated barium concentrations in bottom sediments near a drilling site, attributing this enrichment to the barite present in the drilling mud. Furthermore, Ibrahim et al. [31] demonstrated that barite, when present in drilling fluids, can interact with clay minerals in reservoir rocks, leading to changes in petrophysical properties such as permeability and water saturation. Therefore, it is plausible that the barite identified in our samples may be derived from drilling fluid contamination rather than being a primary constituent of the Coqueiros Formation shales. This possibility should be considered when interpreting the mineralogical data, especially when assessing the paleoenvironmental significance of barite. Further analysis, such as SEM imaging targeting barite crystals, may be necessary to confirm its origin and relationship with the surrounding minerals.
SEM analyses provided valuable insights into the micro-scale texture of the Coqueiros Formation shales, revealing a heterogeneous fabric with grain size, morphology, and porosity variations. These observations are consistent with the descriptions provided by Lima and De Ros [32], who also highlighted the heterogeneous nature of the Coqueiros Formation, with variations in texture and mineralogy. The observed textures are further consistent with the depositional model proposed by [10], which suggests a predominance of storm-influenced processes in shallow lacustrine settings. This textural heterogeneity likely played a role in the complex diagenetic history of the Coqueiros Formation, influencing the distribution and intensity of dolomitization and silicification processes described by Lima and De Ros [32].

5.2. Thermal Maturity, Hydrocarbon Generation Potential, and Implications for the Campos Basin Petroleum System

The Ro% values obtained from Raman spectroscopy (Table 2) consistently exceed 1.00, ranging from 1.03 to 1.40, indicating that the organic matter in the Coqueiros Formation shales has attained a high thermal maturity level, surpassing the conventional threshold for the oil window (0.6 to 1.35) and reaching the condensate wet gas zone (1.35 to 2.0). This high maturity level is consistent with the occurrence of oil in the Campos Basin, a prolific hydrocarbon-producing province, and aligns with previous assessments of source-rock maturity within the basin [13,15]. As noted in the Section 4, sample preparation methods (fresh, macerated, and decarbonated) appear to influence the measured Ro% values, although the magnitude of variation is relatively small. A comprehensive analysis of these variations, considering the heterogeneous nature of the samples and potential impacts of sample preparation, will be addressed in a future study dedicated to the organic thermal maturity evaluation using Raman spectroscopy.
Based on the combined mineralogy, texture, and thermal maturity analysis, the Coqueiros Formation exhibits promising characteristics as a source rock within the Campos Basin petroleum system. The presence of pyrite in both wells, alongside the elevated Ro% values, suggests that the organic matter has experienced sufficient thermal stress to enter the hydrocarbon generation zone (oil window).
The heterogeneous nature of the Coqueiros Formation, as revealed by the variable mineral assemblages and textures, indicates a dynamic depositional environment with fluctuating conditions. This is consistent with the depositional model for the Lagoa Feia Group proposed by Muniz and Bosence [19], which suggests a transition from fluvial-dominated to lacustrine-dominated settings during the basin’s rift evolution. The presence of coquinas, representing high-energy, shallow lacustrine environments interbedded with shales deposited in calmer, deeper water settings, further supports this dynamic depositional model.
The high thermal maturity of the Coqueiros Formation shales aligns with Brownfield and Charpentier’s [33] findings for the Toca Formation, which is considered an analogue of the Coqueiros Formation in the West African Aptian salt basin [34]. The Toca Formation, known for its excellent source-rock potential, exhibits similar high thermal maturity levels, suggesting that the Coqueiros Formation may share a similar capacity for hydrocarbon generation.
Further investigation, incorporating organic geochemical data such as residual TOC and Rock-Eval pyrolysis, will provide a more definitive assessment of the Coqueiros Formation’s source-rock potential. However, based on detailed mineralogical, textural, and thermal maturity analyses, the present study strongly suggests that the Coqueiros Formation shales have contributed significantly to the hydrocarbon accumulations in the Campos Basin.

6. Conclusions

This study provides a comprehensive mineralogical characterization of the Coqueiros Formation shales from two different wells in the Campos Basin, integrating XRD, SEM, and Raman spectroscopy analyses. The results reveal a heterogeneous mineralogical composition, reflecting variations in the depositional environment and diagenetic processes. Quartz, calcite, dolomite, and clay minerals are the dominant phases, with their relative proportions varying between and within the studied wells. The presence of pyrite in several samples, particularly in the deeper well 3-BP-11-RJS, suggests episodes of anoxic conditions that likely favored organic matter preservation.
SEM analyses revealed various textural features, including variable grain sizes, morphologies, and pore types, consistent with a dynamic lacustrine depositional setting influenced by storm events and fluctuations in terrigenous input. The presence of authigenic minerals, such as saddle dolomite, mega-quartz, and various sulfides, provides evidence for possible hydrothermal alteration, likely related to Late Cretaceous magmatic activity in the Campos Basin.
While all analyzed samples exhibit Ro% values within the oil/wet gas window, indicating a high thermal maturity level for the organic matter in the Coqueiros Formation shales, a detailed assessment of thermal maturity and hydrocarbon generation potential requires deeper evaluations, incorporating organic geochemical analyses such as residual TOC and Rock-Eval pyrolysis.
Therefore, further investigation is required to adequately discuss the thermal maturity of the samples, which will be addressed in a future publication. However, the mineralogical data presented herein provides a valuable foundation for future studies, highlighting the complexity and heterogeneity of the Coqueiros Formation and its potential significance as a source rock within the Campos Basin petroleum system.

Supplementary Materials

The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/geosciences14110286/s1, File S1. Mineralogical composition and Ro% of macerated samples from Well 3-BP-11-RJS; File S2. Mineralogical composition and Ro% of decarbonated samples from Well 3-BP-11-RJS; File S3. Mineralogical composition and Ro% of macerated samples from Well 6-DEV-18P-RJS; File S4. Mineralogical composition and Ro% of decarbonated samples from Well 6-DEV-18P-RJS.

Author Contributions

Conceptualization, G.A.B. and A.L.A.; Methodology, G.A.B., V.S.-C. and A.L.A.; Software, T.F.; Validation, G.A.B., F.C.M., D.A.L.A., A.C.S., V.S.-C., T.F., L.F.C.d.O. and A.L.A.; Investigation, G.A.B., F.C.M. and D.A.L.A.; Resources, L.F.C.d.O. and A.L.A.; Writing—Original Draft Preparation, G.A.B.; Writing—Review and Editing, G.A.B., F.C.M., D.A.L.A., L.P.F.P., L.F.M., A.C.S., G.F.S.A., C.M.S.I., V.S.-C., T.F., L.F.C.d.O. and A.L.A.; Visualization, G.A.B., F.C.M., D.A.L.A., L.P.F.P., L.F.M., A.C.S., G.F.S.A., C.M.S.I., V.S.-C., T.F., L.F.C.d.O. and A.L.A.; Supervision, L.F.C.d.O. and A.L.A.; Project Administration, A.L.A.; Funding Acquisition, L.F.C.d.O. and A.L.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received funding from FAPERJ (Rio de Janeiro, Brazil)—projects 205.932/2022 and 205.933/2022.

Data Availability Statement

Data will be available on the journal website.

Acknowledgments

The authors thank the Multi-user Center in Geology and Geophysics at Fluminense Federal University, Spectroscopy and Molecular Structure Center at Federal University of Juiz de Fora for the analytical support, FAPERJ for funding support and ANP for data availability.

Conflicts of Interest

Thiago Feital was employed by the company OPTIMATECH Ltd.a. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Location of Santos, Campos, and Espírito Santo basins, with the location of wells 3-BP-11-RJS (B) and 6-DEV-18P-RJS (D).
Figure 1. Location of Santos, Campos, and Espírito Santo basins, with the location of wells 3-BP-11-RJS (B) and 6-DEV-18P-RJS (D).
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Figure 2. Chronostratigraphic framework, nature of sediment, depositional environment, and lithostratigraphy of the Lagoa Feia Group, Campos Basin. The Coqueiros Fm. was deposited in a lacustrine environment during the Barremian and Aptian periods [11].
Figure 2. Chronostratigraphic framework, nature of sediment, depositional environment, and lithostratigraphy of the Lagoa Feia Group, Campos Basin. The Coqueiros Fm. was deposited in a lacustrine environment during the Barremian and Aptian periods [11].
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Figure 3. Lithological profiles of wells 3-BP-11-RJS and 6-DEV-18P-RJS, highlighting the stratigraphic positions of the analyzed samples. The blue dots indicate the depths of the samples collected for XRD analyses and Raman spectroscopy.
Figure 3. Lithological profiles of wells 3-BP-11-RJS and 6-DEV-18P-RJS, highlighting the stratigraphic positions of the analyzed samples. The blue dots indicate the depths of the samples collected for XRD analyses and Raman spectroscopy.
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Figure 4. Mineralogical composition and equivalent vitrinite reflectance (Ro%) values of macerated samples from well 3-BP-11-RJS at different depths ((A): 6220 m; (B): 6247 m; (C): 6274 m; (D): 6301 m; (E): 6328 m; (F): 6355 m; (G): 6382 m). The seven graphs showcase the relative abundance of different mineral phases identified in each macerated sample, providing a comprehensive overview of the mineralogical variations across the studied depths. The dominant minerals observed are calcite, quartz, dolomite, and Ca-dolomite, with their relative proportions fluctuating throughout the section. Trace amounts of other minerals, such as feldspars, pyrite, and barite, provide further insight into the geochemical conditions and diagenetic processes that affected the shales.
Figure 4. Mineralogical composition and equivalent vitrinite reflectance (Ro%) values of macerated samples from well 3-BP-11-RJS at different depths ((A): 6220 m; (B): 6247 m; (C): 6274 m; (D): 6301 m; (E): 6328 m; (F): 6355 m; (G): 6382 m). The seven graphs showcase the relative abundance of different mineral phases identified in each macerated sample, providing a comprehensive overview of the mineralogical variations across the studied depths. The dominant minerals observed are calcite, quartz, dolomite, and Ca-dolomite, with their relative proportions fluctuating throughout the section. Trace amounts of other minerals, such as feldspars, pyrite, and barite, provide further insight into the geochemical conditions and diagenetic processes that affected the shales.
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Figure 5. Mineralogical composition and equivalent vitrinite reflectance (Ro%) values of decarbonated samples from well 3-BP-11-RJS at different depths ((A): 6220 m; (B): 6247 m; (C): 6274 m; (D): 6301 m; (E): 6328 m; (F): 6355 m; (G): 6382 m). The process highlights the presence and abundance of silicate minerals and clay minerals, revealing a distinct mineralogical profile compared to the macerated samples.
Figure 5. Mineralogical composition and equivalent vitrinite reflectance (Ro%) values of decarbonated samples from well 3-BP-11-RJS at different depths ((A): 6220 m; (B): 6247 m; (C): 6274 m; (D): 6301 m; (E): 6328 m; (F): 6355 m; (G): 6382 m). The process highlights the presence and abundance of silicate minerals and clay minerals, revealing a distinct mineralogical profile compared to the macerated samples.
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Figure 6. Mineralogical composition and equivalent vitrinite reflectance (Ro%) values of macerated samples from well 6-DEV-18P-RJS at different depths ((A): 5324 m; (B): 5339 m; (C): 5372 m; (D): 5405 m; (E): 5414 m; (F): 5444 m). The six graphs showcase the relative abundance of different mineral phases identified in each sample, highlighting the variations in mineralogy across the studied depths. Quartz and calcite are the dominant minerals, with dolomite and calcium-dolomite also present in significant amounts, particularly in the shallower samples. Trace amounts of other minerals, such as feldspars, pyrite, and barite, offer additional insights into the geochemical conditions prevailing during deposition and subsequent diagenesis.
Figure 6. Mineralogical composition and equivalent vitrinite reflectance (Ro%) values of macerated samples from well 6-DEV-18P-RJS at different depths ((A): 5324 m; (B): 5339 m; (C): 5372 m; (D): 5405 m; (E): 5414 m; (F): 5444 m). The six graphs showcase the relative abundance of different mineral phases identified in each sample, highlighting the variations in mineralogy across the studied depths. Quartz and calcite are the dominant minerals, with dolomite and calcium-dolomite also present in significant amounts, particularly in the shallower samples. Trace amounts of other minerals, such as feldspars, pyrite, and barite, offer additional insights into the geochemical conditions prevailing during deposition and subsequent diagenesis.
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Figure 7. Mineralogical composition and equivalent vitrinite reflectance (Ro%) values of decarbonated samples from well 6-DEV-18P-RJS at different depths ((A): 5324 m; (B): 5339 m; (C): 5372 m; (D): 5405 m; (E): 5414 m; (F): 5444 m). The six graphs illustrate the changes in relative mineral abundance after the removal of carbonate minerals. This process highlights the presence and abundance of silicate minerals and clay minerals, revealing a distinct mineralogical profile compared to the macerated samples.
Figure 7. Mineralogical composition and equivalent vitrinite reflectance (Ro%) values of decarbonated samples from well 6-DEV-18P-RJS at different depths ((A): 5324 m; (B): 5339 m; (C): 5372 m; (D): 5405 m; (E): 5414 m; (F): 5444 m). The six graphs illustrate the changes in relative mineral abundance after the removal of carbonate minerals. This process highlights the presence and abundance of silicate minerals and clay minerals, revealing a distinct mineralogical profile compared to the macerated samples.
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Figure 8. SEM micrographs of the 5324 m sample from well 6-DEV-18P-RJS. The sample exhibits a heterogeneous texture characterized by quartz, calcite, and dolomite grains. Illite and chlorite are present as coatings on grains and within the matrix. (A) 400×, (B) 1500×, (C) 2000×, (D) 3000×.
Figure 8. SEM micrographs of the 5324 m sample from well 6-DEV-18P-RJS. The sample exhibits a heterogeneous texture characterized by quartz, calcite, and dolomite grains. Illite and chlorite are present as coatings on grains and within the matrix. (A) 400×, (B) 1500×, (C) 2000×, (D) 3000×.
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Figure 9. SEM micrographs of the 5339 m sample from well 6-DEV-18P-RJS. The sample displays a more homogeneous texture than the 5324 m sample, with a predominance of silt- to fine sand-sized quartz and potassium feldspar grains. Authigenic pyrite is present as small euhedral crystals. (A) 400×, (B) 1500×, (C) 3000×, (D) 4000×.
Figure 9. SEM micrographs of the 5339 m sample from well 6-DEV-18P-RJS. The sample displays a more homogeneous texture than the 5324 m sample, with a predominance of silt- to fine sand-sized quartz and potassium feldspar grains. Authigenic pyrite is present as small euhedral crystals. (A) 400×, (B) 1500×, (C) 3000×, (D) 4000×.
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Figure 10. SEM micrographs of the 5371.5 m sample from well 6-DEV-18P-RJS. The sample is dominated by anhedral calcite crystals, ranging in size from approximately 50 µm to 300 µm. The presence of quartz is less evident. Authigenic dolomite may be present. (A) 200×, (B) 1000×, (C) 1500×, (D) 2000×.
Figure 10. SEM micrographs of the 5371.5 m sample from well 6-DEV-18P-RJS. The sample is dominated by anhedral calcite crystals, ranging in size from approximately 50 µm to 300 µm. The presence of quartz is less evident. Authigenic dolomite may be present. (A) 200×, (B) 1000×, (C) 1500×, (D) 2000×.
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Figure 11. SEM micrographs of the 5372.5 m sample from well 6-DEV-18P-RJS. The sample shows abundant euhedral rhombic dolomite crystals, ranging from approximately 20 µm to 100 µm. Calcite is also present but less abundant than dolomite. Illite/smectite occurs as coatings on grains. (A) 200×, (B) 1000×, (C) 1500×, (D) 3000×.
Figure 11. SEM micrographs of the 5372.5 m sample from well 6-DEV-18P-RJS. The sample shows abundant euhedral rhombic dolomite crystals, ranging from approximately 20 µm to 100 µm. Calcite is also present but less abundant than dolomite. Illite/smectite occurs as coatings on grains. (A) 200×, (B) 1000×, (C) 1500×, (D) 3000×.
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Figure 12. SEM micrographs of the 5404.5 m sample from well 6-DEV-18P-RJS. The sample is dominated by well-crystallized, euhedral rhombic dolomite (Dol) crystals. Authigenic quartz (Qz) and illite (Ill) may be present. (A) 200×, (B) 1000×, (C) 1500×, (D) 2500×.
Figure 12. SEM micrographs of the 5404.5 m sample from well 6-DEV-18P-RJS. The sample is dominated by well-crystallized, euhedral rhombic dolomite (Dol) crystals. Authigenic quartz (Qz) and illite (Ill) may be present. (A) 200×, (B) 1000×, (C) 1500×, (D) 2500×.
Geosciences 14 00286 g012
Geosciences 14 00286 g013
Figure 13. SEM micrographs of the 5413 m sample from well 6-DEV-18P-RJS. The sample exhibits abundant euhedral rhombic dolomite (Dol) crystals, generally smaller than the previous samples, ranging from ~10 µm to ~100 µm. Authigenic quartz (Qz) and illite (Ill) may be present. (A) 200×, (B) 1000×, (C) 3000×, (D) 5000×.
Figure 13. SEM micrographs of the 5413 m sample from well 6-DEV-18P-RJS. The sample exhibits abundant euhedral rhombic dolomite (Dol) crystals, generally smaller than the previous samples, ranging from ~10 µm to ~100 µm. Authigenic quartz (Qz) and illite (Ill) may be present. (A) 200×, (B) 1000×, (C) 3000×, (D) 5000×.
Geosciences 14 00286 g013
Geosciences 14 00286 g014
Figure 14. SEM micrographs of the 5444 m sample from well 6-DEV-18P-RJS. The sample is dominated by anhedral calcite (Cal) crystals of varying sizes and habits. Quartz (Qz) is present as smaller grains. (A) 200×, (B) 1000×, (C) 1500×, (D) 3000×.
Figure 14. SEM micrographs of the 5444 m sample from well 6-DEV-18P-RJS. The sample is dominated by anhedral calcite (Cal) crystals of varying sizes and habits. Quartz (Qz) is present as smaller grains. (A) 200×, (B) 1000×, (C) 1500×, (D) 3000×.
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Table 1. Lithological classification of shale samples from the Coqueiros Formation. The “shales mineralogy” classification is based on the triplot diagram (mineralogy) proposed by [25], while the “petrophysics” classification is based on well log data.
Table 1. Lithological classification of shale samples from the Coqueiros Formation. The “shales mineralogy” classification is based on the triplot diagram (mineralogy) proposed by [25], while the “petrophysics” classification is based on well log data.
WellsDepth (m)Classifications
Shales MineralogyPetrophysics
3-BP-11-RJS6220C1Marl
6247C1Limestone/Shale
6274C1Marl/Shale
6301S1Shale
6328C1Limestone/Marl
6355C1Limestone/Shale
6382C1Shale
6-DEV-18P-RJS5324C2/CLimestone (Coquina)
5339M1Shale
5372S1Limestone (Coquina)
5405S1Marl
5414C1/M1Marl
5444C1Limestone
C—Calcareous shale lithofacies
C1—Silica-rich calcareous shale lithofacies
C2—Mixed calcareous shale lithofacies
M1—Calcareous/siliceous mixed shale
S1—Carbonate-rich siliceous shale lithofacies
Table 2. Equivalent vitrinite reflectance (Ro%) of samples from wells 3-BP-11-RJS and 6-DEV-18P-RJS.
Table 2. Equivalent vitrinite reflectance (Ro%) of samples from wells 3-BP-11-RJS and 6-DEV-18P-RJS.
SamplesDepth (m)Vitrinite Reflectance Ro%
FreshMaceratedDecarbonated
6-DEV-18P-RJS53241.261.251.09
53391.271.181.20
53721.261.031.10
54051.261.071.13
54141.281.241.24
54441.221.221.17
3-BP-11-RJS62201.271.251.26
62471.271.341.24
62741.121.311.28
63011.401.281.28
63281.321.261.37
63551.221.231.27
63821.261.241.27
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Barberes, G.A.; Marques, F.C.; Almeida, D.A.L.; Peixoto, L.P.F.; Maia, L.F.; Sant’Ana, A.C.; Andrade, G.F.S.; Izumi, C.M.S.; Salgado-Campos, V.; Feital, T.; et al. Mineralogical and Maturation Considerations of the Coqueiros Formation (Campos Basin, Brazil): Insights from Multi-Technique Analyses of Source Rocks. Geosciences 2024, 14, 286. https://doi.org/10.3390/geosciences14110286

AMA Style

Barberes GA, Marques FC, Almeida DAL, Peixoto LPF, Maia LF, Sant’Ana AC, Andrade GFS, Izumi CMS, Salgado-Campos V, Feital T, et al. Mineralogical and Maturation Considerations of the Coqueiros Formation (Campos Basin, Brazil): Insights from Multi-Technique Analyses of Source Rocks. Geosciences. 2024; 14(11):286. https://doi.org/10.3390/geosciences14110286

Chicago/Turabian Style

Barberes, Gabriel A., Flávia C. Marques, Dalva A. L. Almeida, Linus Pauling F. Peixoto, Lenize F. Maia, Antonio Carlos Sant’Ana, Gustavo F. S. Andrade, Celly M. S. Izumi, Victor Salgado-Campos, Thiago Feital, and et al. 2024. "Mineralogical and Maturation Considerations of the Coqueiros Formation (Campos Basin, Brazil): Insights from Multi-Technique Analyses of Source Rocks" Geosciences 14, no. 11: 286. https://doi.org/10.3390/geosciences14110286

APA Style

Barberes, G. A., Marques, F. C., Almeida, D. A. L., Peixoto, L. P. F., Maia, L. F., Sant’Ana, A. C., Andrade, G. F. S., Izumi, C. M. S., Salgado-Campos, V., Feital, T., de Oliveira, L. F. C., & Albuquerque, A. L. (2024). Mineralogical and Maturation Considerations of the Coqueiros Formation (Campos Basin, Brazil): Insights from Multi-Technique Analyses of Source Rocks. Geosciences, 14(11), 286. https://doi.org/10.3390/geosciences14110286

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