REVISITING THE MOLECULAR GEOCHEMISTRY OF FLUID PETROLEUM INCLUSIONS: A CASE STUDY FROM THE POTIGUAR BASIN

Introduction

The Potiguar Basin, located on the equatorial margin of Brazil, is characterized by an oil system defined by the Pendência and Alagamar source rock formations. The source rocks of the Pendência Formation (Neocomian) comprise black shales containing organic matter of lacustrine origin and significant terrigenous input. The shales of the Alagamar Formation (Aptian, offshore) have organic matter typical of a lacustrine environment at their base (Upanema Member), which transitions to the top to shales with characteristics of a restricted marine-evaporitic environment (Ponta do Tubarão Layers). These units record multiple events of hydrocarbon generation and migration and are fundamental to understanding the thermal evolution and diagenetic processes of the basin (Pessoa Neto et al., 2007).

In this context, Pestilho et al. (2018) employed an integrated application of techniques to study fluid inclusions, including petrography under ultraviolet fluorescence and fluorescence microspectrophotometry associated with the analysis of the molecular composition (biomarkers) of oil inclusions, to reconstruct the history of hydrocarbon migration in the Potiguar Basin. The study established important geochemical correlations between petroleum inclusions and crude oils from samples taken from two wells located in the Lorena (onshore) and Ubarana (offshore) oil fields. According to the authors, three groups of fluid inclusions were identified: (i) one associated with the Lorena reservoir (Pendência Formation), (ii) another from the siliciclastic Ubarana reservoir (Açu Formation), and (iii) a third from the carbonate rocks adjacent to the Ubarana reservoir (Ponta do Mel Formation, PML). Furthermore, fluorescence microspectrophotometry revealed that the three groups of inclusions contain oils with different API gravities and are not biodegradable (Pestilho et al., 2018).

Despite the relevance of the results obtained by Pestilho et al. (2018), some methodological aspects and interpretations remain open to reevaluation, especially regarding the analytical resolution of the chromatograms presented and the experimental conditions used, and the possibility of analyzing new compounds through the gas chromatography-mass spectrometry (GC-EM), such as identifying aromatic hydrocarbons, in addition to analysis through the gas chromatography with flame ionization detection (GC-FID) (e.g., n-alkanes and isoprenoids). This study presents a reinterpretation of existing data through the application of innovative analytical techniques for hydrocarbon recovery from fluid inclusions, employing open-column liquid chromatography. This method enables the separation of hydrocarbon fractions into saturated and aromatic compounds, allowing for a refined geochemical characterization. By integrating these results with existing literature, we aim to improve the understanding of diagenetic alterations and the thermal maturity evolution of petroleum systems in the Ubarana and Lorena fields.

 

Experimental

In this study, two core samples were selected for analysis based on geological context and the potential occurrence of petroleum-bearing fluid inclusions. The samples were collected from the Lorena field (LOR-60), Pendência Formation, and the Ubarana field (UB-40_2485.55 m), Ponta do Mel Formation (PML). Molecular geochemical analyses of the fluid inclusions were performed using GC-FID and GC-MS.

The samples were initially fragmented and subjected to oil extraction using dichloromethane in an ultrasonic bath. The fragments underwent specific cleaning protocols for siliciclastic and carbonate rocks, following cleaning protocols consistent with those described by George et al. (2007). After thoroughly removing surface contaminants from the grain surfaces, oils trapped in fluid inclusions were extracted through multiple successive extractions, thereby enhancing the concentration of analytes. Both the fluid inclusion oils (FIO) and the recovered oil extracts were fractionated into saturated and aromatic hydrocarbon fractions using open-column liquid chromatography. This method was optimized and adapted for microscale applications using a Pasteur pipette, enabling the analysis of trace-level compounds typically present in fluid inclusions.

 

Results and Discussion

The oil extraction and recovery procedures, followed by fractionation using a silica gel column, enabled the characterization of the saturated and aromatic fractions of the organic extracts from both fluid inclusions and recovered oil extracts from samples LOR-60 and UB-40. GC-FID analysis enabled the characterization of n-alkanes and isoprenoids. At the same time, GC-MS facilitated the identification of molecular biomarkers, including terpanes, steranes, and aromatic hydrocarbons, thereby supporting the geochemical characterization of the samples.

Based on these geochemical signatures, it was possible to correlate the fluid inclusion oils with their respective recovered oil extracts and to compare them with literature data (Pestilho et al., 2018), as evidenced by the mass chromatograms (Fig. 1), where it can be observed that the sample analyzed in this study (FIO (PML)_2485.55 m) demonstrates significant methodological advancements over the approach proposed by Pestilho et al. (2018), particularly in terms of analytical resolution and the diversity of compounds identified.

To assess the thermal evolution of the oils analyzed in this study, several representative thermal maturity parameters were selected based on the distribution of terpanes and steranes, including the Ts/(Ts+Tm) and C29 αββ/(αββ+ααα) ratios, among others. The results are summarized in Table 1, along with the corresponding sample identifications.

 

Table 1. Thermal Maturity Parameters

 (*) From Pestilho et al. (2018). FIO (Fluid Inclusion Oils).

 

Figure 1. Mass chromatograms (m/z 191) of the fluid inclusion oils (FIO) in sample UB-40 (FIO (PML)_2485.55 m) (a), and from sample UB-40 (FIO (PML)_2485.53 m), compiled from Pestilho et al. (2018) (b), with peak identification.

 

The data presented in Table 1 supported the geochemical interpretation through the use of scatter plots. The geochemical parameters analyzed (Fig. 2) indicate that the oils from fluid inclusions (PML and Açu Formations) and the recovered oil extracts lie within the oil generation window, with thermal maturity levels ranging from intermediate to overmature (e.g., FIO (PML)_2438.53 m). The oils exhibit intermediate to advanced thermal maturation, as evidenced by 20S/(20S+20R) ratios (0.41–0.51) and αββ/(αββ+ααα) ratios (0.44–0.55), both of which are consistent with the oil generation window. Low Ts/(Ts+Tm) values (< 0.50) and DIAH/H30 ratios (0.03–0.04) suggest moderate maturity and a carbonate-rich, low-clay source rock. The NORNEO/H29 ratio (0.21–0.28) reflects a slightly reducing depositional environment consistent with restricted lacustrine settings.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2. Thermal maturity indicators for fluid inclusion oils (FIO) and recovered oil extracts from the Ubarana Field (PML and Açu formations). (*) From Pestilho et al. (2018).

 

Conclusions

While the original study focused on conventional biomarker correlations (e.g., hopanes and steranes), this new approach employed optimized preparative techniques, such as sample pre-concentration and silica gel column fractionation, which allowed for the detection of minor hydrocarbon compounds, including n-alkanes, isoprenoids, and diagnostic aromatic hydrocarbons. These results not only complement but significantly expand the interpretations of Pestilho et al. (2018).

The continuation of these investigations is particularly promising for characterizing aromatic compounds, which are sensitive to thermal maturation and redox conditions during deposition. The analysis of these compounds in fluid inclusions in fluid inclusions, still scarcely explored, may allow the integration of molecular geochemical data with fluorescence spectroscopy (Pestilho et al., 2018), contributing to the refinement of maturity parameters and offer a new perspective on the degree of thermal evolution between reservoir oils and fluid inclusion oils from the Ubarana and Lorena fields.

 

Acknowledgements

The authors thank FEC/UFF/CENPES/PETROBRÁS (project 4948) for financial support and CAPES (Code 001 PPG-Geoquímica-UFF).