OFF-LINE STABLE CARBON ISOTOPE ANALYSIS OF GASES FROM FLUID INCLUSIONS

Petroleum fluid inclusions are microscopic cavities within minerals that trap small amounts of water, oil, and/or gas. Fluid inclusions occur in a variety of minerals in sedimentary basins, particularly carbonates, silicates, and evaporites. The isotopic approach consists of releasing and recovering the gaseous phase preserved within fluid inclusions, allowing the determination of carbon isotope ratios of hydrocarbon gases (mainly methane and ethane) and CO2 and, consequently, enabling the characterization of the isotopic signature of gases trapped in paleofluids. This study aimed to analyze gases preserved in fluid inclusions from three pre-salt core samples from the Santos Basin recovered during Petrobras exploratory drilling. Sample preparation initially involved gentle rock fragmentation and extraction of oil adhering to the mineral surface. After elemental and mineralogical characterization by X-ray fluorescence (XRF) and X-ray diffraction (XRD), the mineral fractions were cleaned to remove residual organic contaminants, separated into carbonate and quartz fractions, and subsequently crushed under vacuum using a stainless steel crushing device. The developed off-line crushing device enables the controlled release and recovery of the gaseous phase preserved in fluid inclusions for isotopic analysis by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). Carbonate fractions yielded δ13C values ranging from -35.23 ‰ to -36.38 ‰ for methane (C1), and from -28.44 ‰ to -28.64 ‰ for ethane (C2). The values are consistent with a thermogenic origin of the trapped gases. The isotopic differences Δδ13C(C2-C1) remained close to 7 ‰ in all samples, indicating high compositional similarity among the analyzed gases. In addition, δ13C(C1) values from fluid inclusions (~ -36 ‰) were enriched in 13C compared to values reported for gases currently produced from the reservoir, around -42 ‰ to -41 ‰. This difference suggests that the trapped gases might record conditions that are distinct from those of the present-day petroleum system. In contrast, methane and ethane were not detected above the method detection limit in the quartz fractions. Finally, δ13C values obtained for CO2 in the carbonate fractions ranged from -5.31 ‰ to -6.68 ‰, falling within the isotopic range reported for natural gases from the Santos Basin pre-salt. Similar δ13C(CO2) values were obtained for the quartz fractions, supporting the interpretation that fluid inclusions hosted in both carbonate and quartz minerals recorded fluids from the same geochemical system.