Characterization of light hydrocarbons in subsurface Cenozoic sediments of western and eastern Amazonia drilled by the Trans-Amazon Drilling Project (TADP)

The Trans-Amazon Drilling Project (TADP) provides a unique opportunity to investigate subsurface light-hydrocarbon dynamics in Amazonian sedimentary basins through the integration of continuous real-time gas monitoring during drilling, discrete gas sampling from sediment cores, and laboratory incubation experiments. This study combines gas geochemical data from Cenozoic sediments of the Acre Basin in western Amazonia and the Marajó Basin in eastern Amazonia, to characterize the occurrence, composition, origin, migration of light gaseous hydrocarbons, and potential microbial production of methane (CH₄), within continental siliciclastic successions, contributing to a refined understanding of subsurface carbon cycling.

In the Acre Basin, drilling penetrated a 923-m-thick sedimentary sequence dominated by interbedded claystones, siltstones, and sandstones. Continuous online gas analysis (OLGA) revealed CH₄ as the dominant hydrocarbon throughout the drilled profile, accompanied by recurrent detections of ethane (C₂H₆), propane (C₃H₈), iso-butane (i-C₄H₁₀), and n-butane (n-C₄H₁₀). Elevated concentrations of CH₄, C₂H₆, and C₃H₈ were preferentially associated with sandstone and siltstone layers sealed by claystones, indicating stratigraphic trapping of migrated gas. Bernard parameter values (CH₄/(C₂H₆+ C₃H₈)) range from 2 to 1904, reflecting strong compositional variability and mixing between gas sources. Carbon isotopic signatures of CH4 (δ¹³C- CH4 between −35‰ and −25‰ VPDB) indicate a dominant thermogenic contribution.

In the Marajó Basin, continuous gas monitoring during drilling to 924.3 m depth revealed higher? CH4 concentrations than in the Acre Basin with a general increase toward greater depths. Heavier hydrocarbons (C2–C4) show co-occurring concentration maxima indicating stratigraphically discrete gas migration and accumulation. CH₄ carbon isotopic compositions document a clear vertical transition in gas origin, from microbial hydrogenotrophic methanogenesis in the upper 250 m (δ¹³C- CH4 between −80‰ and −60‰), to mixed microbial–thermogenic gas between 250 and 300 m depth, and dominantly thermogenic gas below 300 m depth (δ¹³C- CH4 approaching −35‰), coinciding with increased C2–C4 concentrations.

Laboratory incubation experiments conducted on core sediment samples from both basins under anoxic conditions reveal a progressive increase in CH₄ concentrations over time, indicating active microbial methanogenesis. Incubation results show higher CH₄ yields in deeper samples, suggesting that, despite the strong influence of migrated thermogenic gas at depth, in situ microbial CH₄ production also contributes to the subsurface methane pool and is modulated by depth, substrate availability, and redox conditions.

Overall, the integrated results demonstrate that light hydrocarbon distributions in both basins are governed by the interaction between upward migration of thermogenic gas from deeper sources, stratigraphic trapping in permeable units sealed by fine-grained sediments, and active microbial processes identified through incubation experiments. The combined use of real-time gas monitoring, isotopic analyses, and incubation experiments provides a robust framework for disentangling gas origin and transformation processes, offering new insights into subsurface carbon cycling in Amazonian sedimentary basins.