Subsurface Gaseous Hydrocarbons and Carbon Dioxide Recorded During the Trans-Amazon Drilling Project (TADP) in the Acre Basin, Western Amazon

The Trans-Amazon Drilling Project (TADP) drilled a sequence of claystones, siltstones, and sandstones in the Acre sedimentary basin, reaching a final depth of 923 m. This study characterizes the occurrence and compositional variation of light gaseous hydrocarbons detected using the online gas analysis (OLGA) monitoring system deployed during drilling, along with methane (CH₄) and carbon dioxide (CO₂) concentrations measured in discrete gas samples extracted from cores during drilling operations. The gaseous hydrocarbons detected by the OLGA system are predominantly CH₄ but with the regular presence of ethane (C₂H₆), propane (C₃H₈), isobutane (i-C₄H₁₀), and n-butane (n-C₄H₁₀). Zones with higher CH₄, C₂H₆, and C₃H₈ concentrations were observed at depth intervals of 250–380 and 420–588 m. These higher concentrations of CH₄, C₂H₆, and C₃H₈ occur in siltstone or sandstone layers capped by claystones, suggesting that these lithological associations act as stratigraphic gas traps. The Bernard parameter (CH₄/C₂H₆ + C₃H₈) varied from a low value of 2 at 466 m depth to a maximum value of 1904 at 621 m depth. Stable carbon isotope ratios of CH₄ show δ¹³C values between −35‰ and −25‰, suggesting the nearly ubiquitous presence of thermogenic gas. The discrete gas samples from cores exhibited CO₂ concentrations between 230 and 1400 ppm in claystones, 850 and 950 ppm in siltstones, and 240–820 ppm in sandstones, indicating higher concentrations in fine-grained sediments. The CH₄ concentration ranges from 2 to 6 ppm in sandstone layers and from 2 to 4 ppm in siltstone and claystone layers. There is no significant correlation between CH₄ and CO₂ concentrations. These results provide evidence of light hydrocarbon migration from deeper thermally mature source rocks, with entrapment in sandstone layers capped by fine-grained sedimentary rock layers. The high concentration of CO₂ relative to CH₄ in fine-grained rock layers points to restricted conditions for microbial gas generation in the drilled sediments, possibly due to a combination of low organic carbon content and oxidizing conditions. This is in accordance with the abundance of reddish fine-grained paleosols in the drilled sedimentary units. The combination of online gas monitoring and discrete sampling methods allowed the comparison between gas collected during drilling and in situ gas, contributing to a better understanding of the processes of the subsurface carbon cycle.