The Impact of Marine Influence on the Hydrocarbon Generation Potential and Process of Upper Paleozoic Coal in the Eastern Ordos Basin, China: Implications for Unconventional Gas Exploration of Coal Measures

Paleozoic coals within the Ordos Basin are believed to be the primary source rock for natural gas produced from these strata. However, the influence of potential marine environments on their hydrocarbon generation potential and processes remains underexplored. This study addresses this knowledge gap by employing organic geochemical methods, high-pressure pyrolysis (gold-tube pyrolysis), and basin modeling. The results reveal significantly higher total sulfur (TS) contents in Taiyuan and Benxi Formation coals compared to those in Shanxi Formation coals. This difference might suggest a strong marine influence on the former two formations. The influence of marine environments on the hydrocarbon generation potential exhibited limited effects across the entire coal sample set, possibly due to the scarcity of samples with Tmax values in the 445-460 degrees C range. However, for coals with similar Tmax ranges (461-470 and 471-480 degrees C), marine-influenced Benxi and Taiyuan Formation coals displayed higher hydrogen index (HI) values compared to nonmarine Shanxi Formation coals. This might suggest a potential extension of the maturation process by marine influence. Benxi Formation coals exhibited a greater hydrocarbon generation potential than Shanxi Formation coals at a thermal maturity below 1.0%. This may be attributed to lower activation energy in marine-influenced coals due to weaker S-S and C-S bonds, facilitating the formation of active sulfur radicals during maturation. Additionally, the rearrangement of sulfur moieties in residual kerogen of Benxi Formation coals might enhance thermal stability and elevate the gas generation potential at thermal maturity exceeding 3.5%. These findings hold implications for coalbed methane exploration in the Shenmu-Fugu region of the Ordos Basin and areas within the Qinshui Basin exhibiting thermal maturity exceeding 3.5%.