Genesis and Accumulation Mechanism of Low-Rank CBM: A Case Study in the Jiergalangtu Block of Erlian Basin, Northern China

Abstract

In order to elucidate the origin of coalbed methane (CBM) in the Jiergalangtu block of Erlian Basin, Inner Mongolia of China, gas components, stable isotope tests of 22 gas samples, radioisotope dating measurements, and water quality analysis of 15 coproduced water samples were evaluated. On account of the geochemical data and genetic indicators, including C1/C1–n , C1/(C2 + C3), and CO2/(CO2 + CH4) (CDMI) values, δ13C­(CO2), Δδ13C­(CO2–CH4 ), δ15N, and 3He/4He combined with vitrinite reflectance (Ro) (0.29–0.48%, avg. 0.35%) of Saihantala formation, the results indicate that methane in the Jiergalangtu block is mostly dominated by primary and secondary biological gas, 40.91% of the gas samples are secondary biogas and primary biogas accounts for 59.19%. Among them, methyl-type fermentation accounts for 31.82%, and carbon dioxide (CO2) reduction makes up 68.18%. CO2 reduction generally occurs region-wide but is mainly associated with the central part of the block, where CO2 depletion and 13C enrichment take place correspondingly. Methane and CO2 δ13C almost tend to isotopically light along the margin of the block, indicating that gas generation is significantly affected by the methyl-type fermentation pathway. Meanwhile, the genesis analysis of other gas components in CBM is also investigated, CO2 is mainly the associated product of microbial methanogenesis, and nitrogen (N2) is primarily from the atmosphere with a little amount from the earth’s crust. Furthermore, the formation time of coalbed water has been dissected based on the hydrogeochemical properties of the coproduced water samples. The coalbed water exhibit a Na–HCO3 and Na–HCO3–Cl type and have a total dissolved solid (TDS) value ranging from 2458.58 to 5579.1 mg/L, with an average of 3440.55 mg/L. Moreover, comprehensive analysis of δD­(H2O), δ18O­(H2O), δ13CDIC, and the radioisotope dating index [3H, 14C­(Fm) and 14C­(BP)] indicates that the coalbed water was formed in the Quaternary Pleistocene and rarely replenished by the present surface water. The mechanism of CBM accumulation is basically sorted out by synthesizing the history of burial, heat, and hydrocarbon generation. The CBM formation can be divided into four stages. That is, microbial gas production approximately began at the beginning of the Early Cretaceous and reached the peak of thermogenic gas production in the middle and late Early Cretaceous. At the end of the Early Cretaceous, strata possibly began to uplift, and denudation led to gas escape. From Neogene to Pleistocene, glacial meltwater tended to penetrate into coalbed on a large scale, and N2 and CO2 also entered the coal seams, stimulating abundant secondary biological gas generation. Since Holocene, geological conditions including temperature and TDS have become hostile to biogas generation, and biogas generation tends to stop. Therefore, the Jiergalangtu block mainly represents sealed primary biological gas and secondary biological gas in CBM reservoirs.