Study on the enrichment mechanism of low to medium rank coalbed gas (CBG) from multiple sources and complex components

Abstract

Unlike high-rank coalbed gas (CBG), which is dominated by thermogenic gas with relatively single components, the low to medium rank CBG has complex components (N2 and CO2 rich CBG blocks are frequently found) and diverse sources (a mixture of thermogenic and biogenic gas, or even dominated by biogenic gas), and the enrichment model of CBG reservoirs is not clear yet. The present study was carried out in the southern Junggar Basin. The field desorption data were utilized to analyze the composition. CH4 was found to be the main component of CBG in the southern Junggar Basin, and high N2 and CO2 were found to exist in the Manasi-Hutubi and Miquan areas, respectively. Gas composition, gas and water stable isotopes were tested to clarify the source and fate of each gas (CH4, CO2 and N2). The results shows that biogenic CH4 is absolutely dominant in the low-medium rank CBG in the southern Junggar Basin, with microbial CH4 accounting for more than 90% of the total in the Liuhuanggou and Miquan areas in particular. Thermogenic, biogenic, and crustal CO2 are all detected in the southern Junggar Baisn. Manasi-Hutubi, Liuhuanggou, Fukang, and Jimushaer CO2 exhibit biogenicity. The intrusion of CO2 from middle to late Miocene crustal sources mainly triggered the anomalously high CO2 in the Miquan area. Excess N2 in the Manasi-Hutubi area originated from the atmosphere and was a consequence of hydrodynamic destruction of CBG reservoirs. In addition, the enrichment pattern of low to medium rank CBG reservoirs with biogenicity as the main source was revealed. Low to medium rank CBG reservoirs could be classified from shallow to deep: (1) Gas weathering zone; (2) Microbial gas zone; (3) Mixed gas zone; (4) Thermogenic gas zone. Microbial gas zone and the mixed gas zone are the enrichment zones of low to medium rank CBG (coal seams from 600 m to 1000 m in the southern Junggar Basin).