Influence mechanism of differences in hydrocarbon generation intensity of deep coalbed methane: a case study of the Daning-Jixian block

Abstract

The deeply buried Upper Paleozoic coal seams in the Ordos Basin are regarded as the principal source rocks for natural gas accumulation. Accurately quantifying their hydrocarbon generation capacity and exploring the influencing mechanism of the difference in hydrocarbon generation intensity are crucial for natural gas resource evaluation. This study utilized a mix of organic geochemical analyses, Ro-gas yield conversion, fixed carbon-TOC regression, confined gold-tube pyrolysis, basin modeling, and gas content calculation to investigate the No. 5 and No. 8 coals in the Daning-Jixian block in the eastern Ordos Basin. The results indicate that both coal seams have high hydrocarbon generation potential. However, their hydrocarbon generation characteristics differ markedly: Coal No. 8 possesses substantially stronger hydrocarbon generation and expulsion capacity than Coal No. 5. Its high hydrocarbon generation intensity zones are primarily concentrated in the western, deeply buried area (cumulative hydrocarbon generation > 10 × 10 8 m 3 /km 2 ), while Coal No. 5 exhibits relatively uniform distribution but lower overall intensity (cumulative hydrocarbon generation > 7 × 10 8 m 3 /km 2 ). Research indicates this disparity is dual-controlled by sedimentary environment and tectonic evolution. The marine-terrestrial transitional sedimentary system differentially influences coalbed hydrocarbon generation capacity. Coal No. 8, formed under marine transgression, exhibits superior hydrocarbon generation potential compared to Coal No. 5. Concurrently, marine-derived sulfur-rich organic matter enriched in Coal No. 8 facilitated early hydrocarbon generation by lowering activation energy. Tectonically, varying burial depths governed the spatial distribution of hydrocarbon generation intensity, with deep high-temperature and high-pressure settings significantly enhancing coal seam thermal evolution and hydrocarbon yield.