基于碳氢同位素的深层煤岩气成因分析及沉积环境响应OACHSSCD

In recent years,significant natural gas showshave been encountered in the deep coal measures of the Permian Longtan Formation in the western Chongqing area,preliminarily confirming considerable resource potential for deep coal-measure gas in this interval.To reveal the geochemical characteristics and clarify the genetic type and origin of the deep coal-measure gas in this region,studies were conducted on the composition of natural gas and the carbon and hydrogen isotopic characteristics of methane,ethane,and CO2.The research results indicate:1)The Longtan Formation coal-measure gas in western Chongqing is predominantly CH4,accompanied by minor CO2 and N2,with low heavy hydrocarbon content.Compared to Permian coal-measure gas from adjacent areas of the Yangtze Platform,the N2 and CO2 contents are significantly higher,indicating a high thermal evolution degree of the coal measures and favorable preservation conditions.2)The carbon isotopes of alkane gases suggest that the deep coal-measure gas is over-mature cracked gas.Hydrocarbon generation history and thermal history analyses indicate that the main gas accumulation in the study area formed during the Early Cretaceous,representing an early charging and accumulation event.3)The δD1 values of the coal-measure gas in the study area exhibit a gradual decrease from bottom to top,a pattern closely related to sea-level fluctuations and paleoclimate variations during the coal deposition period.Spatially,δD1 values in the eastern Sichuan region show an increasing trend from southwest to northeast,overall controlled by the depositional environment and also influenced by the degree of thermal evolution.Under similar thermal evolution conditions,marine δD1 values are more negative than terrestrial values,while the marine-continental transitional facies exhibit a broader range of values.The study demonstrates that the geochemical characteristics of coal-measure gas are closely linked to the formation and evolution of the coal seams,providing a scientific basis for selecting favorable targets and assessing resource potential.