Geochemical characteristics and origin of natural gases from Xuanhan area,eastern Sichuan
Zhu Yangming1, Wang Jibao1, Hao Fang2, Zou Huayao2, Cai Xunyu3
1. Department of Earth Sciences, Zhejiang University, Hangzhou 310027; 2. Faculty of Natural Resources and Information Technology, China University of Petroleum, Beijing 102249; 3. Southern Division, China Petroleum & Chemical Corp., Kunming 650200
Based on the molecular and stable carbon isotope compositions of over 60 gas samples collected from more than 10 exploration wells,in combination with the analytical data of source rocks and reservoir bitumen,the geochemical characteristics and origin of gases mainly from the Puguang and Maobachang gas traps in Xuanhan area,eastern Sichuan,were investigated.The studied results indicated that the natural gases in the reservoirs of traps of Feixianguan-Changxing Formation belonged to dry gas with high amounts of H2S,and showed a molecular composition features of the gas cracking from oil previously reservoired in the traps.The gases were dominated by methane in hydrocarbon component with dryness index mostly more than 0.99,and contained abundant no-hydrocarbon components with CO2 and H2S average concentration being 5.32 and 11.95% respectively.The methane was isotopically heavy(-33‰～-29‰),suggesting a high level of thermal maturity,and the δ13C values for ethane ranged mainly from-33‰ to-28‰ indicative of sapropelic organic source.The studied gases were believed to have a common source with the gases from same reservoir age strata in adjacent gas fields in the eastern Sichuan Basin,but different in origin with Carboniferous reservoired gases due to different molecular and carbon isotope compositions.The high concentration of H2S was considered to be derived from TSR on sulfur isotopes for sulfide and element compositions of reservoir bitumen.Sulfur isotopes revealed that the layered anhydrite was the reactant for TSR,whereas the anhydrite vein appeared to be derived from precipitation of the reactive fluid.It was noted that TSR and derived H2S abundance were possibly associated with the porosity type of their gas reservoirs.H2S in gases from fractured reservoirs was less significantly than one in gases from corroded reservoirs.The carbon isotope correlation between ethane,bitumen and source rock kerogens showed that the gases in the reservoirs of Feixianguan-Changxing Formation were mainly derived from the Permian strata.