川东宣汉地区天然气地球化学特征及成因

Geochemical characteristics and origin of natural gases from Xuanhan area,eastern Sichuan

依据10余口探井60多个气样的化学成份和碳同位素组成数据,结合烃源岩和储层沥青分析资料,系统剖析了四川盆地东部宣汉地区普光、毛坝场等构造带天然气地球化学特征,并探讨了其成因及来源。研究结果表明:这些构造带中飞仙关组—长兴组天然气为高含硫化氢的干气,天然气化学成份表现出古油藏原油裂解气的特点。其烃类气体中以甲烷为主(高于99.5%);富含非烃气体,CO2和H2S平均含量分别达5.32%和11.95%。甲烷碳同位素较重(-33‰～-29‰),表征高热演化性质;乙烷δ13C值主要分布在-33‰至-28‰范围,属油型气。这些天然气与川东邻近气田的同层位天然气具有同源性,而与石炭系气藏天然气在化学成份、碳同位素组成上有所不同,意味着有不同的气源。硫化物硫同位素和沥青元素组成证实高含量的H2S是气藏发生TSR作用所致。δ34S值表征层状沉积成因的硬石膏是TSR作用的反应物,而脉状硬石膏则是其残余物。储层的孔隙类型可能与TSR作用强度和H2S含量高低有联系,裂缝型气层中H2S少,孔洞型储层中H2S丰富。乙烷、沥青和各层系烃源岩干酪根碳同位素对比表明研究区飞仙关组—长兴组气藏天然气主要来自二叠系烃源层。

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 δ^13C 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.