四川盆地川西气田中三叠统雷口坡组气藏气源再认识

Reinterpretation of gas sources in the Middle Triassic Leikoupo Formation in Western Sichuan gas field,Sichuan Basin

天然气探明储量超千亿立方米的川西气田是中国石化继普光和元坝气田之后,在四川盆地发现的第3个大型海相气田。目前对该气田中三叠统雷口坡组气藏的气源认识尚未统一,争议较大,有必要对其开展再分析。对川西气田不同构造雷口坡组四段三亚段(雷四三亚段)上、下产层的天然气组分和烷烃同位素特征分析表明,川西气田同一构造雷四三亚段下储层气藏和上储层气藏具有相同的气源,而不同构造间气源具有差异。其中,石羊场构造和鸭子河构造天然气中H_(2)S和CO_(2)含量高、δ^(13)C_(2)偏轻(均值-32.5‰),以及烷烃气碳同位素倒转,表现出典型的油型气特征;金马构造天然气中H_(2)S和CO_(2)含量相对较低、δ^(13)C_(2)偏重(均值-27.5‰),以及烷烃气碳同位素正序分布,表现出混源气特征。此外,基于研究区天然气运移条件分析,结合前人研究成果认为,川西气田石羊场构造和鸭子河构造天然气主要来自雷口坡组自身烃源岩,而金马构造天然气则主要来自雷口坡组烃源岩和上二叠统龙潭组烃源岩,但不排除上三叠统马鞍塘组-小塘子组烃源岩对其有少许贡献。

The Western Sichuan gas field with proven geologic reserves of over 1000×10^(8) m^(3) of natural gas ranks third in scale in the Sichuan Basin following Puguang and Yuanba gas fields.Recently,there have been some controversies regarding to the sources of gas in the Leikoupo reservoir,rendering the necessity to make further analysis.The natural gas in the upper and lower pay zones in the 3^(rd) submember of the 4^(th) member of Leikoupo Formation(hereinafter referred to as Lei 4^(3))in different structures is analyzed for natural gas components and alkane isotopic compositions,which indicate that the lower and upper gas reservoirs in the Lei 4^(3) shale the same gas source.While the gas sources vary with structural belts.Natural gas from Shiyang and Yazihe structural belts,is characterized by high content of H_(2)S and CO_(2),lowδ^(13)C_(2)(mean:-32.5‰),and carbon isotope reversal of alkane,typical of oil-derived gas.However,in the Jinma Structure,the natural gas features relatively low content of H_(2)S and CO_(2),highδ^(13)C_(2)(mean:-27.5‰),and normal carbon isotope sequence pattern,showing the characteristics of mixed gas.In addition,based on the analysis of gasmigration conditions and previous studies,we consider that the natural gas in the Shiyangchang and Yazihe structural belts is mainly derived from the Leikoupo source rocks,and that in the Jinma structural belt mainly derived from the source rocks of Leikoupo and the Permian Longtan Formations,though without excluding minor contributions by the Ma􀆳antang-Xiaotangzi source rocks.