川西坳陷中三叠统雷口坡组四段气藏气源分析

摘要

A set of thick charcoal grey dolomicrite and micrite source rocks developed in the Middle Triassic Leikoupo Formation in the Western Sichuan Depression, in addition to four sets of marine source rocks in the Permian and its underlying strata. The gas generation intensity of the source rocks is about (1-4)×109 m3/km2, showing a good potential for medium or giant gas fields. According to the gas composition, carbon and hydrogen isotopes, reservoir bitumen and migration conditions, the major sources for the gas reservoirs of the fourth member of the Leikoupo Formation vary in different tectonic zones in the Western Sichuan Depression. The gas from well PZ1 in the Longmenshan piedmont tectonic zone displays high H2 S content, high δ13 C2 value (-26. 4‰) , positive alkane carbon isotopic series and lightδD1 value (-140‰) , consistent with characteristics of mixed gas. The thermal evolution degree reflected by natural gas compositions and carbon isotopes falls between those of Permian and Leikoupo source rocks, which indicates that the gas was sourced from both of them, and the deep faults in front of the Longmenshan Mountains provide favorable conditions for gas migration and accumulation from the source rocks. Meanwhile, the gases from well CK1 in the Xinchang tectonic zone display trace to moderate H2 S content, low δ13 C2 value (-33.2‰) , partial reversal of alkane carbon isotopic series and a light δD1 value (-147‰) , consistent with oil⁃type gas. The thermal evolution degree reflected by natural gas compositions and carbon isotopes is similar to that of Leikoupo source rocks, which indicates that the gases were mainly from the interior source rocks of the Leikoupo Formation.%川西地区海相层系除发育二叠系等烃源岩之外，中三叠统雷口坡组内部还发育一套潟湖相黑灰色泥微晶白云岩、灰岩烃源岩，其生烃强度为（10～40）×108 m3／km2，具备形成大中型气田的气源条件。通过对川西坳陷不同构造带雷口坡组四段气藏天然气组分、烷烃气碳氢同位素特征、储层沥青、天然气运移条件分析认为，不同构造带具有不同的主力气源。龙门山前构造带PZ1井天然气高含H2 S、δ13 C2偏重（－26．4‰）、烷烃气碳同位素呈正序分布、δD1偏轻（－140‰），总体具混源气特征，天然气组分及碳同位素反映的母质演化程度介于二叠系和雷口坡组源岩之间，反映可能为二者的混源气，龙门山前通源断裂的发育为混源成藏提供了条件；新场构造带CK1井天然气微—中含量H2 S、δ13 C2偏轻（－33．2‰）、烷烃气碳同位素局部倒转、δD1偏轻（－147‰），具油型气特征，其与雷口坡组烃源岩大致相当，反映主要来源于雷口坡组内部自身的烃源岩。