Geological characteristics and forming conditions of the platform margin large reef-shoal gas province in the Sichuan Basin

Abstract

The large gas province of platform margin reefs and shoals of the Permian-Triassic ChangxingFormation-Feixianguan Formation in the Sichuan Basin is studied as an example to describe the forming conditions and geological characteristics of large strip-like gas provinces of platform margin reefs and shoals. The Sichuan Basin experienced cratonic rifting and basement faulting during the Late Permian. Within a widespread regional ramp, a paleogeomorphology pattern of “three highs and three depressions” were formed. The three negative relieves are, respectively, Chengkou-E'xi trough, Kaijiang-Liangping trough, and Nanchong-Mianyang intraplatform depression, and the three topographic highs are Fengjie-Zhenba platform flat, Shizhu-Yilong platform flat, and Suining-Moxi platform flat. In the transitional areas between platform flats and troughs, large-scale platform margin reef-shoal complexes were developed under high energy environment. Diagenetic processes, such as early stage dissolution, seepage-reflux dolomitization and burial dissolution, contributed to porosity improvement in high-quality dolomite reservoirs along platform margins. High mature source rocks and faults developed within the platform margin reef-shoal complexes, forming better hydrocarbon accumulation conditions than those of intraplatform reefs and shoals. Therefore, the distribution of platform margins controlled that of gas accumulation zones. Paleogeography background of large platform, extensive dolomitization and solution in reef-shoal reservoirs, good underlying and lateral adjacent source rocks, strip assemblages of lithological, structural, and litho-structural oil and gas traps are the four basic elements that determine the formation and distribution of large strip-like gas provinces. The configuration of the four elements in the studied area shows promising exploration prospects of large platform margin oil and gas provinces. 摘要: 以四川盆地二叠系—三叠系长兴组—飞仙关组台缘带礁滩大气区为例 阐述了台缘礁滩体带状大气区形成条件与地质特征。四川盆地晚二叠世经历了克拉通内裂陷作用和基底断裂活动 在区域大缓坡背景下 形成了以城口—鄂西海、开江—梁平海槽及南充—绵阳台内洼地3个负向单元以及奉节—镇巴台坪、石柱—仪陇台坪、遂宁—磨溪台坪3个古地貌高点为特征的“三隆三凹”的古地理格局 台坪向海槽过渡的高能带环境发育大型台缘带礁滩体 经历早期溶蚀、回流渗透白云石化与埋藏溶蚀等建设性成岩作用 形成了沿台缘带分布的优质白云岩储集层 台缘带礁滩体具有良好烃源条件 断层发育 油气成藏条件明显优于台内礁滩体 台缘带控制天然气富集 即大型台缘古地理背景、规模白云岩溶蚀礁滩储集层、良好的下生与侧向气源条件和带状集群展布的岩性、构造及构造-岩性复合3类圈闭“四要素”有机配置决定带状大气区的形成与分布 展示了台缘带礁滩大油气区的良好勘探前景。图7表3参12 关键词台缘带 礁滩大气区 成藏条件 白云岩储集层 基底断裂 四川盆地-图分类号P618.130.2; TE122 文献标识码A Key words: platform margin, large reef-shoal gas province, hydrocarbon accumulation condition, dolomite reservoir, basement faulting, Sichuan Basin