四川盆地中二叠世岩相古地理及其油气地质意义

Lithofacies palaeogeography of Middle Permian in the Sichuan Basin and its petroleum geological significance

近期四川盆地中二叠统陆续取得勘探发现,展现出很大的勘探潜力。中二叠世该区构造—沉积分异明显,导致油气成藏规律复杂,层序岩相古地理的重建对认识构造—沉积演化、沉积建造和油气勘探具有重要意义。通过综合油气勘探成果、典型钻井及野外露头资料,并以三级层序为最小纵向编图单元,重建了四川盆地中二叠世栖霞—茅口期各层序的岩相古地理。四川盆地中二叠统可划分为5个三级层序,即SQq1(梁山组—栖一段B)、SQq2(栖一段A—栖二段)、SQm1(茅一段—茅二段C)、SQm2(茅二段B—A)和SQm3(茅三—茅四段)。SQq1和SQm1沉积期总体以海侵和填平补齐过程为主,盆内局部古高地存在点状、带状滩体;SQq2、SQm2和SQm3沉积期以持续海退、地貌分异为主,盆缘发育镶边的碳酸盐岩台地沉积体系,台内可形成较大规模多个滩体;SQq2沉积期是中二叠世主要成滩期,较漫长的海退过程和局部块体升降,促使川西台缘、川中南部台内形成较大规模多个滩体;SQm3沉积期在盆地北部沉积分异明显加剧,全面转换为台缘—斜坡—陆棚的隆凹相间沉积分异格局;SQq2和SQm3沉积期高位体系域(HST)的台缘、台内滩体为白云石化、溶蚀等后期建设性成岩改造提供了物质基础。坡折带地貌的刻画和台缘、台内滩体预测应成为下一步勘探研究的重点。

Exploration discoveries have been made in the Middle Permian of the Sichuan Basin, indicating great exploration potential. However, the Middle Permian has obvious tectonic-sedimentary variation, resulting in complexity of hydrocarbon accumulation mechanism. The reconstruction of sequence lithofacies paleogeography is of great significance to understand the tectonic-sedimentary evolution, sedimentary formation and hydrocarbon exploration. In this paper, the lithofacies palaeogeography during the Middle Permian Qixia-Maokou period in the Sichuan Basin was reconstructed with the third order sequence as the minimum longitudinal mapping unit based on oil and gas exploration results as well as typical drilling and outcropping data. Five third-order sequences for the Middle Permian in the Sichuan Basin were supposed in this study including: SQq1(from the Liangshan Formation to the section B of the first member of the Qixia Formation), SQq2(from the section A of the first member to the second member of the Qixia Formation), SQm1(from the first member to the section C of the second member of the Maokou Formation), SQm2(from the section B to the section A of the second member of the Maokou Formation), and SQm3(from the third to the fourth member of Maokou Formation). During the deposition of SQq1 and SQm1, transgression and filling were dominant, and spot and banded beach bodies in some palaeo-highlands of the basin were discovered. During the deposition of SQq2, SQm2 and SQm3, continuous regression and geomorphic differentiation were dominant, the carbonate platforms depositional system with edge were developed in the margin parts of the basin, and large scale multi-stage and multi-zone beach bodies could be formed in the platform. SQq2 was the main shoal-forming stage in the Middle Permian. The relatively long regressive process and local block uplift promoted the formation of large scale multi-stage and multi-zone shoals in the platform margin of the western Sichuan Basin and the southern part of the central Sichuan Basin. During SQm3 period, the sedimentary differentiation in the northern Sichuan Basin was obviously intensified, and it was transformed into an alternate uplift and sag pattern with platform margin, slope and shelf. The platform margin and intra-platform shoal of the HST during the deposition of SQq2 and SQm3 provided material basis for dolomitization,dissolution and other later constructive diagenetic transformation. Therefore, the delineation of slope break zone geomorphology and the prediction of platform margin and intra-platform shoal should be regarded as the focus of further exploration.