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

Abstract

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 [JP+1]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.

Keywords

• third-order sequence
• lithofacies palaeogeography
• petroleum geology
• middle permian
• sichuan basin