Sequence-lithofacies paleogeographic characteristics and petroleum geological significance of Lower Permian Qixia Stage in Sichuan Basin and its adjacent areas, SW China

Abstract

Through the analysis of logging, field outcrops, cores and geochemical data, and based on the study of the relationships between sea level changes, sequence filling, paleo-geomorphy and lithofacies, the sequence lithofacies paleo-geography and evolution process of the Lower Permian Liangshan-Qixia Formation (Qixia Stage for short) in Sichuan Basin and its surrounding areas are restored. The Qixia Stage can be divided into three third-order sequences, in which SQ0, SQ1 and SQ2 are developed in the depression area, and SQ1 and SQ2 are only developed in other areas. The paleo-geomorphy reflected by the thickness of each sequence indicates that before the deposition of the Qixia Stage in the Early Permian, the areas surrounding the Sichuan Basin are characterized by “four uplifts and four depressions”, namely, four paleo-uplifts/paleo-lands of Kangdian, Hannan, Shennongjia and Xuefeng Mountain, and four depressions of Chengdu-Mianyang, Kangdian front, Jiangkou and Yichang; while the interior of the basin is characterized by “secondary uplifts, secondary depressions and alternating convex-concave”. SQ2 is the main shoal forming period of the Qixia Formation, and the high-energy mound shoal facies mainly developed in the highs of sedimentary paleo-geomorphy and the relative slope break zones. The distribution of dolomitic reservoirs (dolomite, limy dolomite and dolomitic limestone) has a good correlation with the sedimentary geomorphic highs and slope break zones. The favorable mound-shoal and dolomitic reservoirs are distributed around depressions at platform-margin and along highs and around sags in the basin. It is pointed out that the platform-margin area in western Sichuan Basin is still the key area for exploration at present; while areas around Chengdu-Mianyang depression and Guangwang secondary depression inside the platform and areas around sags in central Sichuan-southern Sichuan are favorable exploration areas for dolomitic reservoirs of the Qixia Formation in the next step.