Reconstruction and petroleum geological significance of lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in northwestern Sichuan Basin,SW China

摘要

Based on analysis of outcrop,drilling,logging and seismic data,and geotectonic background,the lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in the northwestern Sichuan Basin were reconstructed,and the petroleum geological significance of the lithofacies paleogeography and paleokarst geomorphology were discussed.The Maokou Formation is divided into 3 long-term cycles,namely LSCl,LSC2 and LSC3,which correspond to the Member 1,Member 2 and Member 3 of the Maokou Formation,respectively.Controlled by the extensional structure caused by opening of the Mianlue Ocean in the north margin of the upper Yangtze blocks and basement faults produced by mantle plume uplifting,the area had tectonic differentiation in NWW and NE,and sedimentary basement took on episodic settlement from north to south,as a result,the sedimentary systems of Member 1 to Member 3 gradually evolved from carbonate platform to platform-slope-continental shelf.According to the residual thickness,paleokarst geomorphologic units such as karst highland,karst slope and karst depression at different stages were reconstructed.The karst geomorphological units were developed successively on the basis of sedimentary geomorphology.Sedimentary facies and paleokarst geomorphology are of great significance for oil and gas accumulation.The Maokou Formation in northwestern Sichuan has two kinds of most favorable reservoir zone combinations:high energy grain shoal and karst monadnock,platform margin slope and karst slope.Based on this understanding,the planar distribution of the two kinds of reservoir zones were predicted by overlapping the favorable reservoir facies belt with paleokarst geomorphology.The study results provide a new idea and reference for the exploration deployment of the Middle Permian Maokou Formation in the Sichuan Basin.