Sequence lithofacies paleogeography and favorable exploration zones of the Permian in Sichuan Basin and adjacent areas,China

Based on the sedimentological analysis of outcrops and drilling cores with seismic facies analysis,this study divided the Permian in Sichuan Basin and adjacent areas into five 3rd-order sequences,and prepared four maps of the Permian lithofacies paleogeography of Sichuan Basin and adjacent areas in the 3rd-order sequence stratigraphic framework,i.e.,PSQ1(Guadalupian or Middle Permian Qixia Formation),PSQ2-PSQ3(Guadalupian Maokou Formation),PSQ4(Longtan/Wujiaping Formation,Wuchiapingian Stage of Lopingian or Upper Permian),and PSQ5(Changxing/Dalong Formation,Changhsingian Stage of Lopingian).These showed that the evolution of Permian lithofacies paleogeography in Sichuan Basin and adjacent areas was mainly controlled by the background of regional rifting tectonics and the activities of syndepositional normal faults with an overall increasing trend in the rifting activity.Carbonate ramps mainly occurred in the Guadalupian and evolved into rimmed platform in the Lopingian.The Emei mantle plume activity and basalt eruptions in the end of Guadalupian led to the formation of Kangdian oldland,which became the provenance of terrigenous clastic sediments in the southwestern Sichuan.Through the compilation of lithofacies paleogeography maps,a series of intra-platform reefs and shoals as well as intra-platform depressions was discovered.It further clarified the distribution of middle ramp facies and platform margin facies in the northern Sichuan Basin,and provided guidance for exploration of natural gas in the Permian in Sichuan Basin and adjacent areas.Based on the distribution of main hydrocarbon source rocks and the accumulation conditions,it was proposed that the favorable natural gas exploration zones of Permian carbonate reservoirs in Sichuan Basin included:(1) the dolomitized platform-margin reefs and shoals of the Lopingian around Kaijiang-Liangping basin facies;(2) the dolomitized platform margin reefs and shoals of the Lopingian along the southwest margin of Chengkou-E'xi basin facies;(3) the dolomitized middle-ramp reefs and shoals of the Guadalupian in the northern Sichuan Basin;(4) the weathering crust karst reservoirs of Maokou Formation;(5) the dolomitized intra-platform reefs and shoals of the Lopingian in Changxing Formation;and(6) the fractured hydrothermal dolomite reservoirs of the Guadalupian.