The Evolution Of Deep-water Sedimentary Environment At The Intersection Of P/T In Guangwang Area, Sichuan

Guangwang region is located in the northwest of Sichuan Basin.the marine strata at the intersection of Permian and Triassic have full deep-water sedimentary records.Through the study of the evolution of deep-water sedimentary environment at P/T junction,it has important guiding significance for oil and gas exploration in deep-water sedimentary in this area.The deep-water sediments at the P/T junction in the study area are distributed in Dalong Formation and Feixianguan Formation.Five conodont spine belts were identified through sampling and analysis of Wangcang Yanerya section:C.changxingensis,C.yini-C.zhangi,C.meishanensis,H.parvus and H.lobata.Based on the biostratigraphic division of conodont zones and geochemical characteristics,the redox conditions of deep-water deposits in Yanerya section were determined as follows:Dalong Formation of this section.The overall sedimentary environment of the C.changxingensis zone of the Dalong Formation is characterized by oxygen deficiency.The C.yini-C.zhangi zone at the bottom of the Feixianguan Formation reflects a certain increase in oxygen content.Although the sea level of Feixianguan Formation decreased in the early stage,the whole C.meishanensis showed an outbreak of oxygen-deficient reduction environment when the extinction event approached,which now extends to the whole H.parvus with a sharp decrease in biological content.In H.lobate zone,there is a small fluctuation in the increase of biomass,but it is far from the level before the extinction.The sedimentary facies types of six single fieldplace are determined by facies markers.Based on the relative comparison of sedimentary facies in the study area,the evolution of deepwater sedimentary environment is analyzed.Through the establishment of facies model,it is concluded that the Dalong Formation was mainly composed of deep-water platform basins in the early stage of sedimentation,transited to shallow-water platform basins in the middle and late stage,and evolved into deep-water shelf and transformed into shallow-water shelf in the early stage of Feixianguan Formation.The accumulation pattern inherits the phenomenon of"steep East and slow west" in the late Changxing period.On the basis of summarizing predecessors'understanding of Guangwang deep-water sedimentary area,this paper argues that the statements of "trough","platform shed" or "bay" in Guangwang,Kaijiang-Liangping are inappropriate.The distribution and evolution of sedimentary environment at the P/T junction in the comprehensive area should be "platform trough(platform basin)" deposited in Guangwang area in late Changxing,and can be divided into deep-water platform basin and shallow-water platform basin.Early Feixianguan Formation transited to shelf deposits.Based on the study of the deep-water sedimentary area at the P/T junction of Guangwang Formation,this paper considers that the platform-basin facies of Dalong Formation is favorable for the development of hydrocarbon source rocks,and the shelf facies of early Feixianguan Formation is the secondary favorable exploration area.