Analysis Of Sedimentary Facies And Hydrocarbon Generation Potential Of The Middle Triassic Leikoupo Formation In Northwestern Sichuan

Many gas reservoirs and gas-bearing structures have been discovered in Leikoupo Formation of Sichuan Basin after more than 50 years of exploration.However,the reservoir distribution and reservoir-forming regularity are complex,and the gas source is still unclear,which requires further study.In this paper,taking Leikoupo Formation of Middle Triassic in Northwestern Sichuan Basin as the research object,combining with previous achievements,the petrological,sedimentary and geochemical characteristics of outcrop and core samples of Leikoupo Formation are studied,and the sedimentary facies and hydrocarbon generation potential of Leikoupo Formation are analyzed.According to field outcrop profile measurement,core observation and indoor comprehensive analysis,three sedimentary facies are developed in Leikoupo Formation of Middle Triassic in Northwestern Sichuan Basin:limited platform facies,evaporative platform facies and open platform facies,of which limited platform facies and evaporative platform facies are dominant.The transgression occurred in the first section of Leikoupo Formation,and the sedimentary facies was limited platform facies;the regression occurred in the second section of Leikoupo Formation,mainly limited platform facies,most of which were lagoon microfacies,and the range of platform shoal microfacies was widened;the transgression occurred again in the third section of Leikoupo Formation,mainly limited platform facies lagoon microfacies,and the combination of platform shoal microfacies became larger,resulting in the emergence of new platform microfacies.The fourth member of Leikoupo Formation has retreated,mainly in the limited platform facies sedimentary environment.The central part of West Sichuan Depression has become evaporative platform facies near Longshen 1 Well.The Northwest Township of Guangyuan and the section of Zhuyuanba in the north are affected by the early stage of Indosinian movement,and are severely denuded and the fourth member of Leiyuan Formation is missing.The hydrocarbon generation potential of Leikoupo Formation is analyzed by studying the abundance,maturity,types and paleoproductivity of organic matter in the outcrop and core samples of Leikoupo Formation in the study area.The results show that the samples in this area are low in organic matter abundance,TOC is 0.05%-0.2%.The asphalt components of the samples are mainly massive,vein-like and debris-like asphalt,which is gray-white under oil-immersed reflection light and does not fluoresce.The asphalt has a high maturity,with an average reflectivity of 2.56%and an equivalent vitrinite reflectivity of 2.02%,and is in the stage of over-maturity.Microelement analysis shows that the paleoproductivity of carbonate rocks in Leikoupo Formation is relatively high.The content of Ba in most samples is 5-55 ppm,and the content of biogenic Ba is-14.24-38 ppm,with an average value of 3.69ppm.Compared with the first member of Leikoupo Formation and the third member of Leikoupo Formation,the fourth member of Leikoupo Formation has a higher level of paleoproductivity,which provides a material basis for the development of hydrocarbon source rocks.Although the organic carbon content is low,considering its high conversion rate,it is considered that it has a certain hydrocarbon generation potential.The fourth member of Leikoupo Formation has high Ba content and high paleoproductivity,which can provide material conditions for the development of source rocks.The controlling factors of hydrocarbon generation potential of source rocks in the study area are analyzed in terms of sedimentary environment,characteristics of organic matter and biological productivity.The Leikoupo Formation mainly develops limited platform sediments,which are characterized by weak hydrodynamic conditions,drought and heat,high salinity and anoxic reduction.It is conducive to the preservation and transformation of organic matter in source rocks.Sedimentary environment has obvious control effect on hydrocarbon generation potential of carbonate rocks in the study area.Microbial rocks developed in tidal flat environment have high organic carbon content and relatively large hydrocarbon generation potential.In addition,some intergranular pore of silty dolomite is well developed,and the pore is filled with "migrating organic matter".The organic matter of Leikoupo Formation is all type?1-?2 with high maturity.The influence of organic matter abundance on hydrocarbon generation potential is most obvious.In addition,element analysis shows that the paleoproductivity of Leikoupo Formation is relatively high.It is concluded that sedimentary environment,organic matter characteristics and paleoproductivity play a controlling role in hydrocarbon generation potential,especially in sedimentary environment.