Sequence lithofacies paleogeography,favorable reservoir distribution and exploration zones of the Cambrian and Ordovician in Sichuan Basin,China

Abstract

In this study,a detailed analysis of the field outcrop profiles,drilling cores and well logging data showed that the Sichuan Basin in Cambrian and Ordovician mainly developed restricted platform facies,open platform facies,and platform marginal facies.Combined with seismic data,the Cambrian and Ordovician were respectively divided into five and six 3rd order sequences in accord with the theory and method of sequence stratigraphy.With the 3rd-order sequence as the unit,the map of sequence lithofacies paleogeography was prepared for Sichuan Basin and adjacent areas in Cambrian and Ordovician using the single factor analysis and multifactor comprehensive mapping method.In the Cambrian and Ordovician,terrigenous clastic rocks mainly deposited in the west,and carbonate rocks dominated the east.Regarding the evolution of carbonate platform,clastics-hybrid shelf in the early stage of Early Cambrian(Meishucun Age and Qiongzhusi Age) and carbonate ramp in the middle stage of Early Cambrian(Canglangpu Age) evolved to rimmed platform from the late Early Cambrian(Longwangmiao Age) to the Middle Ordovician.In the Late Ordovician,the combined effects of global sea level rise and tectonic subsidence of Yangtze plate southeast passive continental margin subducted to Cathaysia led to rapid rise of sea level and increase in deposition water depth.As a result,the upper Yangtze(Sichuan) platform was submerged,further developing into a deep-water ramp-basin facies.In the Early Cambrian,terrigenous clastic rocks and carbonate hybrids mainly deposited in the early stage,whereas carbonate rocks mainly deposited in clear water areas in the late stage.A large area of favorable reservoirs(i.e.,intra-platform shoals and intra-platform dolomites) of restricted platform facies was developed in the basin.As affected by the Ordovician transgression,the basin mainly developed open platform facies with intra-platform shoal facies in banded distribution.In the Late Ordovician,the platform was submerged and deposited with micritic limestone,argillaceous limestone,mudstone of Baota Formation,Linxiang Formation,and Wufeng Formation,which became the regional cap rocks throughout the entire basin.Argillaceous limestone and mudstone of Wufeng Formation are the important high quality source rock in Sichuan Basin.The favorable reservoirs of natural gas consist of dolarenite,oolitic dolomite,and silty-fine crystal dolomite,with well-developed dissolution pores(average porosity 2%~6%),large monolayer thickness,and good spatial continuity.Based on the evaluation of favorable reservoirs,four favorable exploration zones of the Cambrian and Ordovician were selected through comprehensive analysis of the hydrocarbon accumulation conditions,and the favorable exploration zones of Leshan-Longnusi Palaeouplift and Luzhou Palaeouplift have developed large-scale structural traps and widely distributed favorable reservoirs,thus are of great exploration potential.