Tectonic Paleogeographic Evolution And Petroleum Geological Significance During Late Sinian-Early Cambrian In Central Sichuan Basin

Since the discovery of Sinian gas field for the first time since 1964 in Weiyuan area,Sichuan basin,Sinian and lower Paleozoic oil and gas exploration in the basin experienced a hard process.Since 2011,Sinian and Cambrian giant gas field is found in paleo uplift,Central Sichuan Basin,whose main gas bearing formations include 2th and 4th Member of Dengying Formation and Cambrian Longwangmiao Formation and geological reserves exceeds more than a trillion cubic meters,which revealed that the deep marine strata has good exploration potential.The dolomite reservoir in SinianCambrian is huge,but it has strong heterogeneity because of multi-stage tectonic movements,which is not conducive to the favorable reservoir prediction and seriously restricts the natural gas exploration,so it has important guiding significance on favorable zone prediction to study tectonic paleogeographic in Late Sinian and Early Cambrian and clear paleogeographic pattern and its effect on oil and gas accumulation.Based on sedimentology,petroleum geology,reservoir geology theory and previous studies,combined with core,logging,seismic and comprehensive analysis of indoor test data,this paper systemically studied sedimentary system and Tectono paleogeography of Upper Sinian-Lower Cambrian in Central Sichuan Basin and the following conclusions are made:(1)Through the core observation and thin section analysis,combined with the color of rocks,sedimentary structure,and its mineral composition and structure characteristics,we found that tithofacies types of Late Sinian and Early Cambrian mainly conclude carbonate rocks which can be further divided into algal dolomite,grain dolomite,micritic dolomite,muddy micritic dolomite,crystalline dolomite and karst breccia dolomite and clastic rocks which can be further divided into shale and mudstone,silty mudstone,argillaceous siltstone,siltstone and fine sandstone.(2)According to core observation,thin section identification and combining petrology characteristics,lithofacies assemblageand logging,we studied sedimentary facies during Late Sinian and Early Cambrian in Central Sichuan Basin and found that carbonate platform and clastic depositional system were developed in the study area.Carbonate platform facies can be further divided into restricted platform,platform margin,slope,shelf and basin facies and clastic facies can be further divided into clastic littoral facies and clastic shelf facies.(3)According to the analysis of sedimentary facies of single well and connectedwell,using quantitative analysis and combined with regional geological background and tectonic evolution,tectonic paleogeographic evolution regular of study area is obtained.During sedimentary period of 1th and 2th Member of Dengying Formation,the rift developed slope-basin facies which to both sides transited platform margin and restricted platform facies and the sedimentary subfacies which concluded algal mound and shoal of platform margin and intra-platform were widely developed in Central Sichuan Basin.During sedimentary period of 3th Member of Dengying Formation,subjected to the rapid transgression,the rift developed deep water shelf facies which to Central Sichuan Basin and Weiyuan-Ziyang area transited to shallow water shelf deposition.The paleogeographic pattern during 4th Member period was similar to that of 2th Member period,but affected by the expansion of the rift,the slope facies boundary moved westward.During Early Cambrian Maidiping-Qiongzhusi Period,clastic shelf facies was widely developed,to Canglangpu Period the sedimentary facies evoluted to clastic shore facies.During Longwangmiao Period,controlled by paleo tectonic pattern of “two uplifts and one depression”,paleogeographic pattern inside Sichuan Basin was characterized by “two banks and one lagoon” and Moxi-Gaoshiti area developed intraplatform shoal.(4)Formation of large oil and gas fields in Central Sichuan Basin was controlled by tectonic paleogeography during Late Sinian and Early Cambrian.The rift controlled hydrocarbon generation center,algal mound and shoal of platform margin reservoir of Dengying Formation on both sides of the rift and grain shoal reservoir of Longwangmiao Formation in Central Sichuan Basin are karst reservoirs of high quality.Besides,hydrocarbon accumulation and preservation conditions in study area is superior.