Sedimentary Facies Of The Lower Silurian Longmaxi Formation In The Sichuan Basin And Its Relationship With Shale Gas

Based on field outcrops and drilling data,this paper is studied on the features of stratigraphic and lithofacies to figure out the sedimentary facies of lower Silurian Longmaxi Formation in Sichuan Basin.Research indicates that Longmaxi can be divided into two sections.Used of "organic matter content + mineral composition Three Unit" to name the lithofacies.Based on the organic carbon content(TOC value)which range divided into three categories:?Rich organic matter(TOC ? 2%);?Containing organic matter(1%? TOC<2%);?Poor organic matter(TOC<1%),and then based on a three-terminal element mineral component(siliceous,clay,carbonate)delineated it into 21 kinds of lithology,nine facies zone.The study shows when Longmaxi Formation deposited was the Shelf in Sichuan Basin.During this period,sea-level was in the level 3 decline stage,with two secondary sea-level change.Each sub sea-level change was began with rapid transgression to the end of the slow regression,forming two corresponding sedimentary cycle.According to the characteristics of the lithology,structure,paleontology,the depositional cycle could be divided into Inner Continental Shelf(ICS)and Outer Continental Shelf(OCS).The ICS could be further divided into five sedimentary microfacies,respectively was Calcareous Siltite Shelf(CSS),Calcareous-mud Siltite Shelf(CMSS),Mud Siltite Shelf(MSS),Argillaceous Limestone Shelf(ALS)and Storm Flow.The OCS consisted of Silty Mudstone Shelf(SMS),Mudstone Shelf(MS)and Contour Current.The sedimentary microfacies distribution was controlled by Leshan-Longnvsi and Qianjiang-Xuefeng ancient uplift.From bottom to top,the particle size of sediments coarsened,argillaceous decreased,silty and calcium content increased.For the direction from the ancient uplift to the ocean,the characteristic was the opposite.Organic geochemical analysis shows that the value distribution range of TOC was 0.06%?9.64%,the average was 1?38%.Among them,the TOC of MS was 0.08%?9.64%,the average was 2.07%,which was far higher than other sedimentary microfacies.Due to the content of TOC were mainly affected by sedimentary environment,the research's results show that the Longmaxi was thrived on planktonic animals and plants.At the same time,long-term tranquil and reduction environment made the abundant of organic matter deposited and saved.The above conditions made the MS become the most favourable exploration area of shale gas in Sichuan Basin.