Lithofacies And Palaeogeography Of The Early Cambrian Longwangmiao Period In The Sichuan Basin

This paper studies the lithofacies palaeogeography of lower Cambrian Longwangmiao Formation on the basis of predecessors’ research,exercising theories of rock stratigraphy, biostratigraphy, carbonate petrology, carbonate sedimentology and carbonate reservoir geology, analyzing data comprehensively from171drilling cores and outcrops in the Sichuan Basin and neighboring regions. Here we perform a comprehensive analysis on data from field outcrops, Loggings, Cores and thin sections on the base of regional geological settings and the thickness of the Longwangmiao Formation. The analysis shows that the basin was surrounded by old lands from north and west with Motianling old land and Hannan old land at north, Kangtien old land at west and underwater palaeouplift from southeast and northeast named Weixin-qianjiang, Shiqian-xiushan-Longshan and Zhenba-wuxi-badong underwater palaeouplift. And two depressions, Langzhong-tongjiang depression and Jiangjin-fengjie depression, were divided by one uplift, medieval Sichuan uplift, inside the basin, distributing approximately at northeast direction, with the existence of Luzhou-junlian-zhaotong inheritance downwarping. Also the basin and its peripheral connected to QinLing Ocean at northeast and Jiangnan Basin at southeast. Single well sedimentary facies classification and even well deposition comparison were made according to rock types, sedimentary structure, sedimentary cycle, the types of paleontology, logging response characteristics, single well sedimentary facies classification, even well deposition, which shows that carbonate platform-slope-basin sedimentary system, with the main body of carbonate platform and identifiable sedimentary facies like mixed deposited tidal flat, restricted-evaporate platform, half restricted to restricted platform and platform margin, were developed in Longwangmiao Age as the consequence of paleogeographic background. Combining this basis and quantitative analysis of single factor like the content of clastic rock and its terrigenous direction.the content of limestone, dolomite and paste gypsum, waters limitations, relative location of platform edge and content of grainstone etc., Lithofacies paleogeographic distribution pattern was recovered and sedimentation model was established. The result shows that the sediment of dolomite and gypsum-salt rock with restricted-evaporate sedimentary environment took the main part of sedimentation inside the basin while gradually transited to the limestone deposition sandwiched with dolomite deposits at peripheral, and water has the tendency of wide and normal salinity. The sea side of Mianyang-zhenba tidal flat, Leshan-kaijiang medieval Sichuan uplift, Weixin-qianjiang, Shiqian-xiushan-Longshan and Zhenba-wuxi-badong underwater palaeouplift and the periphery of Langzhong-junlian-zhaotong depression are grainstone shoal favorable development zone inside of the platform, while Shiqian-sinan-longshan-sangzhi at southeastern edge and Zhenba-xingshan at northeastern edge are the potential area of grainstone shoal. Upper middle Yangtze region is a typical platform-epicontinental sea sedimentary facies model, with development of the carbonate platform-slope-basin sedimentary system, central Sichuan area is restricted-evaporation platform facies, transiting to half restricted-restricted lagoon with grainstone shoal down east and south and high-energy platform-margin belt at southeast and northeast, where further developed into bathyal slope and deep sea basin facies. Particle beach facies reservoir performance is superior. Keeping the palaeogeographical pattern in large area, grainstones in highlands demonstrate a widely covering style of "five lines and one area ". As the consequence of the palaeouplift caused relieved ancient sea bottom shape, change on secondary sea-level affect the lateral continuity.