Study On Lithofacies Paleogeography Of Middle Ordovician In Middle And Upper Yangtze Region

The Middle and Upper Yangtze basin is a large-scale oil-gas bearing basin, were the strata form the Upper Sinian to Middle Triassic are well developed and are both ma-rine deposit. It is famous for natural gas, carbonate rocks, fractured and highly complex structural feature. It has the feature of multiple cyclicity, multi series of strata, multiple source rocks, multiple producing formation, and multi-stage hydrocarbon accumulation etc. After several decades’exploration, large and medium gas fields such as Puguagn, Weiyuan, Daganchi, Luojiazhai and many paleo-reservoir, oil seepages has been found.The study of lithofacies palaeogeography is the basis of the petroleum exploration. According to the detailed and systematic studies, it is hopeful to ascertain the distribu-tion and evolution of the lithofacies palaeogeography, clarify the relation between litho-facies palaeogeography and the development of the resource rocks, reservoirs and cap rocks. Thus, the oil and gas exploration can be guided effectively. In recent ten years, the oil and gas exploration of the Middle and Upper Cambrian in the Middle and Upper Yangtze Region has taken great achievements in the tectonics, lithology, sedimentary facies, source rocks, reservoirs, traps and so on. However, most of the previous litho-facies palaeogeography studies of the Cambrian in the study region are still qualitative, not quantitative. Therefore, it is necessary to conduct a comprehensive, systematic, thorough quantitative study of the lithofacies palaeogeography of the Middle and Upper Cambrian in the Middle and Upper Yangtze Region. Only in this way, the understan-ding of lithology and paleogeography of the study region can be strengthened, and the detailed basic geological data can be provided for the oil and gas exploration.The research object of the dissertation is the Early Ordovician in the Middle and Upper Yangtze Region. Based on the measured section and observed section and the theory of sedimentology, combined with various analyses and tests, the characteristics and origins of the rocks has been studied, and their sedimentary environments has been analyzed. According to the "single factor analysis and multifactor comprehensive mapp-ing method", the lithofacies paleogeography of the Early Ordovician in the Middle and Upper Yangtze Region was studied and mapped. The petroleum geological conditions of the Early Ordovician in the study area have been discussed from the viewpoint of lithofacies palaeogeography. The maior results of the dissertation are as follows.1. According to the study of the measured section and observed sections, combined with the data, the Middle Ordovician in the study area has been divided and correlated. The result indicates that the coctinuity and correlation of the Dawan Stage and Guniutan Stage in the study area are rather good. Dawan Stage can be divided into the Xiaqiaojia Formation, the Dachengshi Formation, the Xiliangshi Formation, the Meitan Formation, the Dawan Formation.the upper part of the Tonggao Formation, the middle part of the Qiaotingzi Formation and, middle part of Ningguo Formation the upper part of the Zhaojiaba Formation, the upper part of the Hongshiya Formation. Guniutan Stage can be divided into the Guniutan Formation, the lower part of Fengdonggang Formation, the lower part of Tanjiagou Formation, the lower part of the Shizipu Formation, the lower part of the Sherenwan Formation, the upper part of the Ningguo Formation and the upper part of the Qiaotingzi Formation2.The dominant rock type of the Middle Ordovician in the study area is carbonate rocks, subsequently the terrigenous clastic rocks, silica rocks and the metamorphic rocks. Limestones is well developed in the carbonate rocks and dolomites is little deve-loped but the dominant rock type is limestones. According to the difference between composition, texture and structure, the limestones can be divided into biograin lime-stones, biograinylimenud limestones, biograin-bearing limemud limestones, limemud limestones, crystal grain limestones and nodular limestones and so on. The dolomites can be divided into putty crystal dolostones, powder crystal dolomites. The clastic rocks can be classified as:sandstones, siltstones, mudstones and shale rocks. The metamor-phic rocks mainly include slates and phyllites.3.The sedimentary environment of Dawan Age and Guniutan Age can be divided into ancient land, elastic coastal, drowned carbonate platform, slope and deep-water basin. The drowned carbonate platform is mainly composed of carbonate shelf, and biology set is developed in the platform.4. This article selects six factors:thickness, shallow carbonate rocks content, deep water sedimentary rock content, penecontemporary doiostone rocks content, bioclast limestone content and relatively deep-water shale.This single factor maps of Dawan Age and Guniutan Age were complied. Based on all this single factor maps, combined with the regional geological data in the study area, and based on a comprehensive analysis and judgment, the quantitative lithofacies paleogeography of Dawan Age and Guniutan Age has been composed. Ancient land, elastic coastal faces, argillaceous shal-low sea shelf, drowned carbonate platform, slope and deep water basin are distributed Successively from the northwest to the southeast both in Dawan Age. Ancient land, dolomitic flat, drowned carbonate platform, slope and deep water basin are distributed Successively from the northwest to the southeast both in Guniutann Age. Through the contrast, it can be found:deep water facies of Guniutan Age inherited the characteri-stics of Dawan Age, There is no much change, but water deepened, slope gradualy becomes more gentle, gravity flow deposits is not developed; Shallow water facies change a lot:as the terrigenous material supply reduce, water is more clear, clastic shore area decreased significantly, dolomitic flat is debeloped in the ancient continental margin; And the content of relatively deep water shale content is rarely, argillaceous shallow sea shelf is not developed, the drowned carbonate platform becomes the main part in Guniutan Age; In addition, the content of bioclastic increases and there are two biological set, named as Chengkou biological set and Nanchuan biological set.5. From the viewpoint of lithofacies palaeogeography, combined with related analyses and tests, the source, reservoir and the cap rock conditions of the Middle and Upper Cambrian in study region have been analyzed. The results indicated that the hydrocarbon generation ability of Ordovician in the area is very limited,the potentional environment of hydrocarbon generation is deep water basin and solpe;the reservation ability of Ordovician is poor,and the reservoir lithofacies palaeogeography unit is clastic shore and dolomitic flat; the sedimentary environment with favorable cap rock condition is mainly platform, the second include argillaceous shallow sea shelf, drowned carbonate platform, slope and deep water basin.