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

The study on lithofacies paleogeography is the basis of oil and gas exploration. It’s helpful in clarifying the evolution and distribution of lithofacies paleogeography and the relationship between source rocks, reservoir rocks and cap rocks. It has very important significance to the forecast evaluation and exploration development of resources pers-pective.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. It is proved that the Middle and Upper Yangtze basin has a better exploration prospects.As one of the important marine strata in the Middle and Upper Yangtze basin, the Lower Ordovician has good reservoir quality. Up to now, many oil gas show of the Lo-wer Ordovician has been found in Sichuan basin, Hunan, Guizhou and other districts. For example, there is natural gas producing in well Nvji, well Weihan17, well Weiji and well Moshen1. Past tens of years, some pregresses has been made in structure, petrolo-gy, sedimentary facies, source rock, reservoir and in many other ways, but the process of exploration was zigzag and has not achieves major breakthrough. Researching the cause, besides the complicated geological conditions, one of the most important points is that there is lack of comprehensive, historical, holistic and profound recognition to the petroleum geological conditions. Many years’ prospecting showd that the types of lithofacies in different geological ages control the distributing and compounding of effective source rock, reservoir and cap rock. The diversity of sedimentary evolution controls the distribution of oil and gas and the types of oil and gas pools. So, it is very hopeful to ascertain the distribution and evolution of the lithofacies paleogeography, clarify the ralation between lithofacies palaeogeography and the development of the source rocks, reserviors and cap rocks.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 Early Ordovician in the study area has been divided and correlated. The result indicates that the coctinuity and correlation of the Xinchang Stage and Hong-huayuan Stage in the study area are rather good. Xinchang Stage can be divided into the Tongzi Formation, the Nanjinguan and Fenxiang Formation, the Guotang Formation, the Panjiazui and Daomayu Formation, the Liuzuiqiao Formation, the Baishuixi Forma-tion, theTangchi Formation and the lower-middle part of Yinzhubu Formation. Hong-huayuan Stage can be divided into the Honghuayuan Formation, the Luohanpo Forma-tion, the Taohuashi Formation, the upper part of the Yinzhubu Formation, the lower part of the Zhaojiaba Formation, the lower part of the Tonggao Formation, the lower part of the Ningguo Formation, the lower part of the Hongshiya Formation and the lower part of the Qiaotingzi Formation.2. The dominant rock type of the Early Ordovician in the study area is carbonate rocks, subsequently the terrigenous clastic rocks, silica rocks and the metamorphic rocks. Limestones and dolomites are both well developed in the carbonate rocks, but the dominant rock type is limestones. According to the difference between composition, texture and structure, the limestones can be divided into grained limestones, particulate matter limestones, containing particles of limestones, plaster limestones, reef limestones, stromatolite limestones, nodular limestones and so on. The dolomites can be divided into putty crystal dolostones, powder crystal dolomites, fine crystal dolomites, medium crystal dolomites, coarse crystal dolomites and grained dolomites. The clastic rocks can be classified as:conglomerates, sandstones, siltstones, mudstones and shale rocks. The metamorphic rocks mainly include slates, phyllites, schists and marbles.3. According to the study of the measured section, observed sections and other ass-istant sections, the sedimentary environment of Xinchang Age can be divided into an-cient land, elastic coastal, carbonate platform, slope and basin. The carbonate platform can further be divided into shoals and dolomitic flat. While the sedimentary environ-ment of Honghuayuan Age can be divided into ancient land, elastic coastal, carbonate platform, slope and deep water basin. The carbonate platform can further be divided into shoals and reefs.4. Take the "single factor analysis and multifactor comprehensive mapping metho-d" as the methodology, based on the field outcrop and drilling profiles,11single factor maps were complied, such as thickness (m), shallow carbonate rocks content (%), deep water shale rocks content (%), particle carbonate rocks content (%), penecontemporary doiostone rocks content (%) and the distribution of reefs. 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 Xinchang Age and Honghuayuan Age has been composed. From the quantitative litho-facies palaeogeography maps can be seen that the Kangdian land, elastic coastal faces, shallow water carbonate platform, slope and deep water basin are distribu-ted successively from the northwest to the southeast both in Xinchang Age and Honghu-ayuan Age. On the whole, it is a process of Transgression from Xinchang Age to Honghuayuan Age, seawater roughly invaded from the southeast to the northwest, and paleogeographic units migrated from southeast to the northwest gradually. Meanwhile, the distribution range of Kangdian land decreased gradually.5. The carbonate rocks of the Lower Ordovician is the important reservoir of Ordovician in the study area. The main reservoir spaces are inter-crystalline hole, inter-crystalline solution pores, intergranular pores, dissolved pores in grains, solution cave and fracture and the main reservoir rock types are fracture carbonate rocks and fracture solutional carbonate rocks. The sandstones in the elastic coastal form the northwest of stdy area are aslo the potential favorable reservoir. The Early Ordovician in the Middle and Upper Yangtze Region can produce some amount of hydrocarbon, but the ability of the oil-generation was limited. The mianly (potential) source rocks are black shale rocks in the Jiangnan shale basin and the deep color carbonate rocks in the carbonate platform. The direct cap rocks in the study area are the mudstones of the Silurian. The cap rocks possess very stronge sealing ability and provide great guarantee for the oil and gas reserving.6. Synthesizing the hydrocarbon generating conditions, reservoir conditions and preservation conditions, the favorable facies belt and region of oil and gas exploration were put forward. Namely the carbonate platform and shoal facies in the central and south sichuan, the carbonate platform, shoal and reef facies in the southern guizhou, the shoal and reef facies in the west, southwest hubei and northwestern hunan. Among all of the favorable facies belts, shoal and reef facies are the most favorable exploration tar-gets.