Study On Lithofacies Palaeogeography Of The Middle And Late Cambrian In The Middle And Upper Yangtze Region

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 distribution and evolution of the lithofacies palaeogeography, clarify the relation between lithofacies 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 lithofacies 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 understanding 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.Associated to the sub-project "Study on accumulation conditions and favorable exploration area of marine carbonate rocks evaluation in South China" of the national science and technology significant project "Large oil and gas fields and coal bed gas exploitation", this paper selectes the Middle and Upper Cambrian as research object. Based on the theory of modern sedimentology, the Petrology, Sedimentary Facies and Paleogeography have been studied. According to the "single factor analysis and multi factor comprehensive mapping method", the lithofacies palaeogeography of the Middle and Upper Cambrian in the Middle and Upper Yangtze Region was studied and mapped. Based on the above, the favorable source, reservoir and the cap rock conditions were analyzed.The study region is part of the Yangtze Region, and the range is almost the same as the Yangtze block. The west boundary of the study region is Red River River fault, Qing River-Xiaojin River fault, Longmenshan fault, the north boundary is Daba mauntain, Xiangfan-Guangji fault, the east boundary is Tuanfeng-Macheng, Xupu- Sibao, Yaziluo, Shizong-Mile fault. The study region includes Yunnan, Guizhou, Sichuan, Chongqing, Shanxi, Hubei, Hunan, Jiangxi and Guangxi Province, and namely, the geographical position of102°-116°E,26°-33°N.According to the regional geological survey, field investigation information and drilling data, the Middle and Upper Cambrian is mainly composed of carbonate rocks, while clastic rocks, metamorphic rocks are also developed, with small quantity of siliceous rocks and volcanic clastic rocks. According to mineral composition, carbonate rocks can be divided into limestone and dolomite. Considering the different structure, construction and the special genesis, limestone can be subdivided into granular limestone, micritic limestone, striped argillaceous limestone and stromatolite limestone. According to the grain size, dolomite can be concluded to clay-crystal powder dolomite and medium to fine crystallized dolomite. On the basis of occurrence and genesis, siliceous rocks include thin-bedded siliceous rocks, massive siliceous rocks and lenticular, crumb, banding siliceous rocks. According to the metamorphic degree, metamorphic rocks can be divided to slate, phyllite and schist. Volcanic clastic rocks mainly consist of tuff. The granular limestone, stromatolite limestone, dolomites, and lenticular, crumb, banding siliceous rocks are mainly developed in carbonate platform; Micritic limestone, striped argillaceous limestone are widespread in restricted platform and slope; Slate, phyllite and schist, thin-bedded siliceous rocks mainly developed in basin; clastic rocks are widely distributed in clastic tidal flat and basin.Based on the study of Petrology, the single factors that can independently reflect some aspects of depositional environmental characteristics have been selected, i.e. thickness, content of deep water sedimentary rocks, content of deep water shale, content of deep water carbonate rocks, content of shallow water carbonate rocks, content of penecontemporaneous dolomite, content of grain carbonate rocks, content of gypsum and halite. And the corresponding single factors fundamental maps have been composed.Then through the superimposition and synthesis of these quantitative single factor maps, combined with regional geological survey and other quantitative and qualitative data, with the dross discarded and the essential selected processes, with the false eliminated and the true retained, quantitative lithofacies palaeogeographic maps of the Middle and Late Cambrian in the Middle and Upper Yangtze Region were composed.The distribution of the lithofacies palaeogeography of the Middle Cambrian in the Middle and Upper Yangtze Region is obvious. From west to east, there are land, clastic tidal flat, carbonate platform, slope, basin, and on the carbonate platform, the dolomite tidal plat, banks, gypsum-bearing lakes were developed. The distribution of the lithofacies palaeogeography of the Upper Cambrian in study region is near to the Middle Cambrian. The difference is that there are no gypsum-bearing lakes on the carbonate platform. In all, the lithofacies palaeogeography pattern of the study region can be summarized as follows:there is land in the west; the main palaeogeography unite is platform; on the platform, banks and tidal flat were developed; there are basins in the east and north of the study region.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 sedimentary environment with favorable hydrocarbon generating condition is mainly basin, the second is slope; the reservoir lithofacies palaeogeography unit is mainly platform, especially bank, then slope; the sedimentary environment with favorable cap rock condition is mainly platform, the second is basin.