The Research Of Sedimentary Character In Late Paleozoic In Hunan And Guangxi

Based on the summary of previous research, sedimentology as the basic theory, outcrop surveys and laboratory analysis as the foundation data, using sedimentary facies analogy mapping method, pattern and evolution of Late Paleozoic lithofacies paleogeography in Hunan and Guangxi was reconstructed accordingly. In the end, this paper gives a brief evaluation of the oil and gas geological conditions (source rock-reservoir-cap associations) in Hunan-Guangxi region.3 facies groups and 15 kinds of sedimentary facies are recognized in Late Paleozoic strata in Hunan-Guangxi region, including alluvial fan facies and fluvial facies in continental facies group; delta facies in transitional facies group; shore facies, tidal lagoon facies, restricted platform facies, open platform facies, inner-platform shoal facies, biological reef facies, platform margin slope facies, continental shelf facies, platform-shelf facies, platform-basin facies, deep basin facies and event deposits in marine facies group.Evolution of Late Paleozoic paleogeography in Hunan-Guangxi region can be divided into three stages, including extensional rifting in Devonian, extension in Carboniferous and development in Permian. Specifically, in Devonian the extensional rifting which resulted to the declination of paleogeography form north to south, changed the palaeogeographic framework to trough-platform pattern, among which the isolated platform is relatively larger and the Qinzhou-Fangcheng trough received successive deposition since the Caledonian. In Carboniferous, shallow-water carbonate platform extended and shoal facies developed, while the distribution of deep-water basin between platforms was relatively limited. The differentiation of trough and platform was not obvious. In Early Permian, transgression occurred, and the whole region was submerged into the sea. And the scope of isolated platform became smaller accordingly. And the trough and platform differentiated significantly. In Late Permian, Qinzhou-Fangcheng trough became uplift according to the compressional uplifting of Dongwu movement. And the deep-water basin contracted accordingly. The trough and platform differentiated significantly. In Early Triassic, carbonate platform transferred to carbonate ramp which is characterized by the development of a large area of oolitic shoal. From Early Triassic to Middle Triassic, all the depressions in Yunnan-Guizhou-Guangxi region merged into a unified depression as Nanpanjiang depression, which is featured by thick turbidite deposition. And at this very time, all the isolated platforms were submerged.Oil and gas geological conditions of Hunan-Guangxi region is featured by the well developed, widely distributed and later severely damage source rock-reservoir-cap associations. Specifically, there are3 sets of source rocks(D,C1 andP) among which, the Devonian and Lower Carboniferous source rocks are better, and are widely distributed, with large thickness and high abundance of organic matter. There are also 4 sets of reservoirs(D2,D3,C2 and P1) and cap rocks(D2-3,C1 andP). The Lower Devonian clastic reservoir and the Upper Carboniferous carbonate reservoir are of better physical properties. And the Devonian shales are widely distributed and have better sealing ability.