A Research Of Devonian Sedimentary Facies And Litho-paleogeographic In Guizhong Depression, Guangxi

Guizhong depression is located in the center of Guangxi Zhuang Autonomous Region with geographic coordinates as 107° 30’ -110° 30’E/23° 00’-25° 00’N. It is seated tectonically in the combined part of Yangtze Para-platform and south China fold zone, bounded to the north by Xuefeng Mountain, limited by Luodian faulted zone to the west and by Dayaoshan uplift and Guilin sag to south and west. From Neopaleozoic to Triassic, sedimentation basin is a continental margin extensional basin formed on the Caledonian folding basement and expanded within the continentduringlate Devonian to Carboniferous period, resulting to a passive continent marginal basin alternated by platform and basin in Guizhong depression.During the basic geographic research and the practices of oil and gas exploration, great amounts of surface oil gas seepages and bitumen were discovered in the central Guangxi area and abundant hydrocarbon shows and bitumen layers were discovered by existing drillings. It indicates abundant sedimentary organic matter and multiple generation, migration, assemblage and accumulation of oil and gas in the Guizhong area, and induced oil and gas potential. Therefore, Guizhong depression is one of the most potential in the south of China.Geological sections were systematically researched such as Liujing in Hengxian, Litang-Guigang, Beiliu-Xiangzhou in east, Guilin in northeast and Nandan-Luofu section in south in this dissertation with field measurement or observation on 16 outcrop sections, accumulated length as over 8,000 meters. Based on the previous research results, regional stratigraphic correlation of Devonian was studied and Devonian was divided as nine Formations based on the lithological assemblage characteristics of different phase regions. They are, in turn, Lianhuashan Fm, Nagaoling Fm, Yujiang Fm, Shanglun Fm, Dale Fm, Tangjiawan Fm, Guilin Fm, Dongcun Fm and Etoucun Fm from the old to the new. The synthetic stratigraphic columns of Devonian in Guizhong depression was established too, with a thickness as 2,691 meters.Outcropped Devonian were mainly discovered in the rim areas of the depression, which is a set of continuous strata with the major part as marine carbonate rocks. The lithology mainly includes bio-limestone, bioclastic limestone, micritic-likened limestone, mud-containing or marl, reef limestone, dolomitic limestone, calcitic dolomite and dolomite with local sandstone, pebbly sandstone, aleuvite, batt, argillaceous limestone, marlstone and silicolites and other clasolite. There are some sedimentary structures in carbonatites such as stromatolitic structure, geopetal and pectinate structure, granophyric and laminated structure, druse massive bedding, horizontal bedding.Organisms and organic detritus are developed well in Devonian carbonate rocks in Guizhong depression, including stromatoporoidea, tetracoralla, tabulata, brachiopoda and blue-green algae and so on. Their significanies in the analysis of sedimentology are multiple. Firstly, as particles, they reflect hydrodynamic conditions of the sedimentary environment by their shapes, size, sorting, roundness and other structural characteristics. Secondly, as creatures, they indicate the living conditions by their abundance, diversion, ecology, preserving state and so on. Therefore, it is particular important in environment indicating.The identification marks of sedimentary facies characterized by petrology and biology feature were used to analyze sedimentary system and sedimentary facies. These marks include rock color, thickness and shape of strata, structural constituent and type, sedimentary structure, type of rocks, components and abundance of organism, diversion, and their assembly and preserving state. Devonian in Guizhong depression contains the bottom Lochko-vian Stage, clastic rocks formation of Pragian Stage and carbonate rocks formation of Famennian Stage. And most of carbonate rocks sedimentation is deposited in the system of rift basin of carbonate tidal flat-platform-bioherm inter platform. There are platform edge bioherm and foreslope between the platform and rift basin. Beyond the slope there is deep basin sedimentation, which can be divided into three phase regions as platform, platform edge and continental shelf-aulacogen. Moreover, nine sedimentary facies beltsare recognized as tidal-flat facies, restricted platform facies, open platform facies, biohermalfacies, algal reef facies, platform edge, continental shelf facies, slope facies and aulacogen.