Shale Gas Accumulation Characters Of Upper Palaeozoic Group In GuangXi Province

In the paper, several big shale gas basins of American have been summarized, and the shale gas accumulation characters of the upper Palaeozoic group in Guizhong depression, including hydrocarbon source rock condition, reservoir characteristics and preservation condition, have been analyzed.First, the reservoir-forming characteristics of shale gas in America were summarized.Shale gas was regarded as a kind of unconventional oil and gas resources, which has the characteristics of the typical "classifies".Its storage state includes the adsorption state, the free state, the disslove state which is rich in organic shale strata.The adsorbed gas and free gas mainly depends on the size of the various geological conditions.It is accounted that the thick shale deposited on a certain depth and hypoxic environment can provide source and reservoir for shale gas. The abundance, type and thermal evolutionary degree of organic matter are main geologic factors affecting shale gas generation.Shale gas reservoir is mainly correlated with surface area, pore and fracture.Dense shale itself can rely on their own ultra-low porosity and permeability as cap rock to keep gases preserved in the reservoirs, which is called"continuous accumulation"play. Shale gas has multiple formation mechanism and it has been associated with the quantity of gas generating and the development degree of fracture.The accumulation mechanism of shale gas, according to the quantity, degree of fracture development can be divided into three phases.The first stage is roughly the same as the coal bed methane which have rich integration hiding mechanism of coalbed methane.The Second stage with piston type transport mode, with the same accumulation mechanism of source-contacting gas.The third stage is similar to conventional entrapped gas accumulation mechanism.Combined with the deposition, distribution, hydrocarbon generation, reservoir, cap rock and preservation features of the upper Palaeozoic group in Guizhong depression, the reservoir-forming conditions were systematically studied.We conclude that the mud shale formation of the Palaeozoic is mainly in the basin facies sedimentary environment,and the deep water is condutive to the accumulation of the shale gas.The shale’s thickness of the Lower Devonian generally is200-500m,and the maximum depth of the shale is3500m.Mainly located in Wuxuan-Xiangzhou-Luzhai,Nandan,Hechi,and other regions.The TOC value’s distribution of the shale of lower devonian is in0.65-4.70%, and the overall performance is II1model of organic matter, the Ro values between1.5-3.0%,which belongs to a mature stage of thermal evolution. The dark shale of the Middle Devonian mainly distributed in the Shanglin-Laibin-Xiangzhou-Luzhai, Nandan, Hechi areas.The thickness of the shale is450-650m,and LuZhai area’s biggest thickness is more than900m, The shale’s TOC content is more than1.0%, mainly is the I-IIi type organic matter, and Ro values is between0.97-3.45%, which belongs to mature-a mature thermal evolution stage.,the Ro values is gradually decreasing from west to the east in the study area. The shale of the Lower carboniferous are mainly distributed in the Luocheng-LuZhai areas and the thickness of the shale is commonly400-500m.The biggest thickness of shale in Luzhai area is more than600m.The TOC content of the Lower carboniferous mainly is0.38-0.38%. the average content is0.86%,which belongs to II type organic matter.The Ro values in the western region is between2.0-3.0%and the Ro values in the north of Songshui-Luzhai-Yongfu areas is less than1.0%which belongs to mature-postmature evolutionary phase. The Permian formation is seriously damaged, and most of the Permian Formation have been eroding away or exposed on the surface.The shale formation is mainly distributed in Wuxuan-Laibin-Xiangzhou area, and the thickness of the shale is generally200-250m.The mud shale’s organic carbon content is very low.Only in the middle of Guizhong Depression is greater than0.6%, and the highest content have reached over1.0%, all other areas are less than0.6%.The shale formation is type II-III type organic matter, and the Ro value is1.02-3.38%, most area’s Ro value is less than2.0%.The shale formation belongs to mature-postmautre phase.Guizhong depression has shale with the potential to produce gas.There are four shales present, lower Devonian Tangding Group, middle Devonian Luofu Group, lower Cambrian Yanguan Group, upper Permian Longtan and Dalong Group, which have rich organic matter, good organic type and high organic degree, generally in super maturation. The shale formation has experienced multiphase tectonic movements, which caused a great quantity of cracks of different direction, conformities and wrinkles, offering sufficient accumulated spaces for shale gas.The characteristics that the low porosity and permeability, strong compact structure of mudstone, medium-large thickness distribution of the shale and a good breakthrough pressure enable shale to be good cap rock. Multiphase movements have not sharply damaged largely the shale-mud formation. Through the comparison with American shale gas, it was found that their geologic features are similar.The shale formation is Mainly distributed in the south of Ziyun-Luodian-Pingtangarea,Yongfu-Liucheng area and the north of the Liujiang Luzhai,and the shale’s thickness in LuZhai area is about600m. The formation in the Depression has been intensely tectonic uplifted and corroded,occurs mainly during the periode of caledonian tectonic movement and the Yanshanian and Himalayan period. The time span of the uplift is long, the distribution is wide, the thickness is large.But eastern yanshan and Himalayan tectonic movement make the north,southeast of depression suffered relatively strong tectonic setting,and the formations have experienced a fold uplift and denudation, the oil and gas preservation conditions are badly affected.Compared to these regions, the shale in the central and the western region of the depression have weak tectonic reworking and good preservation condition.Finally, through the shale index of thickness distribution,Hydrocarbon source rock characteristics, reservoir characteristics and preservation condition in Guizhong depression provide some basis information for further research.