Shale Gas Reservoir Characteristics Of Taiyuan Formation In Southern Qinshui Basin

Southern Qinshui Basin is one key region of CBM development in China,where sedimentparalic faciescoal mud shale. Taiyuan Formatiom is the major shale gas producing reservoir section, which well-developed set of dark shale and has good geological conditions. This paper selects the Taiyuan Formationshale as the research object in Southern Qinshui Basin. A data collection, geological investigation, experimental test and theory researchis used in the study to analy the core scientific issues of Taiyuan Formation shale gasreservoir characteristics. The results showed that:Southern Qinshui Basin is the main part of Qinshui synclinorium, the internal structure is simple and fracture is rare.The Carboniferous Permian Taiyuan formation is the main coal bearing strata whichdevelops dark mud shale,has experienced three tectonic movements transformation of Indosinian period, Yanshan period and Himalaya period,the total thickness of Taiyuan Formation is between 81.61~125.34 m, and the average is 103.96 m.Taiyuan Formation develops barrier island lagoon-tidal flat depositional system composed of barrier island, lagoon, tidal flat, swamp, carbonate platform sedimentary faciesand can be divided into 6 cycles from bottom to bottom.The total thickness of dark mudstone and shale is between 23.71~68.74 m and the average is 47.85 m,develops 5 sets shale system with large thickness and stable distribution,and the thickness of Shangou shale is the largest and the most stable.The depth of the target layer in the study area is gradually increasing from the edge of the basin to the central of the basin, and the buried depth of the main body is located in 484.94~1212.49 m, which belongs to shallow bury.Qinshui Basin shale belongs to type III kerogen,the content of TOC is distributed in 1.31%~3.25%, and the mean value of TOC was more than 2% in most areas.The TOC content of Shangou and Xishan shale in the vertical direction is relatively stable, the change range is small, and the distribution isbetween 1~4%.According to lithofacies combination, shale can be divided into 4 types of lithofacies, such as black common mudstone, silty mudstone, carbonaceous shale and iron mudstone.The mineral composition of shale is mainly composed of clay minerals and quartz, the content of brittle minerals is low, and the content of clay minerals is high.The shale organic matter maturity of Taiyuan Formation is in the range of 1.72%~2.89%, which shows that the organic matter in the geological history has produced a large number of hydrocarbons, which lays the foundation for the formation of shale gas.There are organic matter pore, intergranular pore(brittle minerals, clay minerals), intergranular dissolution pore, pyrite intergranular pore and intergranular pore.The micro cracks typesin the shale are the external and inter granular fractures formed by the external force, and the majority of the material is filled with quartz, organic matter and clay.These micro pores and micro cracks enrich the space of shale gas reservoirs. The shale porosity is 0.66%~4.30%, with an average of 1.50%, and most of the porosity of the sample is less than 2%.From the point of view, the porosity distribution of the Shangou mud shale is relatively stable.Shale pore is mainly distributed between 0.6~1.5nm and 3~4nm, the pore structures are mainly mesoporesaccounting 58.43~72.71%of the total pore volume, the average is 65.20%, the surface area is mainly provided by the 0.6~1.5nm pore.The content of organic carbon and microporous volume content show a good positive correlation, the maturity of the organic matter content is related to microporous and mesoporous content, the content of clay mineral and mesoporous volume content show a certain positive correlation, brittle mineral content is contributive to the macro pore volume.The maximum adsorption capacity of shale is between 0.42 ~1.85m3/t, with an average of 0.73m3/t, and the content of free gas is between 0.13~0.80m3/t, with an average of 0.43m3/t.In different layers of dark shale, the Shangou shale is most stable, distribution is broad, the color is the most deep, organic carbon content is high, the effective thickness is big, the reservoir rock and clay mineral content ratio distribution is good, the porosity is bigger, the gas content is also more stable,is the most meaningfulshale gas reservoir sectionin the Taiyuan Formation, is the first choice for shale gas exploration and development.The transverse adopts comprehensive information superposition method to predict Taiyuan Formation shale reservoir favorable zone is located in the central area of Qinyuan and Hengshui-Heshenmiao area.The research achievements have such a positive significant in the development of transitional facies accumulation theory and form the foundation of exploration deploy of coal strata reservoir in Qinshui basin.