Main Factors Control the Shale Gas Accumulation in the Yudongnan Area, Southwest China

ancient marine shales in southern china, especially in the Jiaoshiba-Fuling area. However, rare succeed exploration and production cases outside Sichuan basin were reported. The main reason for this issue can be attributed to the misunderstandings of the mechanisms of shale gas accumulation and key controlling factors. In order to better understand the mechanisms and discuss the factors of maturity, reservoir property, fluid pressure, gas content, preservation conditions effects on shale gas accumulation, black shale core samples were collected from a well drilled in the Yudongnan area, in southwest China and geochemistry, mineralogy, lithofacies, pore structures, reservoir properties and occurrence state were investigated using shale thin section, X-ray diffraction low pressure N2 and CO2 adsorption analysis and field emission scanning electron microscopy (FE-SEM) observations. The results show that the black shales have total organic carbon (TOC) values ranging from 1.82% to 4.35% and their equivalent vitrinite reflectance values are in the range of 2.5% to 3.5%. Both mineral matrix and organic matter pores are well developed with pore sizes ranging from several to several hundred nanometers observed by FESEM. The total porosity for the samples ranges from 1.60% to 5.78% and the percentages of organic matter porosity are estimated to be in the range of 8% to 32%. The total surface area ranges from 3.1 m/g to 22.56 m/g and the micropore (< 2 nm) surface area estimated ranges from 1.18 m2/g to 12.74 m/g. The TOC values have positive relationships with the total porosity, total surface area and the micropore (< 2 nm) volume and surface area, indicating TOC may be an effective parameter for shale gas reservoir evaluation in the studied area. At a given temperature and pressure, the gas sorption capacities of shales are primarily controlled by the TOC but may be significantly affected by the type and maturity of the organic matter, mineral composition (especially clay content), moisture content, pore volume and structure, leading to different gas sorption capacity for different shales. Under geologic conditions, the gas sorption capacities increases initially with depth due to the predominating effect of pressure, passes through a maximum, and then decreases because of the influence of increasing temperature at greater depth. As temperature and pressure increase and with the presence of moisture, the gas sorption capacities of organic-rich shales are quite low. High contents of free gas in organic-rich shales (47.22%~82.78%) can be preserved in relatively closed system and low diffusion coefficient shales, which have the structure characteristics of flat syncline and low angle unicline, less fracture, low erosion thickness, and thick shale layers. Loss of free gas during post-accumulation tectonic movement and erosion may result in under saturation (the total gas contents lower than the sorption capacity). For the shale gas exploration areas with a strong structure deformation, the preserved condition of the shale system was destroyed to a certain degree with a pressure decrease and loss of large amount of free gas. LI Zhuo, JIANG Zhenxue, LIU Luofu, GAO Xiaoyue, JI Wenming, XIONG Fengyang and SONG Yan, 2015. Main Factors Control the Shale Gas Accumulation in the Yudongnan Area, Southwest China. Acta Geologica Sinica (English Edition), 89(supp.): 263.