Geological conditions of Permian sea-land transitional facies shale gas in the Xiangzhong depression

Abstract: Based on such analytical methods as outcrop core observation, gas content, geochemical and physical property data and isothermal adsorption as well as argon ion polishing plus scanning electron microscopy of Permian black shale in the center of Hunan depression, the authors investigated the geological conditions of shale gas. The results show that the geological conditions of shale in Permian Longtan Formation and Dalong Formation of sea-land transitional facies are beneficial to gas accumulation. The organic carbon content is high than 2%, the grade of maturity distribution is between 1.2% and 1.6%, the main reservoir types are mineral holes with minor organic holes such as residual intergranular pores, intragranular pores and corrosion holes. The porosity is between 0.54% and 5.15%，the average gas content capacity of isothermal adsorption test is between 5.488 and 6.905 m3/t. The thermal evolution analysis shows that Permian shale experienced two times of uplift and denudation, and the second denudation didn’t reach the maximum depth of the first time and there was no secondary hydrocarbon. This might have been the reason which led to the low gas content in the study area. Structural analysis shows that the modification effect of Indo-Chinese epoch and Yanshanian period resulted in the poor preservation conditions that damaged the oil gas obviously and increased the risk of exploration. Using the method of superposition to predict the areas, the authors reveal that Lianyuan-Loudi-Shuangfeng area and Shaoyang-Shaodong area are favorable shale gas areas. The volume method was used to calculate the total quantity of shale gas resource, which yielded (127-425）×108 m3, (the medium value being 254×108 m3)，suggesting a certain potential of shale gas resources. The comprehensive analysis shows that the preservation condition is the key factor for shale gas accumulation in this area, and is also the principal factor for successful exploration.