An analysis of reservoir-forming conditions of shale gas in Middle Jurassic strata of Yuqia area, Qaidam Basin

According to geological drilling, field desorption, and experimental sample analysis, the authors studied the reservoir-form-ing conditions of shale gas in Middle Jurassic strata of Yuqia area. The results show that the shale in Middle Jurassic strata of Yuqia area is a set of high-quality hydrocarbon source rocks comprising mainly braided river delta, braided river plain and lake facies. It is mainly developed in Dameigou(J₂d⁵) and Shimengou formations. The cumulative thickness is 38~300m, TOC is 0.12%~13.23%, R_o is 0.29%~0.89%, quartz content is 18.0%~63.3%, clay mineral content is 10.1%~55.6%, porosity is 1.53%~14.92% and permeability is 0.007×10^-3~6.24×10^-3μm². Scanning electron microscope analysis has revealed micro pores consisting mainly of intergranular pore, dissolved pore, grain pore, with pore diameter being more than 0.3~3μm. Shale gas content is 0.42~0.83m³/t, desorption gas CH₄ content is 8.6%~64.9%, andδ¹³C₁ is-80.59‰~-63.32‰. A comparison with domestic and foreign gas shales shows that the Middle Jurassic shale in the study area is characterized by large thickness, high TOC and medium quartz content; nevertheless, there also exists some disadvantages in the study area such as small distribution area, low maturity and low gas content in addition, the study area has experienced repeated tectonic movement since Cenozoic, which exerted a considerable impact on shale gas. For the exploration prospect of Middle Jurassic shale gas in Yuqia area, much attention should be paid to the study of tectonic preservation of shale gas and shale gas genetic types as well as resource assessment.