Jurassic Terrestrial Shale Gas Potential of the Northern Kashi Sag in the Tarim Basin, Northwestern China

Evaluation of terrestrial shale gas resource potential is a hot issue in unconventional oil and gas exploration. Organic-rich shales are widely developed in the Jurassic strata of Tarim Basin, but their shale gas potential has not been described well. In the study, the Lower-Middle Jurassic fine-grained sedimentary rocks (Kangsu and Yangye Formations) in northern Kashi Sag, northwestern Tarim Basin, were taken as the study object. The comprehensive studies include petrology, mineralogy, organic geochemistry, and physical properties, which were used to characterize the organic matter and reservoir characteristics. Results show that the Jurassic terrestrial shale in the northern Kashi Sag was mainly deposited in lakes, rivers, and deltas. The thickness of black lacustrine shale developed in the Early-Middle Jurassic in the study area is generally over 100 m. The total organic carbon (TOC) content is rich, averaging 2.77%. The vitrinite reflectance (R-o) values indicate that the Lower Jurassic shale organic matter is in the early mature-mature stage, while the Middle Jurassic is in the mature stage. Besides, organic matter is primarily II and III in kerogen types. The whole shale contains a large number of clay minerals, especially illite. The average brittle minerals such as quartz and feldspar are 28.67%, and the average brittleness index is 38.63%. Nanoscale pores containing intergranular pores, dissolution pores, and organic pores, coupled with microcracks, are well developed in Jurassic shale. The sample's average pore volume is 0.017 cm(3)/g, and the specific surface area is 9.36 m(2)/g. Mesoporous contribute the most to pore volume, while the number of microporous is the largest. Both of them provide most of the surface area for the shale. Combined with regional geologic settings, we propose that the Jurassic terrestrial shale has good-excellent shale gas exploration potential and development prospects.