Abstract
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 cm3/g, and the specific surface area is 9.36 m2/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.
Highlights
The remarkable success of shale gas in North America led to the vigorous development of shale gas exploration and increased the investigation of global shale gas potential, changing the world energy pattern [1,2,3,4,5]
Based on the lithologic character from two sections in this study, it was suggested that the strata of Jurassic in Kashi Sag were mainly continental sediments, and the organic-rich shale primarily developed in the Lower Jurassic Kangsu Formation and Middle Jurassic Yangye Formation, which is considered one of the three sets of the potential source rocks in Kashi Sag [27]
The average total organic matter content is greater than 2%, Ro ranges from 0.6% to 1.8%, and organic matter is primarily II and III in kerogen types
Summary
The remarkable success of shale gas in North America led to the vigorous development of shale gas exploration and increased the investigation of global shale gas potential, changing the world energy pattern [1,2,3,4,5]. With the deepening of exploration and development, the pressure to increase shale gas reserves and production in the future is still tremendous. The continental basins in China are widely distributed, and the continental shale is developed in many strata such as Mesozoic-Cenozoic [8,9,10,11], which is one of the required replacement fields for shale gas exploration. Industrial discoveries of continental shale gas are found in several Triassic stratigraphic wells in Ordos Basin and Jurassic strata in Sichuan Basin [12, 13]. Due to the limitation of low exploration degree and complex reservoir conditions [8, 14,15,16], the large-scale exploration and development of continental shale gas still need further research
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