Geochemistry and reservoir characteristics of shale in the upper member of Shimengou Formation,Yuka sag

Yuka sag,located in the middle of the north margin of Qaidam basin,is a key area for continental shale gas exploration in Qinghai province. The upper member of the Shimengou formation of the middle Jurassic is a semi-deep lacustrine to deep lacustrine facies deposit with a dark shale thickness of 10-170 m with an average of 55 m for major formation in shale gas exploration. The organic geochemistry and reservoir characteristics were studied by using nitrogen gas adsorption,total organic carbon content,organic matter maturity,kerogen element,X-ray diffraction analyses and SEM. The results show that the TOC content in Shimengou formation of middle Jurassic is 1. 34% -12. 84% ,with an average of 5. 3% . Kerogen types are mainly type Ⅱ2 ;Ro ranges from 0. 45% to 1. 0% ,which is higher in the west- ern area than that in the eastern area. Clay mineral content ranges from 34% to 58% with an average of 46. 1% . Quartz content ranges from 42% to 55% ,with an average of 48. 3% . The nanopore structure in the Shimengou shale are complex,which can be divided into two types according to the nitrogen adsorption-desorption curves and pore di- ameter distribution. The first type is dominated by the airtight pores with one end closed and the open parallel plate-slit pores,and the pore diameters are in a range of 3-5 nm with a single-peak distribution. The second type mainly con- sists of the airtight pores with one end closed and open inclined slit pores,peaking in a range of 3-5 nm and 6-45 nm with a bimodal distribution. Due to the low maturity,the nano pores of organic matter are not developed,TOC content has a weak positive correlation with mesoporous and total pore volume,but not with micropore volume. Brittle mineral content is positively correlated with porosity,clay mineral content is not correlated with micropore volume,but positive- ly correlated with mesoporous volume and total pore volume. The clay content is a key factor controlling the nanopore development in shale,the pore structure and pore diameter distribution are related to sedimentary environment,and the TOC content and thermal evolution also have some influence.