Abstract:

The Northern Qaidam Basin (NQB) is a typical continental limnic basin and is also one of the basins with shale gas potential. In this paper, the characteristics and influencing factors of nanopores of the shales in the Middle Jurassic Shimengou Formation of Well YQ - 1 in the Yuqia area of NQB were studied by using nitrogen gas adsorption, total organic carbon content, organic matter maturity, and Xray diffraction analyses. The results showed that the nanopore structures in the Shimengou shale are complex, and these nanopores can be subdivided into two types based on the nitrogen adsorptiondesorption curves and pore diameter distribution. The first type mainly consists of the airtight pores with one end closed and the open parallel plateslit pores, and the pore diameters are in a range of 35 nm. The second type mainly consists of the airtight pores with one end closed and the open inclined slit pores, and the pore diameters are characterized by a bimodal distribution, peaking in a range of 35 nm and 814 nm. The micropore and mesopore with an aperture smaller than 50 nm provided the dominating specific surface area and pore volume. The volume of micropores, mesopores and total pores showed a positive correlation with the clay contents. The pores of organic matter were less developed at a low maturity. The H9 shale, characterized by a high TOC content, shows a negative correlation between the TOC content and the volume of micopores, mesopores and total pores, while the H8 shale, characterized by a low TOC content, shows no correlation. Pore structures and diameter distributions are related to the hydrodynamic conditions of depositional environment. The clay content is a key factor controlling the nanopore development in shale, and the TOC content and depositional environment also have a certain influence.

Key words: Northern Qaidam Basin, Middle Jurassic, Shimengou Formation, shale, nanopore