The pore characteristics and its geochemical and mineralogical controls of marine-continental transitional shales from Keluke Formation in the Qaidam Basin, Northwest China

To characterize the pores in marine-continental transitional shales and their geochemical and mineralogical controls, geochemical, mineralogical, scanning electronic microscopy (SEM) and low-pressure gas (CO2 and N2) adsorption experiments were performed on the Keluke shales from the Qaidam Basin, China. The initial shale samples were extracted using dichloromethane/methanol mixture to get the extracted samples and their kerogen samples were also obtained using chemical methods. The results showed that kerogen samples have more CO2 and N2 absorbed amounts and higher pore volumes and surface areas than the initial and extracted samples, suggesting organic matters are more porous than other compositions in our samples. Meanwhile, the extracted samples have higher pore volumes and surface areas than the initial ones, indicating that the extractable organic matters occupied parts of pore space. The weakly positive correlation between total organic carbon (TOC) content and micropore volume means that organic matters contain some micorpores. The correlation between TOC content and micropore volume of our samples is worse than the shale samples containing Type II organic matters, which may be related to the lower hydrocarbon potential of Type III kerogens in them. The quartzes in our samples may be mainly terrigenous. And the positive correlations between quartz content and micropore volume and macropore volume illustrated that terrigenous quartzes can contribute to pore development. Although the clay mineral contents of our samples are high, they have negative correlations with pore volumes. This phenomenon may be related to that kaolinite is the dominant clay mineral in our samples and kaolinite is less porous than other clay minerals.