Geochemical characteristics of marine-continental transitional facies shale and shale gas in Linxing area

In order to reveal the geochemical characteristics of shale and shale gas on the east edge of Ordos Basin, thin section identification, full rock and clay content, scanning electron microscope, organic carbon, rock pyrolysis, kerogen macerals, and kerogen carbon isotopes were tested, and the gas component and carbon isotopes composition of shale were analyzed. The results show that the mineral component of the transitional facies shale in Linxing area is mainly quartz and clay minerals, containing a small amount of calcite, plagioclase, potassium feldspar, dolomite and pyrite. Affected by the mineral genesis and deposition environment, different mineral components and organic matter occur in different ways. There are two deposit patterns of quartz and organic matter, 3 patterns of clay minerals, and 4 patterns of pyrite. The organic matter type is Ⅱ₂-Ⅲ type, the average organic carbon content is greater than 2.0%, and the kerogen carbon isotopes are from –24.5‰ to –23.2‰. The vitrinite reflectivity is from 0.92% to 1.30%, t_max is 427~494℃, and the thermal evolution of organic matter reaches maturity. In shale gas, hydrocarbon gas is mainly methane, containing a small amount of ethane and propane, which is generally dry gas. The mean methane carbon isotope is –40.0‰, between marine shale gas and terrestrial shale gas, ethane carbon isotopes from –26.8‰ to –22.56‰, both more than –29 ‰, showing δ¹³C₁ < δ¹³C₂ < δ¹³C₃ positive carbon sequence. It is believed that the shale in the area has the potential to generate shale gas. The shale gas is mainly derived from humic shale in the upper Paleozoic boundary, which belongs to the organic thermal coal gas generated by the cracking of cheese root.