Origin and isotopic fractionation of shale gas from the Shanxi Formation in the southeastern margin of Ordos Basin

Abstract

The Ordos Basin is abundant in natural gas resources, and the Shanxi Formation with the transitional facies is considered as a potential stratum for shale gas reservoir. However, the origin and geochemical characteristics of the Shanxi Formation shale gas remains unclear. In this study, 22 shale gas samples were collected from the SX-1 well in the southeastern margin of Ordos Basin, and tested for gas components and compositions of stable carbon isotope. Moreover, five rock samples were measured for total organic carbon (TOC), the vitrinite reflectance (Ro), kerogen type, porosity, permeability, and methane adsorption capacity. The shale gas in the Shanxi Formation has a high proportion of methane which is consistent with the high maturity of source rocks. The contradictory relationship between the carbon isotopes of ethane (δ13C2) from the Shanxi Formation shale gas and kerogen types of rock samples may be caused by the complex sedimentary environment, multiple provenance systems, and high maturity. The carbon isotopes of methane (δ13C1), δ13C2 and alkane gas components indicate that the shale gas near coal seams is coal-derived gas, the gas at the bottom of the Shanxi Formation is oil-type gas and the gas in mudstones among the sandstone layer at the top of the Shanxi Formation may be mixed with natural gas migrating from other layers or mixed with gas generated in the later period. Meanwhile, the carbon isotopes of carbon dioxide (δ13CCO2) suggest that the CO2 generated with hydrocarbons in the coal-measures is mainly of organic origin. The carbon isotopic fractionation of methane is commonly observed in shale gas samples. This fractionation is mainly controlled by the diffusion effect which can be promoted by high porosity and permeability. Furthermore, the positive correlation between the TOC values and the degree of fractionations may be attributed to the high TOC values in the samples, which can improve the development of organic pores and increase the content of adsorbed gas.