Unravelling the origin of gas in tight sandstones of the Hangjinqi gas field, Ordos Basin, China: New insights from natural gas geochemistry data

Abstract

The Hangjinqi gas field, a vital tight sandstone gas field on the northern Ordos Basin margin, exhibits a debatable gas origin and accumulation mechanism. Integrating geochemical analyses with gas field data for basin-wide comparisons allows us to resolve the origin of the reservoired gas. Results reveal that the Upper Paleozoic coal-measure source rocks in the Hangjinqi area share similar characteristics with those in other Upper Paleozoic gas fields, where coal seams constitute the primary effective source rocks. However, they exhibit limited hydrocarbon-generating potential coupled with low thermal maturity. Natural gas has a broad range of dryness coefficient values. The carbon-hydrogen isotopes mostly exhibit a positive carbon-hydrogen isotope series, yet 40% of the samples display ethane and propane carbon-hydrogen isotope reversals. The C_5-7 and C₇ light hydrocarbons are dominated by isoalkanes and methylcyclohexane, respectively. The natural gas data follow the aromatic curve in the cross-plot of the heptane and isoheptane values. The natural gas in the Hangjinqi field is typical coal-derived gas, which is derived primarily from the Carboniferous-Permian transitional facies of humic organic matter, similar to Daniudi and other gas fields. Notably, the natural gas in the Hangjinqi gas field has undergone migration and dissipation (mainly including escape and loss). The Hangjinqi gas field presents an apparent contradiction between low maturity source rocks and high maturity natural gas. Its methane carbon isotope values significantly exceed those of the Daniudi gas field but align closely with those of the Sulige and Wushenqi gas fields, which are mature-type gas fields. Natural gas migration-dissipation is not the controlling factor in regional isotopic signatures. Therefore, approximately 40–76 % of the natural gas in the Hangjinqi gas field is exogenous gas. Moreover, these exogenous gases are derived primarily from the Sulige-Wushenqi gas field to its south, indicating strong genetic affinity. Overall, most Upper Paleozoic gas fields in the Ordos Basin have short migration distances, whereas the Hangjinqi gas field represents mixtures of locally generated hydrocarbons with gases migrating longer distances.