Wen Zhang

Interactive depositional systems of marine carbonates and gypsum salt rocks are closely related to natural gas reservoirs. Despite continuous progress in the exploration of new areas of marine carbonate genesis within the Ordos Basin, the source and mechanism of “sub-salt” natural gas genesis remains controversial. In this study, we investigated natural gas genesis through geochemical analysis of Lower Paleozoic natural gas samples from the mid-eastern Ordos Basin, obtaining natural gas composition data and carbon/hydrogen isotope compositions. We found evident differences between the geochemical characteristics of “sub-salt” and “post-salt” natural gas; the methane carbon isotope signature of “sub-salt” natural gas was lighter overall than that of “post-salt” natural gas, while the ethane carbon isotope composition of the former was more widely distributed and partially lighter than that of the latter. Combining these data with the regional geological background and existing geochemical data, it is evident that Ordovician “post-salt” natural gas comprises a composite of Upper Paleozoic coal-type gas and Lower Paleozoic oil-type gas, with the oil-type gas accounting for the largest proportion. In contrast, the “sub-salt” natural gas was formed and preserved within the Ordovician marine carbonates or sourced from deeper and more ancient hydrocarbon source rocks. Geochemical anomalies, including light methane carbon isotopes and ethane carbon isotopes with coal-type gas characteristics, are closely related to the prevalence of thermochemical sulfate reduction during hydrocarbon formation and reservoir formation of natural gas in “sub-salt” strata.

The reservoir of Chang 7 Member of Yanchang Formation in the Ordos Basin is rich in tight oil resources, with well-developed micro-nanopore throats, strong heterogeneity, and complex fluid distribution characteristics. In this paper, a combination of nuclear magnetic resonance and centrifugation experiments is carried out to qualitatively and quantitatively analyze and evaluate the distribution characteristics of the movable fluid in Chang 7 reservoirs in Baibao Block. With different centrifugal forces, the NMR T₂ spectrum characteristics of saturated water state and different centrifugal force state can be obtained, and the movable fluid distribution characteristics under the control of different centrifugal forces and different pore throat intervals can be obtained. The research results demonstrate that the NMR T₂ spectrum of the Chang 7 tight oil reservoir in the study area presents a double-peak or triple-peak shape. With the increase of centrifugal force, the water saturation value of the corresponding sample gradually decreases. The overall movable fluid saturation ranged from 19.07% to 32.52%, with an average of 26.23% and a low degree of movability. The submicron (0.1~1 μm) and nanoscale (0.075~0.1 μm) pore throats were the main pore throat intervals controlling the movable fluid saturation of the studied samples, accounting for an average of 18.31% and 5.57%, respectively, and micron-scale pore throats ( ) accounted for an average of 2.35% of the movable fluid distribution.