Geochemical Characteristics and Oil-Gas Geological Significance of the Cambrian Bituminous Calcite Veins in the Northeast Margin of Ordos Basin, China

The bituminous calcite vein in the basin is an important basis for indicating fluid activity, hydrocarbon migration, and evolution. This paper systematically describes the microscopic petrological characteristics of two types of bituminous calcite veins and surrounding rocks from the Cambrian Xuzhuang Formation in Qingshuihe area, northeast margin of Ordos Basin, and their geochemical characteristics were analyzed by X-ray fluorescence spectroscopy (XRF) content, reflectance R_b, inorganic carbon and oxygen isotope composition (δ¹³C_carb, δ¹⁸O_carb) and organic carbon isotope composition (δ¹³C_org). The Al–Fe–Mn and Mg–Fe–Mn correlation graphs indicate that the two types of calcite veins are calcite formed at low temperatures and are not affected by hydrothermal fluids. The correlation analyses of inorganic carbon and oxygen isotopes indicate that the inorganic carbon isotopes of the two types of calcite veins are significantly lighter, which should be related to the participation of organic fluids during diagenesis, and the forming vein fluids are hydrocarbon-related mixed fluids. The results of equivalent vitrinite reflectance (R_oeq) converted from solid bitumen reflectance (R_b), formation temperature (T_burial), and scanning electron microscopy (SEM) indicate that the bitumen in calcite veins is highly overmature pyrobitumen, which is formed under different temperature conditions compared with calcite veins. By comparing the organic carbon isotope composition (δ¹³C_org) of pyrobitumen in two types of calcite veins with kerogen organic carbon isotope composition of the black gray limestone for surrounding rock, as well as that of the main source rocks of Mesoproterozoic-Lower Paleozoic in the basin, combined with the spatial distribution of the these source rocks, it is indicated that the pyrobitumen in two types of calcite veins is related to the sedimentary organic matter of the surrounding rock. The sources of vein-forming fluids for the two types of calcite veins and pyrobitumen are closely related to the near-source hydrocarbons associated with the organic matter of the surrounding rock, suggesting that the Cambrian in the Ordos Basin develops effective source rocks.