Hydrothermal fluid activity and the quantitative evaluation of its impact on carbonate reservoirs: A case study of the Lower Paleozoic in the west of Dongying sag, Bohai Bay Basin

By integrating the analyses of core samples, thin sections, homogenization temperature measurement of inclusions, stable isotopic compositions of carbon & oxygen, trace element composition, and conventional physical properties, this research identifies the evidence of Lower Paleozoic hydrothermal fluid activity in the western Dongying sag of Bohai Bay Basin, quantitatively evaluates the hydrothermal dissolution strength in various carbonate reservoirs, and analyzes the longitudinal distribution of effective fractures in reservoirs. The markers of hydrothermal dissolution in the study area include the typical hydrothermal mineral combinations such as fluorite-anhydrite-authigenic quartz combination, the homogenization temperature of inclusions in fracture fillings related to hydrothermal fluid of up to 150 °C−210 °C, and the high FeO, high SiO2, high MnO and low Na2O features of carbonate in hydrothermal fluid karst area. Dissolution of hydrothermal fluid can produce not only new fluorite mineral, but also many pores, micropores, small fractures and micro-fractures, which significantly improve the quality of carbonate reservoirs. During the process of hydrothermal fluid dissolution, calcite precipitated and filled structural fractures as the hydrothermal fluid dissolved host rock. As the hydrothermal fluid went up along faults, its velocity and temperature dropped gradually, the intensity of dissolution and calcite filling differ significantly in different formations of the Lower Paleozoic, for example, the Fengshan and Yeli-Liangjiashan Formations have low fracture filling degree and highly developed dissolved fractures and pores, whereas the Majiagou Formation has high fracture filling degree and poorly developed dissolved fractures and pores.