Quantitative Identification of Lithology and Gas-Bearing Properties of Carbonate Reservoirs in the Majiagou Formation, Central Shaanbei Slope, Ordos Basin

The identification of lithology and fluids in reservoirs is the key to the quantitative characterization of gas reservoirs. However, the Ma541 Member of the Majiagou Formation in the Ordos Basin is characterized by strong reservoir heterogeneity, variable lithologic components and complex gas–water relationships. This leads to severe overlapping of conventional logging responses, posing significant challenges to detailed reservoir evaluation. Taking the Ma541 Member in the central Shaanbei Slope of the Ordos Basin as the research object, this study adopts the logging curve superposition and reconstruction method to quantitatively identify reservoir lithology and fluid properties, and establishes a set of identification standards for lithology-fluid logging curve superposition and reconstruction. The results show that the lithology identification plate constructed by introducing new parameters eliminates dimensional differences and effectively highlights the response characteristics of different lithologies. It can rapidly and effectively identify limestone, limy dolomite, dolomite, argillaceous dolomite, and mudstone with an identification accuracy exceeding 90% and an average accuracy of over 92%. In terms of fluid identification, the constructed ΔΦ3–ΔΦ4–ΔΦ5 3D plate successfully achieved the stereoscopic differentiation of gas layers, gas-bearing water layers, water layers, and dry layers. The gas layer identification accuracy reached 93.9%, which is significantly superior to the traditional 2D crossplot method. Applying this model to the plane prediction of lithology in the Ma541 Member of the study area, it was found that the lithology distribution features “pure in the east and mixed in the west.” The central-eastern and southeastern parts of the study area mainly develop high-quality dolomite and limy dolomite reservoirs, making them favorable areas for natural gas exploration. This study provides effective technical support for the quantitative identification of lithology and fluids in non-cored well sections and improves regional exploration and development efficiency.