Seismic pore-type characterization in tight carbonate reservoirs: A case study of the fourth member of Ordovician Majiagou Formation in Ordos Basin, China

Carbonate reservoirs are known for their complex pore structures, which lead to variable elastic behaviors and seismic responses. These variations pose significant challenges for seismic interpretation of carbonate reservoirs. Therefore, quantitative characterization of pore structures is crucial for accurate fluid detection and reservoir property estimation. To address the complexity of pore geometry and the uneven fluid distribution in tight carbonate reservoirs, we develop a triple pore effective medium model by integrating the extended Keys-Xu model with the Gassmann-Hill equation. Comparison between the theoretical modeling results and an available laboratory data set confirms the effectiveness of this model in pore type quantification. Based on this calibrated model, we propose a novel two-step triple pore-type inversion strategy with varying pore aspect ratio via a grid-searching algorithm. We apply this method to well logs and 3D seismic data from the tight carbonate reservoirs of the Ordovician Majiagou formation in the Ordos Basin. The good agreement between pore-type estimates and logging interpretation results suggests that our method significantly improves the accuracy of porosity estimates for different pore types, outperforming the pore-type inversion method with fixed pore aspect ratios. The successful application to seismic data also demonstrates that the proposed method provides a reliable distribution of pore types in tight carbonate reservoirs, confirming its applicability and feasibility in seismic pore-type estimation. This method not only facilitates the recognition of complex pore geometries but also provides valuable insights for accurate detection of high-quality reservoirs.