Pore structures and conductivity of asphaltic carbonate reservoirs： A case study of the Sinian Dengying Formation in the Penglai gas area， Sichuan Basin

Recent exploration practices in the Sichuan Basin reveal the extensive occurrence of bitumen in deep to ultradeep carbonate reservoirs. The existence of asphalt reduces the reservoir capacity， changes the pore-throat structure of the reservoir which leads to the complexity of the conductivity rule of the reservoir. This further affects the difficulty of reservoir logging evaluation of effectiveness and fluid property discrimination. To quantitatively characterize the impacts of bitumen infill on the pore structures and electrical resistivity of carbonate reservoirs， we investigate the Sinian Dengying Formation in the Penglai gas area， Sichuan Basin. Specifically， high-resolution computed tomography （CT） scanning experiments are conducted on full-diameter cores. Based on pore-throat network models， we examine the changes in the reservoir space and pore throats within the cores before and after bitumen infill. Finite element method （FEM） numerical simulations are conducted to assess rock conductivity under varying bitumen infill rates. Accordingly， the variation patterns of the pore structures and resistivity of asphaltic carbonate reservoirs are determined. The results indicate that the grayscale values of bitumen fall between those of pores and rock matrix as seen on CT images. Applying deep learning to the analysis of the infill characteristics and distribution patterns of bitumen in cores and thin sections can significantly enhance the accuracy of CT scanning-based bitumen identification and quantification. Bitumen infill reduces the volume of large pores and the quantity of long， large pore throats. As a result， the quantity of pores with high coordination numbers trends downward. Rock resistivity is positively correlated with the bitumen infill rate， showing a distinct two-stage increase. Notably， the increase in the first stage is attributed primarily to bitumen infill in fractures， while that in the second stage is predominantly due to bitumen infill in pore throats. In both stages， the increasing rate of the resistivity diminishes.