Microconductivity of deep, bitumen-bearing carbonate rocks of dissolved pore and cavity types: A case study of the Cambrian Longwangmiao Formation in the Gaoshiti-Moxi area, Sichuan Basin

Bitumen, which is widely found in the storage spaces of deep carbonate reservoirs, significantly affects the physical and gas-bearing properties of the reservoirs and complicates their resistivity logging responses. This makes it more difficult to evaluate the effectiveness of bitumen-bearing carbonate reservoirs, determine fluid properties, and calculate water saturation. This study focuses on the bitumen-bearing carbonate rocks of dissolved pore and cavity types in the Cambrian Longwangmiao Formation in the Gaoshiti-Moxi area, Anyue gas field, Sichuan Basin. Based on the analysis of the micro-CT scanning data, we characterize the pore structures and bitumen-filling characteristics of the carbonate reservoirs using digital core modeling. As a result, 3D rock conductivity models with varying bitumen content and water saturation conditions are established. Then, numerical simulations of rock conductivity are conducted using the finite element method, and the mechanisms through which bitumen influences the conductivity of carbonate rocks are investigated. The results indicate that the resistivities of bitumen-bearing carbonate rocks of both types exhibit a positive correlation with the bitumen content and a negative correlation with water saturation. Since the narrow throats in dissolved pore-type carbonate rocks are more prone to be blocked by bitumen, the conductivity of these rocks is more subjected to bitumen effect than the dissolved cavity-type carbonate rocks. Under low water saturation, formation water mainly occurs as irreducible water films on storage space surfaces. However, these water films become incomplete in the presence of bitumen, leading to a sharp increase in resistivity for both types of carbonate rocks. Compared to bitumen-bearing dissolved cavity-type carbonate rocks, the dissolved pore-type reservoirs exhibit a higher cementation exponent (m) and a lower saturation exponent (n).