Evaluation of Resistivity and Saturation of Bitumen-Bearing Fracture-Caved Carbonate Reservoir Based on Digital Core Technology

Bitumen is a highly carbon rich polycyclic hydrocarbon compound with low conductivity, similar to the resistivity logging response characteristics of oil and gas. It is easy to mistake the bitumen filled in the pores for oil and gas reservoir during logging interpretation. The fracture-caved carbonate reservoir of Sinian Dengying Formation in Sichuan Basin, China is rich in solid bitumen. The presence of bitumen results in a higher resistivity measured by the target layer resistivity logging, resulting in gas saturation calculated using resistivity being greater than the actual value, which in turn leads to production not reaching the expected output and seriously affects the prediction of natural gas reserves. Therefore, this research proposes a resistivity evaluation method for bitumen-bearing fracture-caved carbonate reservoir based on digital core technology (DCT). Based on CT scanning technology, combined with artificial thresholding and deep machine learning methods, a rock skeleton-bitumen-pore interpretation segmentation technique was established for the first time, effectively establishing two types of digital core models: pore type and fracture type in fracture-caved carbonate rocks containing bitumen. Simulate changes in bitumen content using dilation and erosion algorithms. Based on finite element electrical simulation technology, conductivity simulation experiments with different porosity and water saturation under geological conditions were conducted on the established bitumen containing digital core models. The influence of solid bitumen on formation resistivity has been clarified, and a resistivity correction model for carbonate reservoir containing bitumen has been established. Through rock electrical simulation, the relationship between rock electrical parameters m, n and bitumen content was clarified, and variable rock electrical parameters were established to replace fixed values in bitumen bearing carbonate reservoir. Thus, a gas saturation correction model for the fracture-caved carbonate reservoir of the Dengying Formation was constructed, which improved the accuracy of interpreting the gas saturation of the target layer. At the same time, based on the ideal fracture-caved models containing bitumen, the variation law of electrical resistivity under different bitumen occurrence states and different crack shapes was innovatively explored. The results indicate that an increase in bitumen content will gradually increase the resistivity, and the increasing trend will change from gentle to steep. The resistivity of bitumen-bearing cores has different changing rules under different bitumen occurrence state and different fracture morphology. There is a certain quadratic nonlinear relationship between the rock electrical parameters m, n and the bitumen content. Therefore, fitting models with variable m and n are established to improve the Archie saturation calculation formula. Using the improved formula, the gas saturation of the bitumen-bearing fracture-caved carbonate reservoir in the Dengying Formation of the Sichuan Basin was reevaluated. The results showed that the accuracy of calculating the gas saturation of the interpreted intervals was improved by 10.2%. This demonstrates that DCT provides a new approach for evaluating the saturation logging of bitumen-bearing fracture-caved carbonate reservoir.