Research and comparison of evaluation methods for shale reservoir saturation in Longtan Formation of marine-continent transition facies

As a country rich in shale gas, China has developed relatively mature evaluation systems for marine shale exploration and conducted tentative developments for continental shale. However, limited research has been conducted on marine-continental transitional shale. To address this gap in the research from the perspective of well logging, we have used marine-continental transitional shale of the Longtan Formation in the southeast Sichuan Basin to compare the differences between Longtan shale and marine shale with respect to organic matter, lithology, and logging responses to identify marine-continental transitional shale reservoir features. We also have analyzed the low-resistivity genesis of marine-continental transitional shale. Studies on the Longtan shale have demonstrated that the combination of high clay content, complex pore structure, development of pyrite, graphitization of organic matter, and presence of high-rank coal seams collectively produces low-resistivity properties in the reservoir. Saturation is essential in petroleum exploration and development. However, the low-resistivity characteristics result in low accuracy of gas saturation when calculated by the Archie equation or its electric derivation equations. To improve the efficacy of saturation prediction, we compare and analyze existing nonelectrical models and develop a new process for calculating saturation based on the density-neutron combined model. The errors between the calculation results and core saturation are minor. In addition, the new calculation model exhibits a good application effect in marine shale in the Jiaoshiba area, providing a new approach for saturation evaluation of shale formations.

Automated summary

As a country with abundant shale gas resources, China has made progress in evaluating marine and continental shale. However, there is limited research on marine-continental transitional shale. In this study, we compared the differences between Longtan shale and marine shale using well logging data, and analyzed the low-resistivity genesis of marine-continental transitional shale. Our findings suggest that the low-resistivity properties of the Longtan shale are caused by high clay content, complex pore structure, development of pyrite, graphitization of organic matter, and the presence of high-rank coal seams. To improve saturation prediction, we developed a new process based on the density-neutron combined model, which showed good results in the Jiaoshiba area, providing a new approach for saturation evaluation of shale formations.