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

Abstract

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.