Geological Controlling Factors of Low Resistivity Shale and Their Implications on Reservoir Quality: A Case Study in the Southern Sichuan Basin, China

Abstract

At the Changning block and at the Luzhou block, the genetic mechanism of low-resistivity shale and its impact on reservoir quality are currently a hot topic on a world-wide scale. Shale with resistivity lower than 20 Ω·m is widely developed at the Wufeng-Longmaxi Formation in the Southern Sichuan Basin, bringing a considerable challenge for reservoir prediction using the electromagnetic method. This paper discusses the genetic mechanisms and reservoir qualities of three low-resistivity shale reservoir types in the Southern Sichuan Basin (the Changning block and Luzhou block). Three primary elements controlling low-resistivity shale distribution in the Southern Sichuan Basin have been deduced: widely distributed gravity flow deposits, poor structural preservation conditions and shale graphitization caused by Emeishan basalt. Specifically, (1) the shale reservoir with a resistivity <12 Ω·m was uniformly distributed with gravity flow deposits in the Southern Sichuan Basin. High clay mineral contents (especially illite) in gravity flow deposits increased cation exchange capacity and irreducible water saturation at shale reservoir, decreasing electrical resistivity. (2) The resistivity of the shale reservoir close to a complex fault-fracture zone was generally lower than 20 Ω·m, indicating that poor structural preservation conditions played an important role in the wide distribution of low-resistivity shale. The resistivity of the shale reservoir near NE-trending faults at the Changning block was significantly lower than that in other areas. (3) Emeishan basalt caused extensive shale graphitization at the west of the Changning block, which was limited at the Luzhou block. The shale resistivity at the Luzhou block was not affected by graphitization. Among three types of low-resistivity shale, type III was characterized by high quartz content, high TOC, high porosity, high gas content and low graphitization. Although the resistivity of type III is generally lower that 20 Ω·m, it is still a favorable exploration target in the Southern Sichuan Basin.