Comparison and Significance between Low- and Normal-Resistivity Shales of Wufeng-Longmaxi Formation in Changning Area, Southern Sichuan Basin

Abstract

The shale gas reservoirs with normal resistivity in the Changning area of the southern Sichuan Basin generally have good gas-bearing properties. However, there are notable differences in gas production from low resistivity wells across different regions, including low-yield microgas low resistivity wells and high-yield gas-rich low resistivity wells. Therefore, it is worth investigating the differences between shale reservoirs with low and normal resistivities. To identify their differences, X-ray diffraction, geochemistry, laser Raman spectroscopy, physical properties, electron microscopy, gas content and saturation tests, and well logging data of shale samples from the Wufeng-Longmaxi Formation in the Changning area were comprehensively analyzed. According to the difference in logging resistivity, three types of shale gas reservoirs with ultralow resistivity (R t < 5 Ω•m), low resistivity (R t between 5 and 15 Ω•m), and normal resistivity (R t > 15 Ω•m) were summarized. In ultralow resistivity shale gas reservoirs, total organic carbon (TOC) was generally >2%, Rmc R o > 3.50%, porosity, gas saturation, and gas content were low. Low resistivity shale gas reservoirs typically had a TOC > 4.0%, Rmc R o ranges between 3.40 and 3.50%. Normal resistivity shale gas reservoir had high TOC (>3.0%), Rmc R o < 3.40%. Both types have large values of porosity, gas saturation, and gas content. The ultralow-resistivity shale gas reservoir has the lowest gas content and nearly no industrial productivity because of its deep burial, extensive fracture system development, poor preservation conditions, and strong graphitization degree of organic matter. In comparison, normal-resistivity shale gas reservoirs had the largest gas content owing to medium to shallow burial depth, weak fracture system development, good preservation conditions, and moderate thermal maturation. The regions where low-resistivity shale gas reservoirs are located are generally similar to those of normal-resistivity shale in terms of faulting and preservation conditions, with only some areas showing weak graphitization of organic matter, relatively good gas content, and certain exploration and development potential.