Abstract:
The Longmaxi Formation is one of the major exploration horizons of shale in China at present. The gas-bearing sediments in the Lower Paleozoic Longmaxi Formation in the Yangtze area, South Sichuan are characterized by low resistivity. Therefore, it is difficult to distinguish the gas-bearing sediments from aquifers only using logging response. Given this, this study investigated the genetic mechanisms of low-resistivity gas-bearing sediments in the study area in depth using existing data of conventional logging, core drilling, and production-related dynamic conditions, as well as a large number of core analyses and experiments, including casting thin sections, QEMSCAN, and X whole-rock diffraction. According to the study results, three major controlling factors in the low-resistivity gas-bearing sediments of the Lower Paleozoic Longmaxi Formation in the Yangtze region, South Sichuan include the additional conductivity of clay minerals, the complete graphitization of over-mature organic matter, and the distribution pattern of organic matter laminae. The practical application results indicate that the low-resistivity gas-bearing sediments in the study area were formed due to the organic matter laminae developing and the partial graphitization of organic matter. That is, the low resistivity of the shale gas reservoirs tends to be caused by the mutual superimposition and effects of multiple factors. Therefore, it is necessary to analyze the geological, logging, and core data according to various zones and horizons and employ the correlation between upper and lower horizons and between adjacent wells to determine the major controlling factors of the low resistivity of the shale gas reservoirs.