Electrical properties of Longmaxi organic-rich shale and its potential applications to shale gas exploration and exploitation

Abstract

This paper mainly discussed the experimental relationships between electrical properties and geological parameters such as brittle mineral content, total organic carbon (TOC) content and microscopic pore structure, with examples from Longmaxi organic shale in Sichuan province of China. With results derived from X-ray diffraction, organic geochemical measurements, nitrogen adsorption method and tests of rock electrical parameters, we came to the conclusions as follows: (1) Shale samples dominated by organic pores have the lowest resistivity, medium resistivity was found in samples mainly containing intraparticle pores and intergranular pores, and intercrystal pore dominated samples have the highest resistivity. (2) Brittle mineral content has positive correlation with rock's resistivity, as well as porosity and connectivity of intergranular pores. Low brittle mineral content suppresses electric double layers where ions exchange with clay minerals, ultimately resulting in increasing rock resistivity and decreasing rock polarizability. (3) High TOC content (>3%) is the most critical factor contributing to low organic shale resistivity, as increasing TOC content improves pore structure and reduces resistivity. The isomorphic substitution of the organic matter also serves to enhance shale conductivity and polarization. (4) Pyrite content has negative correlation with resistivity, while positive correlation with polarizability. Our study thus provides a series of basic parameters for shale gas exploration and exploitation, and discusses the geological features of significance for shale gas development in the Longmaxi Formation of this area. This is achieved by the determination of rock electrical parameters, which can be used to judge organic carbon and brittle mineral content to delineate shale areas as ‘brittle shale sweet spots’. In addition, in the process of monitoring hydrofracturing, the electrical characteristics of the low resistivity anomaly correspond to the possible existence of a fracture or fracture zone, which could provide more reliable geological structure information for the later exploitation of shale gas.