Key factors of marine shale conductivity in southern China—Part I: The influence factors other than porosity

Abstract

The complicated response mechanism of shale gas requires that a systematic pilot study is conducted on the influences of resistivity log responses to help with subsequent experimental design, physical derivations and resistivity simulations. The purpose of this research is to clarify the main controlling factors of the resistivity log responses and their semi-quantitative relationship in certain marine shale gas formations. Comprehensive analyses on large core test datasets, including mineralogical, petrological, geochemical and other data, from the Longmaxi-Wufeng Formation and the corresponding resistivity logging curves are carried out in 4 shale gas areas. This research indicates that in low-porosity systems, the mineral composition directly impacts the resistivity. Carbonates with large intergranular pores and even intragranular pores increase the resistivity. The hydrophilic nature of clay decreases the resistivity. When organic matter is not carbonized, the organic matter and biosilica will further increase the resistivity. Pyrite layers will cause a large drop in resistivity (to less than 5 Ω m). The resistivity is lower where organic matter carbonization has occurred in the studied reservoir. Mineral components, especially clay minerals and organic matter, are the main controlling factors of the conductivity of low-porosity systems. In addition, we propose that elemental logging can be used to characterize the comprehensive effect of low-porosity s systems on the resistivity of the reservoir and can accurately reflect the resistivity. These results will help to develop future studies on the relationship between the electrical conductivity and saturation of shale gas rocks, which is of great significance. • Combining the data of 14 wells in four shale gas blocks, the resistivity characteristics of marine shale are systematically and quantitatively analysed. • Shale mineral composition is the non-porous factor that mainly affects resistivity, among which carbonate minerals and combinations of biosilica and (uncarbonized) organic matter will increase the resistivity. • The layered pyrite greatly reduces the resistivity, but the dispersed pyrite has little effect on the resistivity of marine shale. • A semi-quantitative characterization technology for non-porous systems is proposed (ER curve), and the effect of mineral composition on resistivity is semi-quantitatively characterized.