Exploring links between thermal maturity and electrical properties of organic-rich shales

Electrical resistivity is widely used as an effective proxy for detection of organic matter and hydrocarbons in shales. Recently, with more organic-rich shales being studied, it has been noted that resistivity follows a non-linear evolution with thermal maturity, influenced by the changes in conductivity of the liquid and solid phases and their distribution within the rocks. At the high end of thermal maturities relevant for hydrocarbon exploration, the sediments undergo chemical and structural reorganization involving loss of hydrogen and oxygen and aromatization of the organic component leading to a dramatic decrease in the overall electrical resistivity. This influence, especially at thermal maturities consistent with and beyond the gas generation window is not accounted for in petrophysical log interpretation and the petrophysical properties of over-mature shales are poorly documented in the literature. We show examples of hydrocarbon prospective shales characterized by low electrical resistivity of the organic rich sections and explore the mechanisms that influence resistivity. By integrating petrophysical, petrological, and nanoanalytical observations we show that the anomalous resistivity is related to a conductive, connected network of partially graphitised bitumen and not to commonly assumed conductive accessory minerals such as pyrite. This interpretation is verified in several case studies on prospective organic-rich shales that have been exposed to high thermal maturation induced by either deep burial conditions (Appalachian Basin, USA; Sichuan Basin, China), or by contact metamorphism (Beetaloo Sub-basin, Australia).