Evolution of a microfracture network induced by hydrocarbon generation during experimental maturation of organic-rich lacustrine shale

Abstract

Microfracture and matrix-pore networks evolve within organic-rich shales in response to thermal maturation. Here, we report observations on the Triassic Chang 7 Member black shale (total organic carbon [TOC] = 26.8 wt%; Ordos Basin in China) at varying stages of thermal maturation induced by miniature-core-plug gold-tube pyrolysis. Observations via scanning electron microscope and X-ray computed microtomography reveal that microfractures in the highly organic-rich shale are well developed throughout the entire oil-generation window; however, matrix pores are relatively underdeveloped. An abundance of bedding-parallel microfractures and especially lens-shaped microfractures are newly formed at EasyRo = 0.71%−0.91% (EasyRo—equivalent to vitrinite reflectance), whereas bedding-perpendicular microfractures appear only at EasyRo ≥1.14%, both as a consequence of fluid overpressure and mechanical disequilibrium compaction. Compaction is further recorded by morphological variation of Chrysophyceae cyst fossils in the shale upon thermal maturation. The microfracture geometries and abundant oil droplets trapped within the microfractures indicate these open microfractures may potentially serve as oil retention and storage space underground. The later development of bedding-perpendicular microfractures that cut across the bedding-parallel microfractures creates an interconnected three-dimensional fracture network and, therefore, facilitates significant hydrocarbon (HC) expulsion and/or transport via vertical migration from the generation zone. Our experimental findings improve the current understanding of HC primary migration and expulsion in organic-rich shales and suggest a possible new mechanism of shale oil retention and storage in thick high-TOC intervals and/or in overpressured basins. Moreover, our study sheds light on the dynamic process of fracturing that may accompany in situ underground conversion of medium- to low-maturity shale oil.