Types, characteristics and effects of natural fluid pressure fractures in shale: A case study of the Paleogene strata in Eastern China

Taking the Paleogene organic-rich shales from the Dongying Sag and Zhanhua Sag of Jiyang Depression in Bohai Bay Basin and Northern Jiangsu Basin in Eastern China as examples, this paper researches the types and characteristics of natural fluid pressure fractures and their effects on hydrocarbon primary migration. The study shows that fluid overpressure is the main reason for the formation of natural fluid pressure fractures. The natural fluid pressure fractures include three types, early sluiced fractures, bedding-parallel vein fractures, and hydrocarbon generation and expulsion fractures. Early sluiced fractures have the typical characteristics of snaking morphology, bedding-parallel vein fractures are filled with fibrous calcite vein and coexist with organic matter, and hydrocarbon generation and expulsion fractures generated by hydrocarbon-generating pressurization of kerogen are the key to episodic expulsion of organic-rich shale. Fractures of multiple origins, such as natural fluid pressure fractures, bedding fractures and structural fractures, accumulate gradually, forming interconnected fracture networks which are significant primary migration pathways and reservoir space, act as the seepage channel in the process of multi-scale seepage and are the premise of realizing volume fracturing in shale reservoirs.