Fracture-pore facies types and enrichment models of shale oil (tight oil)- A case study of Lucaogou Formation in Jimusaer Sag, Junggar Basin, China

Abstract: In tight reservoirs, fractures and pores play a crucial role in oil migration, accumulation, and production. However, there is a scarcity of research that concurrently examines both pores and fractures to understand their combined impact on the migration and enrichment of shale oil (tight oil). This study, guided by multidisciplinary theories such as petroleum geology, structural geology, and unconventional oil and gas accumulation theory, innovatively introduces the concept of "fracture-pore facies" and categorizes the reservoir fracture-pore facies into 20 distinct types. Through X-ray diffraction, the mineral content of the Lucaogou Formation reservoir was analyzed, and different mineral contents were observed. The fracture-pore coupling relationships in the reservoir were investigated through core observation, high-pressure mercury injection, cast thin sections, scanning electron microscopy, conventional logging, and image logging, identifying the types of fracture-pore facies in the study area. Physical simulation experiments were conducted to observe the migration and enrichment characteristics of oil in different fracture-pore facies. Based on experimental phenomena related to oil saturation, fracture opening, fracture density, permeability, porosity, and physical simulation of oil migration and accumulation, the fracture-pore facies in the study area were divided into three categories: the first category includes seamless-macropore facies and microfracture-medium-large pore facies; the second category includes multistage microfracture-mesopore facies and seamless-small mesoporous facies; the third category includes microfracture-micropore facies and seamless-micropore facies. Through the enrichment and accumulation models of different types of fracture-pore facies, combined with actual geological data from the study area, three oil enrichment patterns were summarized from the spatial perspective of different types of fracture-pore facies coupling: ① Seamless and medium-large pore coupling rich oil; ② Composite fracture-pore coupling rich oil mode; ③ Single microfracture-micropore coupling rich oil mode. The fracture network or fracture-pore network formed by the coupling of fractures with matrix pores in shale oil (tight oil) can significantly improve reservoir properties and control oil migration, enrichment, and exploitation. The types and classification of fracture-pore facies in shale oil (tight oil) provide important guidance for the study of shale oil (tight oil) migration, accumulation, and production.