Fracture characteristics under the coupling effect of tectonic stress and fluid pressure: A case study of the fractured shale oil reservoir in Liutun subsag, Dongpu Sag, Bohai Bay Basin, Eastern China

Based on the integrated study on core description, fluid inclusions and basin modeling, and considering the coupling effect of tectonic stress and fluid pressure, the characteristics of fractures were analyzed in the inter-salt fractured shale oil reservoirs of strong overpressure in upper Sha 3 member (Es3) in Liutun subsag, Dongpu Sag, Bohai Bay Basin. The results show that: (1) Most of fractures in this reservoir are structural or hydraulic, with scattered occurrence, even horizontal hydraulic fractures in strong overpressure area; (2) Overpressure changes the differential stress and principal stress direction by changing the poroelastic response of rock and the seepage force of pore fluid, and thus influencing the type and occurrence of fractures; (3) Structural and hydraulic fractures can be divided into seven types according to their stress states. The development of those fractures includes 3 stages. The transverse tension and longitudinal tension fractures are mainly developed in the first stage, tectonic shear fractures mainly in the second and third stages, and hydraulic fractures mainly in the early phase of the second stage before tectonic uplift. When tectonic stress and overpressure are both strong, the type and occurrence of fractures are diverse.