Characterization, controlling factors and evolution of fracture effectiveness in shale oil reservoirs

Abstract

Fracture effectiveness is of great significance for seeking shale oil exploration targets. The fracture effectiveness and primary controlling factors in the Cretaceous Qing-1 Member shale oil reservoir in Gulong Sag, China, was evaluated based on precise fracture characterization on core samples and thin sections. The evolution sequence of fracture effectiveness was subsequently discussed. Structural fractures, diagenetic fractures and overpressure fractures are found in the study area, where structural fracture is the most important ones, which is characterized by high density, large aperture and weak filling. Structural fractures were formed in three stages and underwent two filling events, while fractures growing at the third stage with no filling are the most effective ones. Diagenetic fractures can be classified into bedding-parallel fractures and stylolites, where the former is partly filled, and the latter is fully filled by dark minerals. Hence, these fractures can act as seepage barriers. The overpressure fractures have large apertures but short length and are insensitively filled. Fracture effectiveness varies greatly as a function of cementation, fracture growth period, dissolution, tectonic activity at later stage, abnormal high pressure and current tectonic stress field. Fractures developing at earlier stage are commonly filled, which tends to be ineffective. Structural inversion at a later stage, abnormal high pressure and dissolution can reopen fractures and improve their effectiveness. The fractures parallel to the current maximum principal stress with high dip-angle have large apertures and good effectiveness. • Proposed a quantitative characterization method for fracture effectiveness. • Evaluated the effectiveness of fractures in shales of Gulong Sag, China. • Discussed the controlling factors and evolution of fracture effectiveness.