Formation of inter-salt overpressure fractures and their significances to shale oil and gas: A case study of the third member of Paleogene Shahejie Formation in Dongpu sag, Bohai Bay Basin

Abstract: Taking the inter-salt organic-rich shales in the third member of Paleogene Shahejie Formation (Es3) of Dongpu sag in Bohai Bay Basin as an example, the origin of overpressure, development characteristics, formation and evolution mechanism, formation stages and geological significance on shale oil and gas of overpressure fractures in the inter-salt shale reservoir were investigated by means of thin section identification, scanning electron microscopy observation, analysis of fluid inclusions, logging data analysis, and formation pressure inversion. The results show that overpressure is universal in the salt-lake basin of Dongpu sag, and under-compaction caused by the sealing of salt-gypsum layer, pressurization due to hydrocarbon generation, transformation and dehydration of clay minerals, and fault sealing are the 4 main factors leading to the occurrence of overpressure. The overpressure fractures are small in scale, with an average length of 356.2 μm and an average underground opening of 11.6 μm. But they are densely developed, with an average surface density of 0.76 cm/cm2. Moreover, they are often accompanied by oil and gas charging, and thus high in effectiveness. Overpressure fractures were mainly formed during two periods of large-scale oil and gas charging, approximately 25–30 Ma ago and 0–5 Ma ago. Inter-salt overpressure fractures play dual roles as the storage space and migration paths of shale oil and gas. They contribute 22.3% to the porosity of shale reservoir and 51.4% to the permeability. They can connect fracture systems of multiple scales, greatly improving the quality of shale reservoir. During the development of shale oil and gas, inter-salt overpressure fractures can affect the extension and morphology of hydraulic fractures, giving rise to complex and highly permeable volumetric fracture networks, improving hydraulic fracturing effect and enhancing shale oil and gas productivity.