Structural, petrophysical and lithological characterization of crystalline bedrock buried-hill reservoirs: A case study of the southern jinzhou oilfield in offshore Bohai Bay Basin, north China

Crystalline bedrock has always been regarded as a restricted area for hydrocarbon exploration due to its large burial depth and deficiency of primary porous. In recent years, bedrock oil and gas have been explored in several basins across the world, while the characteristics and control factors of large-scale bedrock reservoirs have not been systematically studied. Taking South of Jinzhou oil field as an example, we clarified the reservoir characteristics based on the investigation of the cores, thin sections, physical property and well loggings, and discussed the controlling factors of high quality reservoir considering lithology, fault activity and the paleo-topography. The results show that although primary porous are less developed in bedrocks, the tectonic fractures, weathering fractures, pores among crushing particles and corrosion holes developed along fractures constitute rich reservoir spaces in the study area, and the average porosity can reach 6.7%. Dioritic gneiss and granitic gneiss are easier to form reservoirs than diabase for higher felsic contents and more brittleness; reservoirs along the main faults zone with strong activity are thick; in addition, high position of the paleo-topography develops thicker reservoirs due to the release of the stress and the strong weathering. Under the coupling action of weathering and tectonic activity, there developed weathered clay zone, weathered glutenite zone, weathered fracture zone, structural fracture zone, structural breccia zone and tight zone from top to bottom vertically. The present study shows that although there is no primary porosity in bedrocks, large-scale and quality reservoirs could be formed for brittle bedrocks under the coupling effects of strong tectonic activities and weathering. This recognition is of great significance for bedrock hydrocarbon exploration in the basins of high exploration degree all over the world. •The characteristics of crystalline bedrock reservoir are summarized.•The main controlling factors of the formation of high-quality reservoir were discussed.•Reservoir model of crystalline bedrock was established.