Natural fractures in a metamorphic buried hill reservoir, Bozhong 19–6 area, Bohai Bay Basin, China

Abstract

The Bozhong (BZ) 19-6 gas field in the Bohai Bay Basin is a fractured metamorphic reservoir with proven reserves of over 1000 × 108 m3. However, the fracture development in the Archaean metamorphic buried hill reservoir in the BZ19-6 area is still not clear. In this study, cores, thin sections, imaging logs and laboratory measurements were used to elucidate the influences of lithology, tectonics and weathering on the formation and development of natural fractures. Both tectonic fractures and dissolution fractures are well developed, and 96.8% of these are interconnected. Lithology exerts the primary control on fracture formation, while stress and weathering influence the development of the fractures. Leucogneiss, cataclasite and gneiss with high contents of brittle minerals are favorable for fracture formation. Buried hill reservoirs near faults commonly exhibit a high linear density of fractures due to greater stress perturbation. Based on the degree of weathering in the study area, a metamorphic buried hill reservoir may be divided vertically into three zones: a weathered fracture zone, an inner fracture zone, and the basement. Fracture linear density and fracture aperture decrease significantly with a decrease in the intensity of weathering, while paleo-topographic highs with more intense weathering have a higher fracture density. A reservoir evolution pattern is proposed that can be used for targeting the favorable reservoirs in the study area.