Fracture propagation in metamorphic rocks and the influence of mineral composition: a case study of buried-hill reservoir of Taipaleoyu in Jinzhou 25-1 South Oilfield, Bohai Bay Basin

The buried hill has great exploration value and has become a hot spot of exploration in Bohai Bay in recent years. To study the fracture propagation characteristics of metamorphic rocks in buried hill in the Bohai Bay under stress, the Archean metamorphic rocks from the buried hills in the Jinzhou 25-1 South Oilfield were studied. Real-time X-ray CT scanning uniaxial compression tests on three types of rock specimens, i.e., monzogneiss, plagiogneiss, and migmatitic gneiss were performed to analyze the fracture propagation characteristics. Quantitative mineral analyses were conducted on damaged samples to study the influence of mineral composition on fracture propagation. Results indicate that, under the uniaxial compression, vertical high-angle initial cracks at smaller angles relative to the stress direction are more prone to propagation than horizontal ones. The yield deformation stage before peak strength is the crucial phase for fracture propagation during specimen loading. Under the same strain conditions, monzogneiss specimen showed the most developed fractures, followed by plagiogneiss and migmatitic gneiss specimens. The major fractures propagated mostly along minerals and mineral interfaces with low fracture toughness. Additionally, the formation of more weak interfaces of mineral (such as those between potassium feldspar-plagioclase and potassium feldspar-biotite) in monzogneiss favored the fracturing, and the distribution of various minerals with similar contents had a positive impact on fracture propagation. After expansion of microfractures, they can be used as high-quality reservoir space and seepage channel. This study provided a guidance for predicting the degree of fracture development in buried hill structures under lithological control, and a certain reference value for formulating development plan of Jinzhou 25-1 South Oilfield.