The Control Effect and Mechanism of Lithology on Fractures in Metamorphic Rock Reservoirs: A Case Study of The Metamorphic Rock Oil Reservoir in the Bohai Sea

Metamorphic buried hills in the Bohai Bay Basin are widely distributed and possess significant resource potential. Multi-scale fractures are developed in the metamorphic rock reservoirs in the Bohai Sea. The main rocks are plagioclase gneiss and monzonite gneiss. The characteristics of fracture development in the two types of lithology are different. The control effect and mechanism of different lithologies on fractures are not clear, which restricts the study of multi-scale fracture control factors and their spatial distribution law. Therefore, this study takes the Archaeozoic buried hill metamorphic rock in Jinzhou 25-1 South oilfield of Bohai Bay Basin as the research object and uses triaxial rock mechanics experiment and rock mechanics experiment based on CT scanning to study the fracture development characteristics of metamorphic rock samples with different lithology and the influence of mineral composition and distribution on fracture propagation. It is found that the elastic model of monzonitic gneiss is larger, the Poisson’s ratio is smaller, and the brittleness is higher. After the core is compressed, a small-scale fracture is formed at the edge, and then multiple micro fractures are formed in the middle. The connectivity of micro fractures and small-scale fractures is improved due to the interconnection of micro fractures and small-scale fractures; The characteristics of the cracks produced in the experiment are basically the same as those in the underground rock core, and they extend along the edge of the particles in a curved shape when expanding, and are formed in the tensile environment. In three-dimensional space, after compression, the core first forms a simple fracture, then ruptures into a complex fracture, and finally evolves into a very complex fracture network; In the process of fracture evolution, porosity, opening, length, connectivity, fractal dimension gradually increase, and fracture permeability increases. By comparing the development characteristics of fractures in different lithology, it is found that the common form of fractures in plagioclase gneiss is a small-scale fracture along with multiple micro-fractures, while 2 or 3 small-scale fractures coexist in rock, and multiple micro fractures are developed around, showing a more complex network distribution and better connectivity for the latter one. Because the brittleness of K-feldspar is better than that of plagioclase and the content of brittle minerals in monzonitic gneiss is high, the fractures in monzonitic.