Study on fracture distribution law of buried hill in deep metamorphic rock : A case study of Bozhong L oilfield in Bohai Bay Basin, China

AbstractThe buried depth of Archean metamorphic buried hill in Bozhong L Oilfield of Bohai Bay Basin is more than 4000 meters, and the matrix porosity and permeability are extremely low. As an effective reservoir space and seepage channel, natural fractures are the core controlling factors for oil and gas enrichment and high yield in buried hills of tight metamorphic rocks. Due to the influence of multi-stage tectonic movement and weathering, the development of buried hill fractures is complex. At the same time, due to the lack of core and imaging logging data, the study of fracture regularity is not systematic. In this study, the data of core, thin section, scanning electron microscope, imaging logging, conventional logging and production performance were comprehensively used to carry out conventional logging fracture identification, and the vertical and plane distribution of fractures and their influence on productivity were clarified. The conventional logging is calibrated by core and imaging logging, and four logging curves sensitive to fractures, such as resistivity difference, density, acoustic time difference and natural gamma, are optimized. The correlation degree between each parameter and fracture is calculated and weighted, and the fracture indication parameter curve is constructed. Compared with the results of fracture identification such as core and imaging logging in the study area and the dynamic data such as leakage, the accuracy of fracture identification based on conventional logging is more than 85 %. In the longitudinal direction, the strong weathered zone is dominated by weathering fractures, with high degree of fracture filling and strong reservoir heterogeneity. The sub-weathered zone develops structures and weathering fractures, which are transformed by dissolution and have high porosity and high permeability. The tight zone only develops regional structural fractures with low porosity and low permeability. The inner fracture zone is dominated by fault-related structural fractures, with low porosity and high permeability. The degree of fracture development is sub-weathered zone > strong weathered zone > inner fracture zone > tight zone. On the plane, fracture development is mainly controlled by faults. With the increase of distance from faults, the degree of fracture development decreases exponentially. The degree of fracture development is significantly positively correlated with productivity. The more developed the fracture is, the higher the productivity is. The research results can provide reference for the characterization of natural fractures in deep metamorphic buried hill reservoirs, and provide geological basis for the efficient development of deep metamorphic buried hill reservoirs in this area.Key wordsMetamorphic buried hills; Conventional logging; Fracture identification; Distribution law; Capacity; Bohai Sea