Characteristics and main controlling factors of structural fracture development in deep buried hill reservoirs of basement metamorphic rocks: a case study of B block, Bohai Bay Basin

Structural fractures are pivotal in enhancing the physical properties of buried hill reservoirs within basement metamorphic rocks, thereby increasing oil and gas productivity. However, there is limited comprehensive, multi-scale investigations into the development characteristics and main controlling factors of such fractures. Taking B block in the Bohai Bay Basin as a case study, this research systematically synthesized thin-section, core, and imaging logging data to delineate the characteristics of structural fractures in buried hill reservoirs within basement metamorphic rocks. This study identified the principal controlling factors and revealed the influencing factors and development patterns of these fractures. The study area mainly developed shear fractures, followed by tensile fractures, with lesser occurrences of oblique and vertical fractures. The fractures exhibited a high overall filling degree, primarily filled with mud followed by carbonate. Four groups of structural fractures in near-EW, NE-SW, NW-SE and NNW-SSE directions were identified. Among them, the near-EW fractures were more developed, indicating that the strong compressive environment during the early stage of Indosinian was crucial for widespread fracture development. The development of structural fractures in the study area was mainly controlled by rock mechanics properties, tectonic activity, weathering processes, reservoir physical properties, and lithology. The effectiveness of structural fractures was mainly controlled by factors including the angle between the maximum horizontal principal stress direction and the orientation of structural fractures, tectonic movement, fracture filling and dissolution. Vertically, the influence of weathering decreased with depth in buried hill reservoirs, with local development of dissolution pores along faults in the internal zones. Rocks with different mechanical properties showed different degrees of fracture development. Rocks with higher brittleness index exhibited higher degree of fracture development. Laterally, the line density of structural fractures in the core of the anticlines was greater than that in the flanks, providing favorable conditions for the development of structural fractures when reservoir porosity and permeability were within suitable ranges.