Research on the Formation Mechanisms and Characterization of Fractures in Deep Metamorphic Rock Buried Hills in the Bohai Sea: A Case Study of Bozhong a Oilfield

The Bozhong A oilfield is the first deep metamorphic rock buried-hill reservoir with 100 million-ton reserves discovered in the Bohai Sea in recent years. Influenced by tectonic movements, this oilfield exhibits complex fracture formation mechanisms and distribution patterns. To address the challenges of multi-phase tectonic evolution, strong reservoir heterogeneity, and complex dominant reservoir distribution in the study area, this study integrates logging and seismic data to analyze the relationships between fracture formation stages and tectonic stresses on the basis of the regional tectonic evolution study, and describes the mechanism of fracture formation under the control of complex faults, further revealing the distribution characteristics and development patterns of buried hill reservoirs in the study area. The research results indicate the following: (1) Affected by multiple tectonic movements, fractures mainly formed during the Indosinian and Yanshanian periods. Fracture density shows a significant negative correlation with the distance to Indosinian or Yanshanian faults, with differential controls by paleogeomorphology. Highland-slope transition zones exhibit the most intensive fracture development; (2) A steepness attribute method constrained by paleogeomorphology enables quantitative characterization of weathering zone fractures, allowing subdivision of reservoirs in well blocks 2/4 into three categories: Class I reservoirs (distributed near paleo-high aeras and activity faults), Class II reservoirs (distributed near paleo-slope areas), and Class III reservoirs (distributed near paleo-low areas and staed away from active faults). The research results have guided the efficient implementation of 23 wells, achieving an average daily oil equivalent of 300 tons per well, which has important guiding significance for the deployment of well locations in the region and the study of similar buried-hill reservoirs.