Prediction of effective fractured reservoirs in metamorphic buried hills based on paleo-present stress field coupling: a case study of Bozhong 19-6 Gas Field, Bohai Bay Basin

The Archean metamorphic buried-hill reservoirs in Bozhong 19-6 Gas Field, Bohai Bay Basin are deeply buried, with complex fracture formation mechanisms and distribution patterns. Research and prediction methods for effective fractured reservoirs are still lacking, seriously restricting the efficient development of this gas field. Based on seismic, core, well logging, and other data, fracture development mechanisms and distribution characteristics of reservoirs were clarified through multi-stage paleo-tectonic stress field analysis. The current stress distribution characteristics in the study area were simulated using the finite element method. Finally, a prediction method for effective buried-hill fractured reservoirs was established based on the coupling of paleo-present stress fields. The results showed that: (1) Fracture development in the study area was controlled by multi-stage tectonic movements. Indosinian-Yanshanian compression formed the initial fractures, and Himalayan extension and strike-slip movements reactivated and modified the early fractures. (2) Structurally, the study area could be divided into western compression and eastern strike-slip zones, with the core of the compression zone and the strike-slip zone being favorable areas for fracture development. (3) The current stress field characteristics showed that current maximum horizontal principal stress direction in the study area was generally NEE75°-SEE105°. Influenced by regional dextral strike-slip faults, the current maximum horizontal principal stress direction showed a SEE-E-NEE directional change from west to east. (4) Based on the prediction using paleo-present stress field coupling, effective fractured reservoirs were classified into three categories. Class Ⅰ area, distributed near the core of the compression zone and the strike-slip zone, showed the most developed effective fractured reservoirs. Class Ⅱ area was located in high positions of eastern well area near the strike-slip zone. Class Ⅲ area, situated in structurally lower positions, exhibited relatively poor development of effective fractured reservoirs.