Mesozoic-Cenozoic tectonic controls on the formation of large-scale metamorphic rock buried-hill reservoirs in Bozhong Sag, eastern China

Buried-hill reservoirs are important targets for petroleum exploration. The number of buried-hill oil-gas fields with reservoirs hosted in metamorphic rock is low, and metamorphic rock buried-hill reservoirs deeper than 4,500 m (ultra-deep) are even rarer. Ultra-deep metamorphic rock buried-hill reservoirs are characterized by highly heterogeneous lithologies and structure and have complex formation mechanisms, but such reservoirs have received relatively little research attention compared with reservoirs hosted in other rock types and at shallower depths. In the Bozhong Sag in the Bohai Bay Basin, Archean metamorphic rocks are buried at depths of 4,500-6,000 m, and their anticlinal structures are thus defined as ultra-deep buried hills. Owing to their burial depth and compacted overburden, tectonism has been a key control on the formation of high-quality reservoirs by causing deformation such as fractures. In this study, using results of drill-core analyses, thin-section imaging, and rock mechanical tests, we investigated the distribution and formation mechanisms of high-quality reservoirs within the Bozhong 19-A metamorphic rock buried hill. We also examined the formation mechanism of fractures in the Bozhong 19-A buried hill with respect to early-stage initiation by compression and late-stage reactivation by transpression. Results show that intense compression-related overthrusting caused by collision between the South China and North China plates during the Indosinian (252-237 Ma) orogeny generated initial fractures in the metamorphic rock buried hill and the formation of numerous NW-trending nappe structures. Subsequent Yanshanian (163-66 Ma) sinistral transpression not only reactivated pre-existing fractures, but also resulted in the development of numerous new fractures. In addition, in low-stress regions such as anticline flanks and the lower parts of structures, compression closed the pre-existing fractures, leading to lower fracture densities. Results of acoustic emission mechanical testing indicate that Himalayan (66-0 Ma) extensional stress was insufficient to produce large-scale fracture systems in the Archean metamorphic rock, but was crucial for reactivating pre-existing early-stage fractures. Thus, the Himalayan period was a key interval for the formation of effective fractures. Therefore, rocks that underwent large-scale overthrusting during early-stage (Indosinian) fracturing and subsequent late-stage (Yanshanian and Himalayan) reactivation favoured the development of ultra-deep metamorphic rock buried-hill reservoirs. This study should provide guidance for petroleum exploration in buried hills in superimposed basins that underwent early-stage compression, late-stage transpression, and final extension.