Types and development patterns of traps in the Pinghu Formation of the Kongqueting Area in the Xihu Depression
摘要
In recent years, the Paleogene strata in the Kongqueting Area of the Xihu Depression in China have become a hot spot for oil and gas exploration. In this study, a comprehensive analysis of the distribution of sand bodies in the Pinghu Formation and the types of traps and oil and gas enrichment patterns controlled by faults in the Kongqueting Area of the Xihu Depression was conducted. This analysis was systematically carried out using 1200 km2 of 3D seismic data, well logging data and core data from the Kongqueting Area, along with a combined approach of core observation, well logging, and seismic interpretation. The study indicates that the Pinghu Formation in this area is primarily characterized by a deltaic sedimentary system influenced by tidal action, with two third-order sequence boundaries developed internally. The fault systems are categorized into three distinct patterns: parallel and co-directional, co-directional overlapping, and counter-directional overlapping, with the parallel and co-directional configuration being the most prevalent. Syndepositional fault activity significantly influences the direction and distribution of deltaic extension. Four types of traps have been identified, which are: lithologic traps with sandstone updip pinch-out, structural-lithologic traps with sandstone updip pinch-out and sealed by lateral faults, structural-lithologic traps formed by fault-related flexural zones, and structural-lithologic traps formed by topographic flexural zones. The presence of oil-source faults in the downdip direction of the trap is the fundamental condition for the oil and gas enrichment in lithologic traps. The depositional fill structure of the prototype basin during the Pinghu Formation period is well-preserved, hence the formation of lithologic traps is primarily a product of the structural morphology and depositional characteristics caused by syndepositional faults. The vertical migration of oil and gas along oil-source faults and short-distance lateral migration within fault-bounded fluvial sand bodies are the main pathways for hydrocarbon migration and accumulation. Influenced by the varying distribution characteristics of faults, differential activity, fault combination features, and their correlation with paleogeomorphology during the depositional period of the Pinghu Formation, the fault-fold belt in the southwestern region is the main controlling factor for the development of fault-lithologic traps. The stepped distribution of faults provides an excellent vertical conduit for the migration of oil and gas. In the lower slope zone, the development of up-dipping pinch-out lithologic traps is influenced by the presence of oil-source faults on the down-dip side, and the up-dip development of lithologic pinch-outs is a key controlling factor for oil and gas enrichment. In the higher slope zone, where fault activity is intense, the sealing capacity of faults on the up-dip side of structural-lithologic traps is the decisive factor for natural gas enrichment.