Accumulation models and exploration practice of tight oil and gas in the deep sag areas of continental fault basins:a case study of Huanghua depression in Bohai Bay Basin
Fault basins are the important oil-gas production bases in China.With the deepening of exploration work,the proven con-ventional oil-gas resources in the middle and shallow layers(depth＜3 500 m)exceeds 70％,the difficulty of discovering large-scale oil-gas reservoirs increased,and the exploration focus gradually shifted to deep sag area.Based on studying the genetic mechanisms,development conditions of lithologic reservoirs,and hydrocarbon generation conditions in the deep sag area of Huanghua depression,the paper systematically summarizes the accumulation conditions of tight oil and gas in the deep sag areas of fault basin.The research suggests that the fan delta plains,fan delta fronts,and nearshore underwater fans are developed near the provenance,as a result of being controlled by fault activities,lake level fluctuations,and source supply intensity.Multiple types of lithologic reservoirs are de-veloped in the central part of the lake basin,including sublacustrine fans,slope fans,far-shore underwater fans,slump turbidites,and regressive delta fronts.When the burial depth is greater than 3 500 m,the reservoirs are gradually densified under the control of compaction.Deep sag areas have a large accommodation space during lake flooding,which is conducive to the formation and preser-vation of organic-rich shale,and thus has better source rock conditions.The accumulation models for Kongxi slope,Banqiao sag,and Qikou main sag are established,respectively,and it is proposed that the fan delta front in Member 2 of Kongdian Formation in Kongxi sag,the basin-floor fans and slope fans in Member 3 of Dongying Formation in Qikou main sag are favorable target areas.Well CT1 and Well HT1 were deployed and drilled in 2023,from which a high-yield industrial oil flow of 118.9 t/d and 33.2-35.7 t/d were obtained respectively,indicating two fields of hundred million ton scale for increasing reserves were discovered.This confirms the exploration potential of large-scale tight lithologic oil-gas reservoirs in the deep sag areas of Huanghua depression,providing a crucial resource base for the sustainable development of old oil fields.
Huanghua depressionPaleogenedeep sag areagenetic mechanismlithologic oil-gas reservoirhydrocarbon accumula-tionexploration practice
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