Study On Preservation Conditions Of Mesozoic And Paleozoic Buried Hill Reservoirs In The Bohai Bay Basin

The Mesozoic and Paleozoic buried hill reservoirs in the Bohai Bay Basin are widely distributed and experienced multi-stage tectonic processes.The study of reservoir preservation conditions is of great significance to further oil and gas exploration of buried hill.On the basis of previous research results,this paper analyzes the distribution and source of oil and gas in Mesozoic and Paleozoic buried hill in the Bohai Bay Basin by geological,geochemical,seismic and logging data,discusses the key preservation factors such as caprock,tectonic uplift and faulting,analyzes the dynamic evolution characteristics in caprock and fault sealing of typical buried hill oil and gas reservoirs,and establishes a comprehensive evaluation standard of preservation conditions.The study shows that the oil and gas Phase states and sources of Mesozoic and Paleozoic buried hill in the Bohai Bay Basin are different.The Mesozoic and Lower Paleozoic are dominated by oil reservoirs,mainly from Paleogene source rocks,while the Upper Paleozoic are dominated by gas reservoirs,mainly from Carboniferous-Permian source rocks.There are five sets of mudstone caprocks including the Carboniferous-Permian Benxi-Shanxi Formation and the Upper Shihezi-Shiqianfeng Formation,Mesozoic,Paleogene and Neogene.The Carboniferous-Permian caprocks have strong sealing ability,while the Neogene mudstone has poor sealing ability and can only seal the petroleum reservoir.The early oil and gas reservoirs suffered from late tectonism and were seriously lost.The buried hill reservoirs were mainly formed at the end of Paleogene or Neogene-Quaternary.Faults affect the integrity of caprocks,and the configuration and sealing of faults and caprocks control the vertical distribution and preservation of buried hill reservoirs.Among them,the C-type fault-cap combination pattern is most conducive to the enrichment and preservation of buried hill reservoirs.Select the Wangguantun Middle and Paleozoic buried hill and Beidagang Ordovician buried hill oil and gas reservoirs in Huanghua Depression to analyze their preservation conditions.Both types of oil and gas reservoirs belonged to closed-type fault-cap configuration buried hill reservoirs in the early accumulation period,and belonged to unclosed-type fault-cap configuration buried hill reservoirs in the late accumulation period.The overlying mudstone caprock of buried hill oil and gas reservoir is pure in quality,and the displacement pressure is relatively high nowadays.The chemical characteristics of formation water reflect the overall good preservation conditions of oil and gas reservoir.The formation of sealing ability of Kongdong Fault rock in Wangguantun area during the late accumulation period is consistent with the period of hydrocarbon generation and expulsion,which can effectively seal oil and gas.In the late stage,the activity rate of Gangxi Fault in Ordovician buried hill of Beidagang is relatively high in most areas,which mainly plays the role of oil and gas migration.By selecting parameters such as cumulative thickness of mudstone,mud-to-ground ratio,displacement pressure,fault sealing,formation water type and mineralization degree,combined with the specific characteristics,the corresponding weights are given,the weight coefficients of each evaluation parameter are determined,and the comprehensive evaluation criteria for the preservation conditions of buried hill reservoirs are established.Among them,the preservation conditions of Wangguantun Permian and Mesozoic buried hill oil and gas reservoirs are evaluated as good,and the preservation conditions of Ordovician buried hill oil and gas reservoirs in Beidagang are evaluated as medium.