Formation Mechanism Of High-Pressure Thin Reservoirs In The Tertiary Saline Lacustrine Facies In The Puwei Area,Dongpu Sag

Thin reservoirs are the main reservoirs in the Dongpu Sag,Bohai Bay Basin,which is a typical Tertiary saline lacustrine facies hydrocarbon-rich sag in China.However,the formation mechanism of thin reservoir is complex.Conventional geochemical methods including Rock-Eval,gas chromatography-mass spectrometry?GC-MS?,compound specific carbon isotopic analysis and the latest electrospray ionization?ESI?Fourier transform ion cyclotron resonance mass spectrometry?FT-ICR MS?combined with geological and well-logging analyses were used to reveal the formation mechanism of different types of typical thin reservoirs in the Puwei Sub-sag,which was the main subject of the study.There are four types of thin reservoirs in the Puwei Sub-sag,such as subsalt fault-lithologic reservoir with normal temperature and pressure in the middle-shallow slope of the Sub-sag,subsalt tight fault-lithologic reservoir with high temperature and pressure in the deep sub-sag,lithologic reservoir with normal temperature and pressure in the shallow of the sub-sag and intra-salt tight fault-lithologic reservoir with high temperature and pressure in the slope of the sub-sag.It was observed that the crude oils bearing typical characteristics of saline lacustrine oils such as predominance of phytane to pristine and C₃₅-hopane to C₃₄-hopane,enrichment of gammacerane,?-carotane and long-chain n-alkanes(nC₃₇ and nC₃₈),and low extent of isomerization of steranes.The oils are also featured by two-stage carbon isotope distribution of n-alkanes.Abundant sulfur compounds are detected from the oils analyzed by positive-ion ESI FT-ICR MS for the first time.The composition and distribution as well as thermal evolution characteristics of the sulfur compounds in the oils with different maturities are revealed,which were confirmed having a close genetic relationship with the immature oils in the area.Three oil families have been recognized based on detailed oil-oil and oil-source rock correlations.Crude oils in the high position of Wenmingzhai-Weicheng Structure are immature and mixed oils primarily derived from Es₃^L and Es₄^U source rocks.Tight reservoir oils,such as oils from Pushen 20 and Wei 42 wellblocks,are moderate-high maturity oils mainly from Es₃^M source rocks with weakened water salinity and/or stratification during deposition.Crude oils in the Pucheng area,which are low-moderate maturity oils with relatively low sulfur content,are mainly from Es₃^M source rocks mixed with a small amount of Es₃^L and Es₄^U.The quality of source rocks,the distribution of gypsum-salt rock,physical properties of reservoir,sandstone thickness,faults and the related fractures are the main controlling factors of hydrocarbon migration and accumulation in the Puwei Sub-sag.Controlling factors varied with different types of thin reservoirs.The quality of source rocks,physical properties of reservoir and sandstone thickness are controlling factors in the formation of tight reservoir with high pressure developed in the deep sub-sag.Four accumulation models are developed for the typical thin reservoirs.The first model is characterized by“slope”,“mainly lower-generation and upper-accumulation”,“fault-sandbody transportation system”,“multiperiod charging”,“normal temperature and pressure”and“mixed oils”.The second model is featured by“deep sub-sag”,“limited fluid dynamic field”,“mainly self-generation and self-accumulation”,“high temperature and pressure”and“tight reservoir”.The third one is characterized by“shallow sub-sag”,“early charging”,“fault and subtle pathway transportation system”and“secondary lithologic reservoir”.The last one is featured by“slope”,“salt-salt sealing”,“limited and free fluid dynamic fields”,“self-generation and self-accumulation”,“high temperature and pressure”and“tight reservoir”.