Geochemical And Hydrocarbons Accumulation Characteristics For The Saline Lacustrine Oils From The West Slope Of Dongpu Sag,Bohai Bay Basin

Dongpu Sag is a typical Tertiary saline lacustrine sag with a great petroliferous potential in thesoutheastern margin of the Bohai Bay Basin,Eastern China..The origin and formation mechanism of the hydrocarbons are complex.Conventional geochemical methods such as Rock-eval and GC-MS combined with ESI FT-ICR MS were utilized to unravel geochemical and hydrocarbons generation and accumulation features of the crude oils in the West Sope of the Dongpu Sag.The distinguished geochemical characteristics of the oils are as the following:?1?Typical features of saline lacustrine oils such as predominance of phytane to pristine and C₃₅-hopane to C₃₄-hopane,enrichment of gammacerame,?-carrotane and Long-chain n-alkanes(nC₃₇7 and nC₃₈);?2?The majority of the crude oils discovered are less mature oils situated in the middle-shallow horizons while the minority of the oils discovered are high to over-high mature,located in deep horizons;?3?Partial of the deep oils were experienced thermochemical sulfate reduction?TSR?.Two oil families can be subdivided,one is the crude oils in the Mazhai Tectonic Belt,characterized by strong reducing,saline and/or water column stratification;another one is the crude oils in the Huqing-Xingzhuang and Liutun tectonic zone,featured a little different from the Mazhai,a relatively weak reducing,saline and/or water column stratification.Oil-oil and oil-source rock correlation show that the crude oils in the Mazhai,Huqing and Xingzhuang-Liutun tectonic belts were mainly derived from the Es₃^LEs₄^U source rocks with a burial depth>3000m in the adjacent sub-sages?Mazhai,Haitongqi and Liutun Sub-sag,respectively?,and the oil mixing with the Es₃^UEs₄^M genetic oils couldn't be excluded for partial of the oils.Hydrocarbon accumulation features for the crude oils in the West Slope mainly include:?1?Types of the oil reservoirs are diverse,including fault blocks reservoir,fault block combined with lithologic reservoir,burial hill reservoir and unconventional compacted reservoir.?2?The reservoir is generally thin interbed with the deep reservoirs compacted and tight oils developed probably.?3?There are two stages of hydrocarbons accumulation with the early one dominated.?4?Gypsum salt rocks have a strong controlling on hydrocarbons migration and accumulation as well as reservoir temperature and pressure.Totally two hydrocarbons migration and accumulation models were established:?1?"Proximal and early oil-charging-fault transportation-fault-associated trap model"for the middle to shallow oil pools;?2?"Self-generation and self-accumulation tight oils"model for the oil pools in deep horizons of depocenter.This study is helpful for revealing hydrocarbons generation and accumulation mechanisms for the crude oils in the Dongpu West Slope and instructive for the further petroleum exploration in the area.