Tracing Pathways And Accumulation Mechanisms Of Multi-source And Multi-period Hydrocarbons In The South Slope Of Dongying Sag

In continental rifted basins,the high-frequency nature of sedimentary cycles and the diversity of sources ultimately result in the complexities of hydrocarbon migration and accumulation processes.This complexity is primarily characterized by the multi-source nature of reservoir composition,multi-stage nature of charging,multi-mechanism nature of migration and accumulation,and multi-type nature of traps.Although the Dongying Sag has entered the stage of high-ultra-high exploration level,the understanding of the hydrocarbon charging episodes and the nature of crude oil from different episodes in the south slope of Dongying Sag is still poor,especially the contribution proportion of the Es3 and Es4 source rocks,and the mechanism of hydrocarbon migration and accumulation.All these elements have seriously restricted the further detailed exploration and discovery of remaining hydrocarbon resources in this area.This paper takes the south slope of Dongying Sag as an example,based on the theories of the hydrocarbon migration and accumulation dynamics,and integrates geology-geochemistry,diagenetic product and numerical simulation to establish the mass fraction maturity evaluation model to clarify the crude oil charging characteristics of typical reservoirs,and apply multi-methods and multi-approaches to tracing the hydrocarbon preferential migration pathways.Based on this,the influence of early crude oil charging on later crude oil accumulation is elucidated,and the mechanisms and models of multi-source and multi-stage hydrocarbon accumulation are summarized.The crude oil maturity in the Ek interval is generally high,and the early products of the generation window are not captured in the present traps,and the crude oil maturity zone is dominated by the products formed in the maturity zone from 0.85%to1.0%.The maturity of crude oil in the Es4 interval varies widely,and some crude oil can be regarded as early maturity oil,and the maturity range of crude oil is between0.6%and 0.9%,which refers to a continuous charging process.The crude oil in the Es3interval is dominated by peak oil.The maturity range is narrow,mainly between 0.6%and 0.75%,and is also a continuous charging process.Some oil samples,such as C104,show normal maturity characteristics in terms of relative biomarker concentrations but high maturity characteristics in terms of absolute biomarker concentrations,which are a mixture of episodic and continuous charging processes.The crude oil in the Ek interval mainly originates from the Ek source rocks,and the biomarker concentration and maturity parameters show that its maturity is much higher than that of the Es crude oil.The crude oil from the Es4 and Es3 source rocks are distinctly characterized,with the former predominating.The crude oil in the Es4 interval primarily comes from the Es4source rock,which is rich in gammacerane and C₃₅ homohopane,and developed in a strongly reducing environment.The crude oil in the Es3 interval is mainly from the Es3source rocks,developed in a brackish-freshwater environment.The tricyclic terpene ratios in the Es3 source rocks are also lower than those in the Es4 source rocks,except for the lower concentration of gammacerane and C₃₅ homohopane.The crude oil in the Es2 interval is mainly derived from the Es4 source rocks.There are mainly two episodes of hydrocarbon charging in the study area.Among them,the early crude oil charging occurred during the deposition stage of the Dongying Formation（32 Ma-25 Ma）,with a relatively small amount of charged hydrocarbons;the late crude oil charging occurred from the end of the deposition stage of the Guantao Formation to the present day（8 Ma-present）,which corresponded to the peak of hydrocarbon generation and expulsion of source rocks.The transmissibility of the sand bodies was quantitatively evaluated by sandstone percentages.Except for the Niuzhuang Sag and the Chenguanzhuang Fault Zone,the sand bodies in the Es3z interval are basically stacked and connected,and the fluid connectivity between the sand bodies is good,which can be regarded as a good carrier bed.The faults are strongly active during the deposition stage of the Guantao Formation-Minghuazhen Formation,and have a good matching relationship with the hydrocarbon charging time,providing a conduit for the vertical migration of hydrocarbons.Based on Monte Carlo simulation,a new method is established to realize the prediction of hydrocarbon column height sealed by the fault,which is in good agreement with the actual hydrocarbon column height sealed by the fault.Comprehensively using logging data,formation water chemical analysis,organic geochemical analysis and numerical simulation,there are four hydrocarbon preferential migration pathways in the study area.Among them,the migration pathways from Niuzhuang Sag to Wangjiagang Fault Zone,Guangli Buried Hill Draping Structure Zone and Bamianhe Fault Zone have the strongest ability to supply and capture hydrocarbons,while the migration pathways from Niuzhuang Sag to Chenguanzhuang Fault Zone is more likely to develop small fault blocks due to the low connectivity probability of sand bodies.The porosity of reservoirs that have experienced early and late crude oil charging is higher than that of reservoirs with or without first-stage crude oil charging,but the reservoirs with first-stage crude oil charging have the highest relative permeability.The charging of crude oil can alter rock wettability,and early crude oil charging facilitates late hydrocarbon migration and accumulation.Mature source rocks control the origin of hydrocarbons,preferential migration pathways control the distribution areas of reservoirs,and differences in fault sealing control the location of reservoirs.These factors have resulted in the distribution pattern of hydrocarbons showing the characteristics of“preferential migration,and differential enrichment”.Crude oil from the Ek interval is characterized by the accumulation in deep-buried reservoirs,in which hydrocarbons migrate along the sand body-unconformity toward the basin margin and accumulate in inverse fault-hosting traps.The crude oil generated from the Es4 source rocks migrates upward and outward along the fault-sand body and accumulates in the fault or stratigraphic traps.