Charging Mechanism And Accumulation Model Of Mesozoic And Paleozoic Hydrocarbons In The Huanghua Sub-basin

The Mesozoic-Paleozoic(M-P)in the Huanghua Sub-basin underwent muti-stage tectonic evolution,with hydrocarbons heterogeneous enriched in multi-layers.This paper deeply discussed the charging mechanism and established the accumulation model starting from the differential enrichment of M-P hydrocarbons and taking the coupling relationship between hydrocarbon charging dynamic and resistance as the core.Based on the origin of buried hill and trap features,the M-P reservoirs are divided into four types,i.e.the fault block-landform type,fault block-nose type,anticline type and fault block-anticline type.The fault block-landform reservoir is mainly distributed around the Indosinian paleo-uplift in the northern area,and O is the main production layer with good physical properties of crude oil,high salinity of formation water and humid gas.The fault block-nose reservoir is widely distributed with hydrocarbons enriched in multi-layers and great changes in fluid properties.While the anticline and fault block-anticline reservoirs are concentrated in the southern areas,in which the Mesozoic(Mz)is rich in oil with poor physical properties,the Permian(P)is enriched both in oil and gas with good physical properties of oil and good preservation conditions,and the Ordovician(O)mainly produced natural gas.The genetic types of oil and gas were classified using carbon isotope,biomarker parameters and spectrogram data,then oil-and gas-source correlation were carried out combined with the geological and geochemical features of source rocks.The results show that3 sets of source rocks(i.e.Es₃,Ek₂ and C-P)correspondingly formed 3 types of oil and 2 types of gas.Lower Paleozoic hydrocrabons in fault block-landform reservoir are sourced from Es₃source rocks.M-P hydrocarbons are supplied dominantly by Paleogene source rocks and supplemented by C-P source rocks in the fault block-nose reservoir,moreover,oil and gas are both sourced from Es₃ source rocks in the north,while oil from Ek₂and gas from C-P source rocks in the south.Whereas Paleozoic oil and gas are coal-derived oil and gas from C-P source rocks in the anticline reservoir,and M-P oil sourced from Ek₂ source rocks and gas from C-P source rocks in the fault block-anticline reservoir.Furthermore,influenced by the formation temperature and pressure and evolution degree of source rocks.Oil-type gas is almost formed by kerogen pyrolysis,while coal-type gas can be formed by oil and kerogen pyrolysis.Based on the hydrocarbon transportation framework and the relationship between source and reservoir,this paper divided hydrocarbon charging models and established the quantitative methods to calculate the charging dynamic and resistance for different models,and then clarified the charateristics of charging dynamic and resistance and their coupling relationship in typical buried hills.The results show that there are three charging models developed in the M-P reservoirs in the sub-basin including the vertical charging model under fault-controlled pressure difference(I),the lateral charging model under fault-controlled pressure difference(II)and the vertical charging model under source-controlled pressure difference(III).The dynamic force consists of buoyancy and overpressure,and the maximum buoyancy of M-P oil and gas is 0.044 MPa and 0.048 MPa respectively.Meanwhile,charging dynamic varies among different cahrgign models.The model I is mainly developed in fault block-nose reservoirs with the pressure between the pressure after hydrocarbon migration along fault and the reservoir as the charging dynamic,which can be calculated by Bernoulli equation.The model II widely developed in reservoirs except anticline type and the charging dynamic is the pressure difference between Paleogene souece rocks and M-P reservoirs.While the model III is only developed in the anticline reservoir,and the excessive pressure difference between coaly source rocks and M-P reservoirs provides dynamic for hydrocarbon charging.The M-P reservoirs are with low-porosity and(especially)low-permeability.Dissolution pore and micro-fracture are well developed in Lower Paleozoic carbonate rocks,so the capillary force can be neglected and reservoir formation pressure is the main resistance for hydrocarbon charging.Meanwhile,the pore throat structure is well developed in Mz and C-P sandstones,therefore,except for formation pressure,the capillary force generated by pore throat structure is the important charging resistance,determined by interfacial tension,wetting angle and pore throat radius.Combined the thermal evolution of source rocks,fault activity,coupling of charging dynamic and resistance and charging periods,this paper restored the hydrocarbon accumulation process and established accumulation models.The qianmiqiao buried hill as the fault block-landform type only has one accumulation stage at Neogene-Quaternary(N-Q),and the accumulation model is“Paleogene source-fault controlled pressure drive-late period accumulation-rich in the hydrocarbon supply window”.Hydrocarbons occured two charging periods at Middle Triassic and N-Q and have undergone many secondary changes including crude oil cracking,biodegradation and oxidation,and the accumulation model is“double sources-mixed drive-two charging periods-rich in multi layers”in the Beidagang buried hill as the fault block-nose type.While the Wangguantun buried hill as the fault block-anticline type occurs two periods of oil and gas at late Early Cretaceous(LEC)and N-Q,and the accumulation model can be summarized as“double sources-fault controlled pressure drive-two charging periods-rich in the hydrocarbon supply window”.And the Wumaying buried hill as the anticline type has the same charging perieds with Wangguantun buried hill and the accumulation model is“coaly source-cource controlled overpressure drive-two charging periods-rich in Paleozoic”.