Dynamic Process Of Hydrocarbon Migration And Accumulation Of Lenticular Sandstones Of The Third Middle Member Of Shahejie Formation In Ninzhuang Subsag, Dongying Depression

Niuzhuang subsag,one of the biggest lithologic reservoir distribution area in Dongying depression,has a potential reserves of 8.287×10⁷ t.Lenticular sandbody reservoir is an important type of lithologic reservoirs.However,the particular hydrocarbon accumulation conditions of Niuzhuang create a unique hydrocarbon accumulation characteristic of the critical petroliferous formation of the third Middle Member of Shahejie Formation.The filling and accumulation mechanism of these reservoirs are so complicated,and the distribution is hard to controlled that restricts the development of petroleum exploration.This dissertation,based on the results of predecessors' research:(1) by integrating the systematic fluid inclusion analysis with delicate burial history analysis,analyzed the phases of hydrocarbon charging and determined the charging time of Dongying sag;(2) by using the fluid inclusion PVT modeling,reconstructed the paleo-pressure of hydrocarbon charging,analyzed the evolution of paleo-pressure;(3) based on the former analysis,reconstructed the paleo-fluid potential of different periods of hydrocarbon filling and accumulation;(4) integrating geochemistry,analyzed oil and source rock correlation; (5) at last,on the basis of the results of aforementioned research and the analysis of the porosity and permeability of sand bodies,integrating the seismic interpretation,discussed the charging forces and process of hydrocarbon charging and the controlling factors of oil saturation of the sand lens of the third middle member of Shahejie formation in Niuzhuang sag,established hydrocarbon accumulation model.The main conclusions are as follows:Three events of hydrocarbon charging had occurred here:the first phase is between 26.2-24.8Ma,the second is between 13.8-8.0 Ma,and the third is between 8.0-0.0Ma, respectively.The main periods of hydrocarbon charging,however,were in the second and the third stage.Especially,the ultimate distribution of oil and gas here is controlled by the third hydrocarbon charging.The uplift of strata restrained the hydrocarbon generation,therefore no oil accumulated in the sandbodies between 24.8-13.8Ma.The third phase was only detected in the central of Niuzhuang subsag reflects the source rocks in the center turn into hydrocarbon generation and expulsion period earlier than the rocks over the edge of the subsag.Overpressure in the Niuzhuang subsag began around 11.4Ma,and continued to this day.The evolution of paleo-pressure experienced three phases,corresponding to the three hydrocarbon accumulation.The first phase of 26.2-24.8Ma is characterized of normal system,and the second phase of 13.8-8.0Ma began to appear low abnormal overpressure.The third period of 8.0-0.0Ma presented medium-strong abnormal overpressure.The accuracy of the temperature of fluid inclusions and PVT modeling proved to be reliable. The homogenization temperature of the third phase of hydrocarbon charging is about 22℃higher than present-day formation temperature,reflects Niuzhuang subsag experienced a automatic cooling geological process in the thermal subsidence phase.The third phase of PVT modeling results has the same linear relationship with present-day formation pressure.The three phases of paleo-fluid potential are different.Lenticular sandbody reservoir is distributed in the relatively lower potential zone of 4-7KJ/Kg,3-9.5KJ/Kg,and 12-21KJ/Kg respectively for the three hydrocarbons charging.For each phase of accumulation,higher potential area is often in the center or around the center,and the lower potential zone is at the edge of tectonic belt in the subsag.Hydrocarbon accumulation zone is in lower potential areas, as well as relatively low potential areas and even higher potential areas.The main controlling factors of lenticular sandbody reservoir in the middle section of Sha 3 Member in Niuzhuang subsag is source rock,faults,sedimentary facies,fluid potential.Source rock is the primary factor,and there is poor accumulation in the depth below the limits of mature source rocks.The oil has a character of mixed source,and the first and second charging is from the bottom section of Sha 3 Member or the upper section of Sha 4 Member through the faults or sub-seismic faults,however,the third charging is also from the source around itself.The oil and gas is controlled by faults or sub-seismic faults obviously.On the premise of plenteous source and effective transport,facies and potential coupling determine the oil saturation of sands.The coupling relation between them is more adequate,the oil saturation is higher.The petroliferous sands are mainly distributed in the porosity of 12%-24%and a penetration rate of 2×10^-3μm² -100×10^-3μm² within the reservoir,and the better properties of sands,the higher degree of saturation.Fluid potentials and sub-seismic faults also have a coupling relation on the hydrocarbon saturations of the lenticular sandy bodies.Sub-seismic faults can create a relatively lower fluid potential lithological trap,which are always favorable of hydrocarbon migration and accumulation.There are three reservoir forming patterns for lenticular sandbody reservoirs in the middle section of Sha 3 Member:fault-sands pattern,sub-seismic faults pattern,and sands pattern. Fault-sands pattern was developed in non-mature source rocks,and sub-seismic faults pattern and sands pattern was developed in the mature source rocks.