The Characteristics Of Pore Development And Evolution Of Shale In The Fourth Submember Of The Third Member Of The Shahejie Formation In Shichang Sub-sag,nanpu Sag

The unconventional oil and gas resources,regarded as the important alternative energy of the conventional oil and gas resources,have great resource potential.One of the core tasks of unconventional shale oil and gas exploration is to search for reservoirs.Hence,the difficulties and key points of shale reservoir studies are the identification,classification and quantification of shale micro-nanopores,and the comprehension of pore development and evolution laws,which are of great significance to clarify the occurrence mechanism and laws of shale oil and gas.In this paper,taking the Es₃⁴ organic-rich shale in the Shichang sub-sag of the Nanpu Sag in the Huanghua Depression of the Bohai Bay Basin as an example,field emission scanning electron microscopy,high-pressure mercury intrusion,and nitrogen adsorption experiments were used in combination with organic rock identification,rock pyrolysis,X-ray diffraction(XRD)and testing of oil and gas family components to study the characterization of shale pore development characteristics,focusing on the morphology,structure,size,and pore size distribution of shale reservoirs.The pore space of the Es₃⁴ shale in the Shichang sub-sag of the Nanpu Sag can be divided into mineral matrix pores(intergranular pores,intergranular pores,dissolved pores),organic pores,and fractures.Shale porosity is between 5.8%-12.1%,with an average of 8.8%;permeability is between 1.27-4.63m D,with an average of 2.59m D.The main distribution range of pore diameter is 4-100nm;the average pore throat radius is 0.044-0.359?m,and the average is 0.16?m.High-pressure mercury intrusion analysis reveals that the pore size distribution is 0.005-100?m and the average pore throat radius is 0.044-0.35?m.Low maturity shale samples from the study area were selected to conduct a semi-closed and semi-open hydrocarbon generation thermocompressure simulation experiment,combined with scanning electron microscope observation and high-pressure mercury injection-nitrogen adsorption method,which were used to observe the samples after the thermal simulation experiment at six temperature points under the electron microscope and to characterize the full aperture.The qualitative and quantitative aspects of the characteristics of shale pore evolution during the thermal simulation evolution were studied systematically.At the same time,the organic geochemical analysis tests such as rock pyrolysis were performed on the samples after the thermal simulation experiment to better connect the pore evolution withhydrocarbon generation thermal evolution.Further the controlling factors of the pore development and evolution of shale were discussed.Combined with the pore development characteristics and the pore evolution characteristics of the samples after thermal simulation experiment,the shale pore-developmentof the Es₃⁴shale of the Shichang sub-sag was established,which can be divided into four stages:immature-low maturity stage(Ro<0.7%)is controlled by compaction and the volume of the pore system is reduced;mature stage(0.7%<Ro<1.2%)is controlled by compaction and the hydrocarbon generation of organic matter together,and the total pore volume decreases first and then increases.The retention effect of liquid hydrocarbons and asphalt has a significant effect on the pores.At the high maturity stage(1.2%<Ro<2.0%),the pore system is controlled by the hydrocarbon generation of organic matter and mineral transformation,and the total pore volume and specific surface area rise slowly first,and then rise rapidly;the pore system tends to stabilize in the over-maturity stage.