Quantitative Evaluation Of Shale Reservoir Space In The Shahejie Formation Of Dongying Sag

In recent years,with the successful exploration and development of shale oil and gas in the field of unconventional oil and gas geology in the world,the storage space of shale reservoirs has become a hot research topic.Studying the pore characteristics of shale reservoirs and the vertical evolution of pores can provide geological bases for storage properties of shale reservoirs,the occurrence of shale oil and gas and the evaluation of resources,and the prediction of favorable accumulative intervals.Therefore,accurate holographic characterization of shale reservoir spatial structure characteristics and the evolution of pore structure during diagenesis have become key issues in reservoir research.The Shahejie Formation shale in the Dongying Sag is not only a good source rock in the continental basin of eastern China,but also a favorable shale oil reservoir.Based on the summarization of domestic and foreign scholars'research results,this paper takes the shale reservoir of the upper member of the fourth member of Shahejie Formation in the third member of Shahejie Formation in Dongying Sag as the research object.Based on the original natural evolution shale samples after extraction,the shale reservoir characteristics,pore structure characteristics,pore structure evolution and controlling factors of pore development were studied by using reservoir characterization techniques,such as slice observation,TOC analysis,rock pyrolysis,XRD diffraction,field emission scanning electron microscopy,low temperature N₂ adsorption,high pressure mercury,nuclear magnetic resonance,etc.The research results show that the Shahejie Formation shale has the characteristics of“three highs,two lows and multiple types”.Carbonate minerals and clay minerals and organic carbon are high in content.The degree of thermal evolution and the content of detrital minerals are low.The types of organic matter are mainly type II₁.Field emission scanning electron microscopy revealed six kinds of pore fractures in the Shahejie Formation shale,including intergranular pores,intragranular pores,intercrystalline pores,dissolution pores,organic nanopores and micro-cracks.Among them,intergranular pores and the intergranular pores of clay minerals are the most important pore types.The pore morphology is complex,and the slit-like pores and the narrow-necked ink-bottle pores are relatively developed.The microporous structure of shale was characterized by high pressure mercury injection,low temperature nitrogen adsorption and nuclear magnetic resonance.The results show that the shale microstructure is gradually deteriorated by the layer?layered?block.The stratified shale has a wide pore size distribution and develops many micron-level interstitial seams.The bulk rock pore size is mainly distributed below 100 nm.The full pore size distribution curve has a three-peak characteristic,and the peak pore size is mainly distributed between 2?6nm,20?80nm and 5000?50000nm.It is shown that the pores with pore diameters in these three intervals are relatively developed.Pores having a pore size of less than 25 nm provide a pore volume of about 50%and a specific surface area of 93.3%,followed by 25-100nm,the 100-1000 nm pores are poor.The shale porosity,unit TOC pore volume and unit TOC specific surface area of the study area are similar in shape with depth,showing an"S"shape.From shallow to deep,it has experienced the processes of continuous decrease,increase,and then decrease.In addition to the difference in the increase or decrease,the volume of TOC pores in the four types of pores(micropores,small pores,mesopores and macropores)varies with depth.Throughout the evolution phase,the main specific surface area is provided by the micropores.In addition to the main pore volume provided by micropores and macropores during the peak period of hydrocarbon generation,the contribution of micropores to pore volume is dominant in other stages.The pore development of shale reservoirs is mainly controlled by the mineral and organic matter content.The pore volume and specific surface area were linearly positively correlated with clay mineral content and organic matter content,and negatively correlated with carbonate mineral content.The abnormal high-porosity zone at the depth of3400?3750m is a favorable accumulation section of shale oil.It is recommended to consider this interval when drilling shale oil wells.