| In this paper,according to the classification of oil content grade of oil shale,three types of oil shale of high grade,medium grade and low grade are selected.The oil shale samples under different heating stages are obtained through the semi-open system thermal simulation experiment,and analyzed by X-ray diffraction,Rock pyrolysis,TOC,high-pressure mercury intrusion test and low-temperature nitrogen adsorption test to study the variation of mineral composition,thermal evolution of organic matter and evolution law of reservoir characteristics of oil shale and the difference between different grades of oil shale.Rock pyrolysis results show that the organic matter type of high-grade and medium-grade oil shale is type II1,the organic matter type of low-grade oil shale is type I,and the organic matter of the three original oil shale samples is in the immature or low-mature stage.X-ray diffraction analysis results show that in the original samples,the high-grade oil shale mineral composition in the original sample is mainly quartz(46.5%)and clay minerals(46.1%),with a small amount of pyrite(5.4%),feldspar(1.0%)and dolomite(1.0%),clay minerals are composed of illite(43%)and kaolinite(57%);the mineral composition of medium-grade oil shale is dominated by pyrite(56.8%),followed by quartz(18.6%)and clay minerals(19.8%),contains a small amount of feldspar(4.8%),clay minerals are composed of illite(100%);low-grade oil shale minerals are mainly composed of clay minerals(42.0%)and quartz(40.7%),contains a small amount of pyrite(5.6%),feldspar(8.8%),and calcite(2.9%).All grades of oil shale clay minerals lose weight at room temperature?185°C and475°C?520°C,and their content decreases.The relative content of illite and kaolinite in high grade oil shale,illite in medium grade oil shale,chlorite in low grade oil shale have little change with good thermal stability.The illite/smectite mixed-layer in low-grade oil shale is dehydrated at 475??520?to transform into illite,the relative content of the illite mixed layer decreases and the relative content of illite increases.During the heating process,the content of detrital minerals such as quartz and feldspar in high-grade and low-grade oil shale fluctuates up and down due to dissolution and the weight loss of other minerals.Pyrite in medium-grade oil shale transform into pyrrhotite by thermal metamorphism desulfurization at 400°C?520°C,pyrite content decreases and pyrrhotite content increases,other detrital minerals content fluctuated up and down during heating.According to the variation of TOC and rock pyrolysis parameters in each heating stage,the thermal evolution of organic matter in oil shale can be divided into three stages.Low temperature heating stage(room temperature?300°C):organic matter in oil shale is in immature or low-mature stage,a small amount of organic matter consumption;The main hydrocarbon-generation stage(300°C?475°C):organic matter is in the mature or over-mature stage,and the consumption of organic matter reaches the maximum at 400°C?475°C.When heated to 475°C,the organic matter in oil shale is basically consumed completely;High temperature heating stage(475°C?520°C):organic matter is in the over-mature stage,a small amount of generation and expulsion of hydrocarbon,and when heated to 520°C,all grades of oil shale basically no longer have the hydrocarbon-generation potential.The analysis results of high-pressure mercury intrusion test and low-temperature nitrogen adsorption test show that with the increase of heating temperature,the porosity,total pore volume,and average pore diameter of oil shale continue to increase,while the mesopore pore diameter increases and gradually transforms into macropores.In the high-temperature heating stage,the pores in oil shale are mainly macropores.There is a good coupling between the evolution of oil shale reservoir characteristics and the thermal evolution of organic matter:the total pore volume and porosity of oil shale increase rapidly during the main hydrocarbon-generation stage(300°C?475°C),and increase slowly in the low temperature heating stage(room temperature?300?)and high temperature heating stage(475??520?)with less organic matter consumption.As the maturity of organic matter increases,the consumption of organic matter improves the reservoir properties of oil shale.The total pore volume,porosity and average pore size of pore structure parameters are negatively correlated with TOC and Pg,and positively correlated with Tmax.After entering the over-mature stage,when the hydrocarbon-generation potential of organic matter is exhausted,the total pore volume,porosity and average pore diameter of the oil shale with better grade are larger,which indicates that the oil shale with higher organic matter abundance and stronger hydrocarbon-generation potential has stronger effect on improving reservoir properties during heating process.The semi-coke pore imaging characteristics of oil shale at different heating stages under SEM show that the influencing factors the evolution of oil shale reservoir characteristics are mainly the dehydration of clay minerals and the thermal evolution of organic matter.Dehydration reduces the volume of clay minerals and destroys the structure,forming inter-granular pores,marginal shrinkage pores,intra-granular pore and microcracks of clay minerals.The improvement of oil shale reservoir properties by thermal evolution of organic matter is mainly in two aspects.On the one hand,with the increase of organic matter maturity,the generation and expulsion of hydrocarbon from organic matter form pores and cracks within organic matter and organo-clay complex.On the other hand,the pyrolysis of organic matter generates organic acids to dissolve detrital minerals,forming dissolution pores and cracks,and with the pyrolysis of organic matter between detrital minerals,the pores between clastic minerals gradually develop.With the deepening of thermal evolution,the number of above-mentioned pores and cracks increases,the pore size and pore volume increase,and the connectivity becomes better. |
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