Geochemical Characterization Of Hydrocarbon Generation And Expulsion From Thermal And Pressure Simulation Of Coal

One coal sample collected from the Middle Jurassic Xishanyao Formation of Zhundong Beishan coal mine,Xinjiang Uygur Autonomous Region has been performed pyrolysis simulations in semi-open system in order to explore the characteristics of hydrocarbon generation and expulsion from coal during thermal evolution.The target temperatures were settled at 250°C,275°C,300°C,320°C,340°C,350°C,360°C and375°C with 1°C/min heating rate and 48 hour holding time.The pressure was adjusted accordingly based on empirical correlation in natural burial situations.The gaseous products from coal simulation were qualitatively and quantitatively analysed by gas chromatography-mass spectrometry.Quantity of hydrocarbon and non-hydrocarbon gases is small at low mature and mature stages,while the percentage of hydrocarbon gases increases sharply at high maturity stage,especially the percentage of methane.The?¹³C values of the methane and ethane decrease initially,then increase as the maturity increasing.The?¹³C values propane increase continuesly,while stable carbon isotope values of CO₂ changes with fluctuation.The pathway of thermal evolution could be divided into three phases based on the yield of pyrolysis products.Phase 1?250°C?T?275°C?was characterized by low oil yield,which mainly indicated remain hydrocarbons formed during geological history?thermal evaporation?.The newly generated hydrocarbons and pre-existing hydrocarbons were expelled by overcoming the intermolecular forces.Phase 2?275°C?T?320°C?reflected simultaneous hydrocarbon generation process.The oil yields increased with heating temperatures and reached the maximum yield at 320°C with expelled and residual oil yield of 3.18 and 13.31 mg/g-coal,respectively.Phase 3?T>320°C?was characterized by decreasing hydrocarbon generation potential with minimum expelled oil yield of 2.58 mg/g-coal and residual oil yield of 7.01mg/g-coal present at 360°C.Large quantity of biomarkers was generated from coal kerogen at phase 2,and these compounds were largely decomposed at phase 3.The maturity parameters,such as Pr/n-C17,Ph/n-C18,rearrangement of sterane,C29 sterane??/???+???,Ts/Tm,alkylnaphthalenes,alkylphenanthrenes,alkyldibenzothiophenes and alkylchrysenes,have good correlation with heating temperatures,which increase rapidly at high mature stage.The results indicated that coal maturation process under experimental conditions has some similarity with traditional source rocks,but it also has its own particularity.Laboratory simulation has provided a practical tool to investigate hydrocarbon generation potential and evolution mechanism of coal source rocks.