Relationship Between Bond Structure Evolution And Gas-liquid Products During Thermal Simulation Of Source Rocks

As the source of oil and gas,high-efficient development and utilization of source rocks is very important.Thermal simulation of source rocks is one of the commonly used methods to study the evolution of source rocks.There is a difference between the thermal simulation of source rocks under high temperature and short time and the geological evolution of source rocks under low temperature and long time.This study took marine source rocks as the research object,and analyzed the thermal evolution process of source rocks from multiple perspectives.Such as the covalent bond structure of source rocks,the concentration of free radicals in source rocks,the composition of thermally simulated oil and gas products and their relationship with bond structures.And the difference between the geological evolution process of source rock and the thermal simulation process was compared.The main conclusions are as following:（1）Both thermal simulation and geological evolution process are dominated by free radical reactions,and there are free radical-induced behaviors in the thermal simulation process,which leads to differences between thermal simulation and geological evolution.During the thermal simulation,350?C may be a characteristic or transition temperature in the thermal simulation process.Before and after this temperature,the evolution of covalent bonds has undergone a precise jump.Compared with aromatic carbon-related covalent bonds in source rocks,aliphatic carbon-related covalent bonds are more likely to be induced by free radicals generated during thermal simulations,so it reaches a level at a lower temperature（such as 350?C）similar to that in the later stage of geological evolution.Aromatic carbon is more stable,and its corresponding covalent bond is not easily affected by free radical induction during thermal evolution,so the evolution law in the higher temperature range is similar to the geological evolution stage.（2）The DTG curve of thermal evolution of marine source rocks is divided into five sub-peaks,corresponding to five different types of chemical bonds.Among them,the peak area ratios of aliphatic carbon-heteroatom bonds and aliphatic carbon-aliphatic carbon bonds decreased with the increase of temperature,aliphatic carbon-aromatic carbon bonds showed a convex curve decreasing,and aromatic carbon-hydrogen bonds showed an increasing trend.The difference between the geological evolution process of source rocks and the thermal evolution process is that the aromatic carbon-hydrogen bond decreases with the increase of maturity,which indicates that thermal simulation cannot completely reproduce the geological evolution process.The process of gas bond breaking in the evolution of source rocks is mainly related to the third and fourth stages of covalent bond breaking.The formation of methane is mainly the breaking of aliphatic carbon-aliphatic carbon bonds and aliphatic carbon-aromatic carbon bonds,while the hydrocarbons（C₂-C₄）and aromatics are mainly formed by aliphatic carbon-aliphatic carbon cleavage.（3）In the initial stage of the thermal simulation,gases are first generated,mainly oxygen-containing inorganic gases and sulfur/nitrogen-containing gases.When the thermal simulation temperature is higher than350?C（before 450?C）,a large amount of hydrocarbon gas begins to be produced,in which the methane content is the highest.And the proportion of methane in the gas increases with increasing temperature.The production process of oil is different from that of gas,before 350?C was the main temperature stage of oil production.Compared with the gas generation process,the evolution of the covalent bond structure of the source rocks involved in the generation of oil is more severe.In the higher thermal simulation temperature stage（greater than 400?C）,the polycondensation reaction between the generated oil and gas products begins,resulting in a decrease in the oil yield.The production of isoparaffins in the thermal simulation process shows that the carbon ion reaction also occurred during the evolution of the source rock due to the existence of inorganic substances.However,the content of normal paraffins in the thermal simulation stage is more than that of isoparaffins,regardless of gas or liquid.It shows that the thermal simulation process of source rock is dominated by free radical reaction.