Quantitative analysis of variation of organic carbon mass and content in source rock during evolution process

According to the mass conservation principle of organic carbon, a mathematical model of organic carbon mass compensation coefficient is established to indicate the variation of organic carbon mass. Considering the influence of organic carbon hydrocarbon generation, hydrocarbon expulsion and rock weight loss, a mathematical model for calculating changes in organic carbon content is also established. Forward method is used to simulate the compensation coefficient of organic carbon mass and the organic carbon content value of source rocks in different evolution stages and hydrocarbon expulsion efficiency. The compensation coefficient variation depends on organic matter type, maturity degree and expulsion efficiency. The maximum compensation coefficients of types I, II1, II2 and III organic matter are 2.104, 1.360, 1.169 and 1.099, respectively. There exists a threshold value in hydrocarbon expulsion efficiency in source rocks, type I is 20%, type II1 is 30%, type II2 and type III is 60%. When the expulsion efficiency is less than this threshold value, the residual organic carbon content of the source rock is generally higher than the original organic carbon content; when the expulsion efficiency is greater than the value, the residual organic carbon content is generally lower than the original organic carbon content. Under the conditions of full expulsion, the maximum organic carbon reduction of types I, II1, II2 and III muddy source rocks is roughly 43%, 20%, 10% and 10%.