Abstract: The abundance and composition of organic carbon in source rocks continue to change with the increase of thermal evolution. In order to study the change rule of organic carbon composition, the concept of “organic carbon composition” was proposed, and a method was developed to calculate the relative contents of residual effective carbon, residual ineffective carbon, original effective carbon and original ineffective carbon in source rocks based on rock pyrolysis parameters. Based on low-cost rock pyrolysis analysis, this method can calculate the organic carbon composition of various source rocks in a fast and quantitative manner. This method was applied to the study of organic carbon composition of low-mature Mesozoic and post-mature Paleozoic source rocks in Yin′gen-Ejinaqi Basin, and achieved outstanding results. The results showed that for the low-mature source rocks, the residual organic carbon composition was not changed much compared with the original organic carbon composition, and the proportion of effective carbon only decreased slightly, while the proportion of ineffective carbon only increased slightly, indicating that the hydrocarbon conversion rate of low-mature source rocks was low, and only a small amount of effective carbon was converted to hydrocarbons; for the post-mature source rocks, the hydrocarbon conversion rate of effective carbon was extremely high, and most of the residual organic carbon was non-pyrolysable “dead carbon”, while only a very small proportion of effective carbon was converted to hydrocarbons. This study has realized the improvement of studies on organic carbon composition of source rocks from qualitative to quantitative, and can provide a basis for the study of thermal evolution characteristics of organic carbon in source rocks.

Key words: rock pyrolysis, thermal evolution characteristics, organic carbon composition, low-mature source rock, post-mature source rock