Evolution characteristics of saturated hydrocarbons of enclosed organic matter in carbonate minerals in Tieling limestone under high temperature and high pressure

Abstract

The enclosed organic matter chiefly releases lower carbon-number n-alkanes under high temperature and high pressure, while the kerogen mainly produces higher carbon-number n-alkanes. The residual hydrocarbons generated by both kerogen and enclosed organic matter in the Tieling limestone contain abundant tricyclic terpanes, pentacyclic triterpanes and steranes, but the contents of tetracyclic terpanes and 25-norhopane are lower. The residual enclosed organic matter shows the same distribution characteristics of n-alkanes, steranes and terpanes as that of the original bitumen A, i. e., the higher contents of triterpanes and tetracyclic terpanes, the higher ratios of 25-norhopanes over regular hopanes and markedly degraded steranes. By comparing the residual hydrocarbon, residual enclosed organic matter and original enclosed organic matter, it can be concluded that steranes and terpanes in the residual hydrocarbons are produced mainly by the kerogen and subordinately by the residual enclosed organic matter, the steranes and terpanes do not enter into the residual enclosed organic matter, and the thermal evolution of the residual enclosed organic matter maintains its unique character. Furthermore, pressure retards the pyrolysis of higher carbon-number alkanes and influences the isomerization ratios of C₂₉-steranes, making 20S/(20S+ 20R) lower under the higher pressure than that under lower pressure. Higher pressure retards the thermal evolution of organic matter.