Carbon isotopic characteristics of hydrocarbon gases from coal-measure source rocks—A thermal simulation experiment

Abstract

Gaseous hydrocarbon geochemistry research through a thermal simulation experiment in combination with the natural evolution process in which natural gases were formed from coal-measure source rocks revealed that the δ¹³C₁ values of methane vary from light to heavy along with the increase of thermal evolution degree of coal-measure source rocks, and the δ¹³C₂ values of ethane range from −28.3‰ to −20‰ (PDB). δ¹³C₂ value was −28‰ ± (R₀ = 0.45% − 0.65%) at the lower thermal evolution stage of coal-measure source rocks. After the rocks entered the main hydrocarbon-generating stage (R_o = 0.65% − 1.50%), δ¹³C₂ values generally varied within the range of −26‰ – −23‰ ±; with further thermal evolution of the rocks the carbon isotopes of ethane became heavier and heavier, but generally less than − 20‰

The partial carbon isotope sequence inversion of hydrogen gases is a characteristic feature of mixing of natural gases of different origins. Under the condition of specially designated type of organic matter, hydrogen source rocks may show this phenomenon via their own evolution.

In the lower evolution stages of the rocks, it is mainly determined by organic precursors that gaseous hydrocarbons display partial inversion of the carbon isotope sequence and the carbon isotopic values of ethane are relatively low. These characteristic features also are related to the geochemical composition of primary soluble organic matter.