Geochemical evolution of kerogen type III during hydrous pyrolysis: A case study from the Damoguaihe Formation, Hailar Basin, China

Abstract

In order to evaluate changes in geochemistry, organic petrology, and chemical composition of petroleum and byproducts, an immature sample from the Damoguaihe Formation in Hailar Basin in China was examined using hydrous pyrolysis (HP) at the following temperature steps: 300 °C, 325 °C, 350 °C, 365 °C, 400 °C, and 450 °C. The final product from each pyrolysis step was assessed with several analytical methods including programmed pyrolysis, Gas Chromatography (GC), Gas Chromatography-Mass Spectrometry (GC-MS) and organic petrology. Rock-Eval pyrolysis revealed that the geochemical parameters including TOC, S2, HI, and Tmax changed as thermal maturity advanced. Both un-extracted and extracted organic matter (OM) showed a steady decrease in TOC, HI, and OI as the temperature increased. On the other hand, a general increase in Tmax values with increasing temperature was observed. Regarding GC analysis, fluctuations were observed in n-alkanes during pyrolysis. In general, lower molecular weight n-alkanes of <C17 increased between 325 °C and 400 °C, and heavier n-alkanes fluctuated with increasing temperature. Similarly, GC-MS results showed variations in biomarkers with increasing pyrolysis temperature. These variations could stem from the GC-MS instrument's inability to detect light hydrocarbons, specifically at higher temperatures beyond the oil generation window into the gas window. Some biomarkers, including Ts/Tm, C29H/C30H, Gammacerane Index (GI), C23 tricyclic terpane/C24 tetracyclic terpane showed that the Damoguaihe Formation has a carbonate lithology and was deposited in a freshwater environment with no water stratification. Based on organic petrography, an increase in temperature caused optical and structural changes in the macerals. Organic petrography also showed that the Damoguaihe Formation contains different maceral groups such as liptinite, vitrinite, and inertinite, and hence a combination of kerogen types II, III, and IV. Based on these observations, a deltaic depositional setting is envisaged for the Damoguaihe Formation in the study area, which is also confirmed by previous studies.