Maturation of Green River Shale Kerogen with Hydrous Pyrolysis: Characterization of Geochemical Biomarkers and Carbon Isotopes

Abstract:

To fully understand controlling factors of organic compound generation during oil shale maturation, and systematically assess associated carbon isotope values, a series of hydrous pyrolysis experiments are performed. Kerogen was isolated from Green River shale by a set of acid treatment. Experiments are conducted at 350 ^°C and 300 bars of total pressure with running time of 24, 48 and 72 hours, respectively. In each experiment, the reactor contains 1.5 grams of kerogen and 30 grams of deionized water. After experiments, gaseous products are removed under cryogenic conditions for chemical and carbon isotope analyses (GC-IRMS). The bitumen product is retrieved and separated into saturated hydrocarbons, aromatics, resins, and asphaltenes (SARA) by HPLC before subsequent analyses (GC, GC-MS, and IRMS).

The gaseous compounds from experiments consisted of CO₂ and C₁ to C₄ hydrocarbons. Semiquantitative analysis indicates the yield of n-alkanes decreases with carbon number, with CO₂ being more abundant than all alkanes. The δ¹³C value of alkanes increases with molecular weight, with CO₂ having the highest value. Methane and ethane become enriched in ¹³C with time.

In bitumen products, gravimetric analysis has shown that the abundance of aromatics increases with time, while that of asphaltenes decreases. After 72 hours, the weight percentages of saturated hydrocarbons, aromatics, resins and asphaltenes are 2.6, 42.3, 40.1, and 15.0, respectively. High resolution GC-MS results indicate low kerogen maturation after 72 hours using saturated biomarker compounds as thermal maturity indicator, such as 22S/(22S + 22R) of C₃₁ to C₃₅ homohopanes, tricyclics/17(H)-hopanes, and Ts/(Ts + Tm). Bulk carbon isotope value of bitumen decreases with time, with 2.5‰ lighter than original kerogen after 72 hours. In terms of different groups, saturated hydrocarbons and resins become depleted in ¹³C with longer reaction time, while aromatics and asphaltenes become enriched in ¹³C.

Experiments with longer reaction time and under different physicochemical conditions are underway. That would facilitate a better understanding of oil and gas generation and carbon isotope systematics during kerogen maturation processes for effective conventional and unconventional exploration.