Laboratory and theoretical constraints on the generation and composition of natural gas

Abstract

Hydrous pyrolysis experiments were conducted at 125 to 375°C and 350 bars to constrain factors that regulate the generation and relative abundance of hydrocarbon and nonhydrocarbon gases during thermal maturation of Monterey, Eutaw, and Smackover shale. Thermogenic gas was generated at temperatures as low as 125°C and increased in abundance with increasing temperature. The relative abundance of individual hydrocarbons varied substantially in response to increasing time and temperature reflecting the chemical processes responsible for their formation. The hydrocarbon fraction of low maturity gas produced via primary cracking of kerogen was composed predominantly of methane. With increasing thermal maturity, the onset of bitumen generation produced longer-chain hydrocarbons causing a decrease in the relative abundance of methane. At high levels of thermal maturity, the absolute and relative abundance of methane increased due to decomposition of bitumen. In all experiments at all temperatures, carbon dioxide was the most abundant volatile organic alteration product. Carbon dioxide was produced directly from kerogen at low thermal maturity and via the decomposition of bitumen and/or kerogen at high thermal maturity. During early stage alteration, kerogen likely represents the dominant source of oxygen in carbon dioxide while at high thermal maturities water may represent an abundant and reactive oxygen source. Hydrogen released during the disproportionation of water is likely consumed during hydrocarbon generation. Theoretical reaction path modeling suggests that the precipitation of calcite may effectively remove carbon dioxide from natural gas if a source of Ca is available within the rock. Thus, carbon dioxide-rich natural gas may be relatively pristine while methane-rich natural gas may reflect the occurrence of secondary reactions involving inorganic sedimentary components. Kinetic analysis of the experimental data indicates a narrow range of activation energies for the generation of C 1-C 4 hydrocarbons from the Monterey, Smackover, and Eutaw shales. Carbon dioxide generation from the Monterey and Eutaw shales is accounted for by a substantially broader range of activation energies. Application of these data to predict gas formation at temperatures and time scales typical of subsiding sedimentary basins suggests that C 1-C 4 generation is restricted to relatively high temperatures while carbon dioxide generation occurs at both low and high thermal maturities. Thus, in contrast to the bulk of C 1-C 4 generation which is predicted to occur after peak bitumen generation, production of carbon dioxide will occur before, during, and after the generation of liquid hydrocarbons.