LABORATORY AND THEORETICAL CONSTRAINTS ON THE GENERATION AND COMPOSITION OF NATURAL-GAS

Hydrous pyrolysis experiments were conducted at 125 to 375 degrees C a nd 350 bars to constrain factors that regulate the generation and rela tive abundance of hydrocarbon and nonhydrocarbon gases during thermal maturation of Monterey, Eutaw, and Smackover shale. Thermogenic gas wa s generated at temperatures as low as 125 degrees C and increased in a bundance with increasing temperature. The relative abundance of indivi dual hydrocarbons varied substantially in response to increasing time and temperature reflecting the chemical processes responsible for thei r formation. The hydrocarbon fraction of low maturity gas produced via primary cracking of kerogen was composed predominantly of methane. Wi th increasing thermal maturity, the onset of bitumen generation produc ed longer-chain hydrocarbons causing a decrease in the relative abunda nce of methane. At high levels of thermal maturity, the absolute and r elative 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 produ ced directly from kerogen at low thermal maturity and via the decompos ition of bitumen and/or kerogen at high thermal maturity. During early stage alteration, kerogen likely represents the dominant source of ox ygen in carbon dioxide while at high thermal maturities water may repr esent 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 prec ipitation of calcite may effectively remove carbon dioxide from natura l gas if a source of Ca is available within the rock. Thus, carbon dio xide-rich natural gas may be relatively pristine while methane-rich na tural 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 generati on of C-1-C-4 hydrocarbons from the Monterey, Smackover, and Eutaw sha les. Carbon dioxide generation from the Monterey and Eutaw shales is a ccounted 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 ca rbon dioxide generation occurs at both low and high thermal maturities . Thus, in contrast to the bulk of C-1-C-4 generation which is predict ed to occur after peak bitumen generation, production of carbon dioxid e will occur before, during, and after the generation of liquid hydroc arbons. Copyright (C) 1998 Elsevier Science Ltd.