Hydrocarbon generation from calcium stearate: Insights from closed-system pyrolysis

Abstract

The total organic carbon of carbonate sediments includes a 5%–45% contribution from acid-soluble organic matter, particularly carboxylic acid salts. However, there is little information available on hydrocarbon generation from carboxylic acid salts. The objectives of this study were to investigate the extent and timing of hydrocarbon generation from calcium stearate. Thus, gold-tube nonhydrous pyrolysis experiments of calcium stearate were conducted from 250 to 600 °C at 50 MPa. Infrared spectroscopy and δ13C testing were also performed on solid pyrolysate and gaseous products, respectively, to trace changes of functional groups and stable carbon isotopes.Pyrolysis of calcium stearate produced hydrocarbons, CO2, CaO, CaCO3, coke, and other inorganic compounds of low molecular weight. A Van Krevelen diagram indicated that calcium stearate falls in the same region as hydrogen-rich type I kerogen. The maximum yield of methane generated from calcium stearate was as high as that from North Sea oil. Hydrocarbon generation from calcium stearate was related to cracking of the alkyl moiety and similar to that occurring in pyrolysis of n-C18. As a consequence of kinetic carbon isotope fractionation, with increasing temperature CO2 becomes enriched in 13C and CaCO3 becomes enriched in 12C. Notably, CaCO3 with very low δ13C values is probably an effective proxy indicating hydrocarbon generation from carboxylic acid salts, if no other CaCO3 is produced via thermochemical sulphate reduction processes. The optimized kinetic parameters demonstrate that calcium stearate had higher methane generation activation energies than those of n-C18 and type I kerogen. Hence, methane generation from calcium stearate is delayed when extrapolated to geological conditions. This retarded methane generation illustrates the vital role of carboxylic acid salts in the generation of deep oil and gas. Future research should concern the abundance of carboxylic acid salts in carbonate formations and the depositional environments that facilitate the formation of carboxylic acid salts.