TG/plus—a pyrolysis method for following maturation of oil and gas generation zones using Tmax of methane

Thermogravimetric Fourier transform infrared spectroscopy (TG-FTIR) analyses were carried out on two sets of isolated kerogens covering a wide maturity range from low mature (0.46% R_o) through the end of oil and gas generation (maximum R_{o = 5.32%}). Data onweight percent and T_max for evolution of methane, volatile tars, ethylene, SO₂, NH₃, CO₂, and CO are reported. The T_max of methane shows the most consistent response to increasing maturation in both sets of samples. Results are comparable to those of whole rocks from an Alaskan North Slope well analyzed previously. The collective data for both whole rocks and isolated kerogens shows a generally linear correlation between %R_o and T_max of methane, with the exception of R_o of about 2.0% where a dip in the curve occurs. The slope of the correlation line was steeper for the predominantly terrigenous Wilcox kerogen than for more marine Colorado kerogen or for the Alaskan North Slope whole rock samples, probably reflecting differences in the chemical nature of various kerogen sets, which is also reflected by differences in the shapes of the pyrolysis curves of SO₂, CO₂, CO, H₂O, and ethylene. These preliminary data indicate that T_max of methane is a good maturation indicator for whole rocks and isolated kerogens up to an R_o of about 4%, which includes all of the wet gas and a considerable portion of the dry gas generation zones. This correlation was also observed for samples containing migrated bitumen, where it was not possible to obtain a reliable T_max for the volatile tar (S2) peak. The more terrigenous Wilcox kerogens also showed a good correlation of the T_max of ethylene with %R_o. T_max of ammonia evolution did not correlate with maturity and occurred 100–200°C lower than previously found for whole rocks, consistent with a whole-rock source of pyrolytic ammonia for Alaskan whole rock samples. HI and OI indices were calculated in several ways and plotted to reflect kerogen type as well as both the residual oil and gas generation potential. The ratio of pyrolyzable to combustible sulfur (evolved as SO₂) was independent of maturity and showed a clear difference between the more terrigenous Wilcox kerogens and the more marine Colorado kerogens.