Isotope rollover in shale gas observed in laboratory pyrolysis experiments: Insight to the role of water in thermogenesis of mature gas

Abstract

Pyrolysis experiments on type II kerogen with maturities equivalent to vitrinite reflectance (Ro) between 0.7% and 1.4% were conducted in the presence and absence of added water to determine whether the isotope rollover effect observed in natural highly productive shale gas plays can be reproduced in the laboratory. Isotope rollover occurs when isotope δ values (i.e. measures of the abundance of heavy stable isotopes 2H or 13C) initially become less negative, as expected, with increasing thermal maturity but later display a reversal in trend becoming more negative with increasing maturity. It was found that H2O had a pronounced influence on the composition, yields and carbon and hydrogen isotopic ratios of thermogenic gases. Compared to gases from confined pyrolysis experiments without added water, lower overall quantities of hydrocarbon gases were generated from experiments with water added, although the relative abundance of methane among other gas species was higher in the experiments with water introduced. Methane, ethane and CO2 generated from pyrolysis experiments with added water exhibited the carbon isotope rollover while the corresponding gases produced from pyrolysis experiments without added water did not show carbon isotope rollover. The rollover point was observed after 7days of heating (Easy Ro=1.49%) of the least mature kerogen but shifted to nearly 15days (Easy Ro=1.65%) when heating more mature kerogens. The least mature kerogen was able to generate the largest amount of late gas in the presence of H2O. In contrast to carbon, hydrogen isotope rollover was not observed in either methane or ethane generated during the pyrolysis experiments no matter if water was added or not. Confined pyrolysis experiments with added water generally yielded more negative δ2HCH4 values than pyrolysis experiments without added water, indicating that water promoted the secondary cracking of C2+ hydrocarbons to methane. Our results confirm that (1) carbon isotope rollover in productive shale gas plays is not an oddity but can be reproduced in the laboratory and can be explained by chemical mechanisms operating at elevated temperatures, and (2) the presence of H2O promotes secondary cracking of C2+ hydrocarbons to methane and generates isotopically lighter gas accumulatively to present isotope rollover in a closed system.