Isotope Rollover in Shale Gas Observed in Laboratory Pyrolysis Experiments: Insight to the Role of Water in Thermogenesis of Mature Gas

Gao, Ling; Schimmelmann, Arndt; Tang, Yongchun; Katz, Barry J.; Mastalerz, Maria

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 highly productive shale gas plays can be re-produced in the laboratory. Isotope rollover occurs when isotope values initially become less negative as expected increasing thermal maturity, then display a reversal in trend becoming lighter with increasing maturity. It was found that H2O has a pronounced influence on the composition, yields, and carbon and hydrogen isotopic ratios of thermogenic gases. Compared to gases from anhydrous pyrolysis experiments, lower overall quantities of hydrocarbon gases were generated from hydrous pyrolysis experiments, although the relative abundance of methane among other gas species was higher in hydrous pyrolysis experiments. Methane, ethane and CO2 generated from hydrous pyrolysis experiments exhibited the carbon isotope rollover while the corresponding gases produced from anhydrous pyrolysis experiments did not show carbon isotope rollover. The rollover point was observed after 7 days of heating the least mature kerogen in the study but shifted to nearly 15 days 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 either hydrous or anhydrous pyrolysis experiments. Hydrous pyrolysis experiments generally yielded more negative δ2HCH4 values than anhydrous pyrolysis experiments, indicating that water promoted the secondary cracking of C2+ hydrocarbons to methane. The small change in δ2HH2O values during hydrous pyrolysis experiments suggests that isotopic exchange between H2O and organic pyrolysates was limited. 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.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013