Release of bound aliphatic biomarkers via hydropyrolysis from Type II kerogen at high maturity

At relatively low maturity, kerogen-bound aliphatic biomarkers invariably display retarded maturity compared to their "free" counterparts as a result of the protection afforded by the macromolecular structure. This study investigates the release of hopanes and steranes at elevated (post oil window) maturity from Type II kerogen. Results for a suite of North Sea petroleum source rocks (Kimmeridge Clay Formation) indicate that, below a hydrogen index (HI) value of ca. 300, differences between the "free" and "bound" phases for hopane and sterane side chain isomerisation are small. However, ring isomerisation within the kerogen-bound phase is still more retarded. For a corresponding series of kerogen samples artificially matured via hydrous pyrolysis, both "free" and "bound" phase side chain isomerisation reactions are significantly retarded relative to the naturally occurring samples. This is consistent with earlier studies, but the extent of retardation for ring isomerisation in the "bound" phase is even more pronounced than for the "free" phase. (C) 2008 Elsevier Ltd. All rights reserved.