Comparison of covalently-bound aliphatic biomarkers released via hydropyrolysis with their solvent-extractable counterparts for a suite of Kimmeridge clays

Fixed-bed hydropyrolysis at pressures greater than 10 MPa has the unique ability to maximise the yields of covalently-bound alkanes without adversely affecting their stereochemistries. In order to compare the yields and compositions of the solvent-extractable and kerogen-bound hopanes and steranes for source rocks of varying maturity, six Kimmeridge clay samples (Jurassic, Type II kerogen) have been subjected to successive dichloromethane extraction and hydropyrolysis. The ratios of free to kerogen-bound aliphatic biomarkers increase markedly at relatively low vitrinite reflectance (ca. 0.45–0.50%) with large reductions occurring in the concentrations of bound hopanes and steranes. However, the fact that the total concentrations of observable hopanes and steranes (i.e. free plus kerogen-bound) do not decrease until the vitrinite reflectance is greater than ca. 0.50% provides evidence that neither oil migration nor aromatisation has begun to occur to a significant extent below this maturity level. To the authorsʹ knowledge, this represents the first study to observe a systematic change in epimer ratios with increasing maturity for hopanes and steranes covalently bound in kerogens. Indeed, the kerogen-bound hopanes and steranes undergo the same epimerisation reaction pathways as their free counterparts in the bitumen, but they are generally less mature in terms of isomerisation at both ring and side-chain chiral centres. Indeed, the range of the moretane/hopane index for the hydropyrolysis oils is much larger than that for the bitumens (ca. 5 times), indicating a much greater sensitivity to relatively small changes in maturity. The covalently-bound hopanes and steranes released via hydropyrolysis have enabled the maturity of a contaminated core from the North Sea (Central Graben) maturity to be assessed and have showed that it is compatible with the Kimmeridge formation.