Relative thermodynamic stabilities of biomarkers – Evidence from drill-bit metamorphism in the Iving Discovery wells 25/8-19 S & A, Norwegian North Sea

Abstract

In this study, the effects of drill-bit metamorphism (DBM) on oil in a kerogen-free system is examined to provide an insight into the relative thermodynamic stabilities of biomarkers and aromatic hydrocarbons, free from the contributions of newly generated hydrocarbons that complicate studies based on source rock intervals. Mud gas from the upper 20 m of fractured basement in the 25/8-19 A well of the Iving Discovery exhibits compositional variations attributable to DBM, such as abundant alkenes and neopentane, and C isotopic fractionation. Molecular, and particularly biomarker, distributions from corresponding core extracts are compared to those in unaltered test oil from immediately overlying Triassic sandstone, after normalization to diahopane, which is suggested to be relatively resistant towards thermal degradation. The variations in these relative abundances, determined by GC–MS analysis, correspond to some of the previously observed or theoretically postulated thermodynamic stability trends among biomarkers, such as depletion increasing with C-number among extended hopanes, and enrichment of rearranged vs regular hopanes. Some other trends are not as expected, such as the similar sensitivities of diasteranes towards removal as regular steranes, and greater depletion of short-chain triaromatic steroids compared to their C26–C28 homologues. Triaromatic dinosteroids and their 28-nor counterparts are enriched relative to diahopane, whereas during normal maturation their concentrations have been found to decline towards the end of the oil window in a similar way to biomarkers in general. Among cheilanthanes, C29 and C30 members are particularly depleted when compared to C28. Methylated naphthalenes are more readily removed than corresponding phenanthrenes and dibenzothiophenes, but all such polycyclic aromatics are at least as depleted as regular hopanes and steranes, suggesting a level of thermal destruction similar to that observed above ∼1.2% vitrinite reflectance under natural maturation conditions. The influence of oil-based mud (OBM) on core extracts is limited, although contributions are recognized for C29 and C30 17α-hopanes and 17β-moretanes, 5α,14α,17α-20R isomer of the C27–C30 steranes, C20 and C21 cheilanthanes, gammacerane and 3β-methyl-17α-hopanes.