Effect of igneous activity on molecular-maturation indices in different types of organic matter

Although there are many data in the literature relating to changes of certain relatively simple chemical parameters of kerogens promoted by igneous activity, there have been few reports of how the distributions of polycyclic alkanes and aromatic hydrocarbons respond to geologically rapid heating. Based on a large sample suite of Carboniferous coals and sediments from the Midland Valley of Scotland, this report demonstrates that extensive and important modifications to molecular distributions are caused by the rapid heating of igneous activity compared with the relatively slow heating of burial metamorphism. Many samples of high rank, established on the basis of optical assessment (vitrinite reflectance), are shown to display immature molecular signatures. Reversals in the trends of molecular-maturity parameters, based on both saturated and aromatic hydrocarbons are observed, as is the failure of certain biomarker-maturation indices to reach equilibration if subjected to rapid heating. Through the investigation of molecular distributions of type I and type III organic matter of different sediments, a type dependency is inferred for hopane/moretane ratios, Tm/Ts ratios and ratios based on distributions of methylphenanthrene isomers. The importance is stressed of a completely integrated optical and geochemical approach for hydrocarbon appraisals in sedimentary basins, particularly those in which there has been widespread igneous activity.