Vitrinite reflectance and molecular geochemistry of Jurassic sediments: the influence of heating by Tertiary dykes (northwest Scotland)

Abstract

Vitrinite reflectance measurements from sedimentary rocks adjacent to six dykes from sites across the Inner Hebrides indicate that the lateral extent of thermal alteration is reasonably similar and proportional to dyke thickness (d.t.), with alteration starting to take effect at 70% d.t. Detailed sampling of a Jurassic siltstone adjacent to a 0.9 m dyke reveals that the zone of maximum extractable organic matter (EOM, wt/g rock), equivalent to the “oil window”, is relatively narrow ( ca. 25% of the d.t.). The position of this “oil window”, relative o the vitrinite reflectance profile, is comparable with that from burial maturation. Gas chromatograms of the aliphatic hydrocarbon fractions reveal: (i) a shift from branched- and cyclic-dominated hydrocarbons to an increasing predominance of straight-chain components; and (ii) the carbon number distribution of the n-alkanes shifts towards the shorter chain homologues, with decreasing distance from the dyke contact. Apart from the methylphenanthrene index (MPI1), changes in each of the molecular maturity parameters are initiated by the the thermal effects of the dyke between 80 and 70% d.t. from the contact. The MPI1 parameter was the most sensitive maturity indicator in that it responded to the dyke emplacement even in the distal samples (i.e. distance from dyke contact ≈140% d.t.). A significant reduction in TOC at the dyke contact suggests that the “synthesis gas” reaction may have occurred, i.e. the reaction of organic matter with steam, resulting in the formation of carbon monoxide. This reaction may be associated with the vitrinite reflectance reversal. This study suggests that these particular dykes have had only a local effect on the maturity of the intruded sediments.