Carboniferous rocks of the eastern and central Midland Valley of Scotland :organic petrology, organic geochemistry and effects of igneous activity

Abstract

An integrated organic petrological and geochemical approach has been adopted to evaluate the variation in the type of organic materials contributing to the Carboniferous succession in the eastern and central Midland Valley and to assess the extent to which both extrusive and intrusive igneous activity have modified the optical and geochemical properties of organic matter in this province. The type investigation reveals that the succession is extremely rich in organic matter, much of which originated from the vascular tissues of higher plants. The profound influence of terrestrial sedimentation throughout the Carboniferous is evident from the marked terrestrial overprinting of many known marine horizons. Much of the accumulated organic matter has been subjected to varying degrees of microbial reworking. In addition to the characterisation of organic matter in oil shales (lamosites and torbanitic shales), humic coals, sapropelic coals, marine bands and carbonaceous clasts incorporated in volcanic vent deposits, each of which bear different petrological and/or geochemical signatures, a detailed evaluation was undertaken of the Lower Carboniferous (Vis6an) sequence in east Fife, since the algal-rich shales in the Visean have long been considered as the most likely source rocks for the oil shows and modest petroleum accumulations which occur in the Midland Valley. Although many of the oils and bitumens studied have suffered varying degrees of biodegradation and/or water washing, these early suspicions have been confirmed by the close correspondence between the biomarker profiles of the oils/bitumens studied and those of the non-marine, lamositic shales of the Lothians Oil- Shale Group (Lothians oil shales). Notwithstanding the close spatial relationship which exists between oil shows and igneous materials in many areas of the Midland Valley, a hightemperature origin for all of these hydrocarbons need not be invoked because consideration of regional maturation levels indicates that the Lower Carboniferous succession attained maturities conducive to the generation of liquid (and gaseous) hydrocarbons through the "conventional" course of burial metamorphism by the end of the Carboniferous period. For the rank investigation, maturation profiles (based on vitrinite reflectance) for nearly forty boreholes and field sections in the Midland Valley were constructed and rank maps were prepared for different horizons throughout the Carboniferous. Lateral and vertical patterns of paleaoheatflow were monitored both before and after the intrusion of the Midland Valley Sill. The higher regional levels of organic maturation recorded in the west of the Midland Valley, at all levels in the Carboniferous, correspond to an increased thickness of the volcanic pile in the central and western parts of the province, suggesting that a greater regional heat sink existed in the west during the Carboniferous. The optical and textural properties of vitrinite have been used to constrain the timing of sill emplacement with respect to the imposition of regional levels of coalification, from which deductions have been made about the importance of igneous activity in modifying background maturation profiles, not only in the Midland Valley, but elsewhere in the world. The early Permian tholeiitic Midland Valley Sill, which underlies most of the eastern and central Midland Valley, has caused the development of extensive thermal aureoles, in marked contrast to the preceding Lower to mid-Carboniferous alkaline-dolerite intrusions which have failed to induce any widespread alteration of organic matter. This major difference in organicmatter response is explained in terms of differing rank levels of organic matter at the time of intrusion and to differences in the degree of sediment compaction and water saturation, factors which probably also strongly influenced the rank level of organic matter prior to invasion by magmas. Maturation of organic matter closely associated with extensive Surtseyan-type volcanicity in the eastern part of the Midland Valley displays no relationship to the regionally developed coalification in associated sediments. Maturation levels of clasts in pyroclastic vent deposits have generally not been raised much above the regional reflectance level, suggesting the incorporation of organic matter in ash streams in which water was an important coolant and protective agent. The effects that different heating rates have had on commonly used molecularmaturity indices, based on both saturated and aromatic hydrocarbons, in different types of organic matter were also investigated. Results show that molecular-maturity parameters may significantly underestimate the maturities of samples in which the organic matter has experienced high rates of heating, either through igneous activity or a high background geothermal gradient, and that distinct "reversals" occur in many ratios at elevated ranks. Type influences are inferred for Tmas and hopane/moretane ratios and for parameters based on distributions of methylphenanthrene isomers. Anomalous assessments of sample maturity may additionally derive from the lowering (suppression) of vitrinite reflectance which invariably occurs in samples containing abundant liptinitic materials, principally sporinite in the Carboniferous of the Midland Valley. The results presented in this thesis carry important implications for hydrocarbon exploration. Accurate appraisals will only be achieved through the complete integration of organic petrological, geochemical and geological data in sedimentary sequences containing abundant hydrogen-prone organic components and/or in basins in which there has been widespread igneous activity.