The influence of igneous activity on the generation and accumulation of petroleum in the central area of the Midland Valley of Scotland

Abstract

The thesis comprises an investigation of the effects of igneous intrusive activity on petroleum generation and accumulation in the Carboniferous of the central area of the Midland Valley of Scotland. Samples were collected principally from outcrop, but sidewall cores and cuttings from the Inch of Ferryton borehole were also analysed. The maturity of intruded sediments was primarily determined using optical petrological methods. Bulk geochemical and organic geochemical techniques allowed corroboration through maturity indicators and provided information on the dominant type and richness of organic matter in the sediments. The maturity of Carboniferous sediments, unaffected by igneous intrusions, in relation to petroleum generation under conditions of normal burial, particularly in the deeper basins, is discussed. The majority of the fine-grained sediments are type III kerogen-bearing mudstones and siltstones and arc dominantly gas-prone. The type I kerogen-bearing oil shales of West Lothian and Fife represent a rich, potential oil-source. Previous work on the relative sizes of thermal aureoles associated with the igneous intrusions in the Midland Valley is reviewed and further corroboration through other examples is recorded. Possible reasons for the differences arc discussed, including the water- saturation of the sediments and the maturity of the organic matter at the lime of intrusion. The importance of the lime interval between deposition of the sediment and intrusion in determining the size of thermal aureoles is considered. The high maturity of intruded sediments indicates that considerable amounts of petroleum may have been generated by the heat of igneous activity. The amount of oil and gas that could have been generated by the Midland Valley Sill is calculated approximately. The relative sensitivity of optical and organic geochemical parameters under conditions of rapid heating arc considered. Hopanc, sterane and aromatic steroid maturity ratios arc evaluated, as arc those based on the distribution of alkylated naphthalene and phenanthrene isomers. The main controls (organic richness and kerogen type) on the stable isotope composition (8”C and 8D) of some oil-shale samples intruded by a quartz-dolcritc dyke arc considered in relation to any isotopic fractionation caused by maturity variation. The possibility of petroleum accumulation in the Midland Valley was assessed by determining the diagenetic and petrophysical properties of the Carboniferous sandstones. The effect of diagenetic cements and dissolution episodes on porosity and permeability and the influence of igneous intrusions on the reservoir properties of intruded sandstones arc recorded and discussed. A number of black sandstones found throughout the Midland Valley near both quartz-dolcritc and alkali-dolcritc intrusions contain an opaque, solid bitumen and arc believed to represent thermally metamorphosed oil reservoirs.