97/02551 Variation in the chemistry of macerals in coals of the Mist Mountain Formation, Elk Valley coalfield, British Columbia, Canada

Abstract

Vitrinite reflectance profiles have been constructed for twelve borehole sequences from the Carboniferous of the eastern and central Midland Valley of Scotland. These successions are intruded by the early Stephanian, tholeiitic, (quartz-dolerite) ‘Midland Valley Sill’, or by Namurian/early Westphalian alkaline (olivine-dolerite or teschenite) sills, or by both alkaline and tholeiitic sill types. Patterns of reflectance variation in sequences invaded by the Midland Valley Sill are similar to those observed in Carboniferous successions in the north of England, where they are intruded by the ‘Great Whin Sill’, which is comagmatic with the Midland Valley Sill. The reflectance patterns all reveal extensive thermal aureoles that have modified organic matter for many tens of metres above and below the sill-sediment contacts. In contrast, no aureoles of similar scale appear to have developed, either above or below the alkaline sills. Reflectance values only rise to substantial levels in the immediate vicinity of the sediment-intrusion contacts. The most likely explanation for the radical contrast between the reflectance patterns produced by the two sill types, must be sought in the physical state of the sediments at the time of intrusion. The degree of sediment compaction, the volume of pore water in the sediments and the initial maturation level of the organic matter at the time the magma invaded the sediments will be the most significant factors, as opposed to the temperature levels of the magmas, which were probably similar (T ≈ 1000 °C), but which, because of the physical condition of the sediments, produced little rise in temperature of the organic matter even very close to the alkaline sills. The data suggest that relationships between reflectance (thermal) gradients, stratigraphic level of sill intrusion and the timing of intrusion should be carefully examined to ensure successful geothermal evaluation of sedimentary basins in which the burial history has been interrupted by intrusive igneous activity. Geochemical/petrological appraisals of sedimentary basins for hydrocarbon prospects must also be influenced by these results.