Black sandstones in the Midland Valley of Scotland: Thermally metamorphosed hydrocarbon reservoirs?

Carboniferous sandstones from within the thermal aureoles of igneous intrusions in the Midland Valley of Scotland sometimes have a strong, black colour and are termed ‘black sandstones’. Tait (1926) concluded that the black sandstones are the products of the thermal metamorphism of petroliferous sandstones, a theory which is discussed here in the light of modern petrological and geochemical techniques. The black colour of the sandstones is due to an amorphous, opaque bitumen which usually coats illitic clays, fills porosity and was mostly emplaced at a fairly late diagenetic stage. This solid bitumen has a low reflectance (c. 0·15% R₀), no fluorescence under blue-light excitation, is insoluble in organic solvents, is isotopically heavy and has a very low H/C atomic ratio. These data, together with the field relationships of the black sandstones and igneous intrusions, suggest that the bitumen was formed by the thermal alteration of hydrocarbons, as described by Tait (1926), rather than by other possible mechanisms such as the deasphalting of an oil, or the generation of hydrocarbons from organic-rich rocks heated by igneous intrusions, followed by fractionation during migration. This conclusion suggests that by the time of emplacement of the quartz-dolerite intrusions, and some of the alkali-dolerite sills, there had been widespread generation and migration of hydrocarbons which possibly could have been preserved to the present day where not thermally altered.