The organic geochemistry of the Minch Basin Jurassic shales
The Jurassic sediments of the Minch Basin were deposited in a series of small interconnected basins that became the centre of extensive igneous activity during the Lower Tertiary. Outcrop samples have been collected from throughout the Minch area, particularly from the islands of Skye and Raasay, and aliphatic and aromatic hydrocarbon fractions analysed for biomarker compounds by GC-MS. Sample suites collected from the margins of Tertiary igneous dykes of varying sizes have shown that the dykes matured the immature sediments in a manner analogous to burial. Biomarker reactions however occurred later relative to kerogen breakdown reactions, so that the hopane isomer ratio does not reach completion until the peak of oil generation. Steroid aromatisation occurs at a similar rate to isomerisation, suggesting heating rate is not the only control on relative rates of reaction. The sills seen in the youngest Jurassic sediments have not had the same effect, but show anomalous η-alkane distributions with heavy preference in the most heated samples. The biomarker isomer ratios are not at equilibrium values, possibly due to high temperature cracking. These unusual effects are interpreted as the result of intrusion into wet unconsolidated sediments. The central intrusive complexes have also had the effect of maturing the country rock sediments, with the oil window seen on Raasay between about 5km and 8km from the complex margin. No thermal effect is seen beyond about 15km. Maturity appears to decrease radially away from the complex, and no anomalies can be definitely ascribed to postulated hydrothermal circulation systems. The temperature of the peak of oil generation is estimated as 130°C. Biomarker assemblages in immature sediments from all Jurassic formations have been considered in relation to proposed depositional environment and palynofacies. A number of unusual features have been noted including abundant heavy anteisoalkanes in shales associated with corals, very abundant 4-methyl steranes in the Cullaidh Shale, and unusual assemblages in the lagoonal Duntulm Formation. A dark brackish marine shale contained virtually nothing other than C_25 and C_27 η-alkanes and C27 steranes, yet the palynomorphs consisted almost entirely of a monotypic dinoflagellate assemblage. A cryptic algal source is suggested for the η-alkanes. 4-methyl sterane distributions in all formations have been plotted on triangular diagrams and separate into distinct fields. This may be due to different dinoflagellate input. Finally, the petroleum potential of the Jurassic sediments in the basin has been assessed. Although potential source rocks occur, it seems unlikely that they can have been matured by burial in the Minch, and the Central Intrusive Complexes can not have matured a significant volume of source rock. It is possible however that large sills concordant with bedding could have matured source rocks over a wider area, and it is suggested that this has occurred at Invertote, where the black sandstone beneath the Cullaidh Shale is believed to be an exhumed reservoir.