Organic geochemistry applied to petroleum source potential and tectonic history of the Inner Moray Firth Basin
Upper Jurassic sediments from the Inner Moray Basin have been analysed using a variety of organic geochemical techniques. Shales belonging to the Kimmeridge Clay and Heather/Brora Formations may be readily distinguished using molecular parameters, bulk geochemical data and the results of transmitted light kerogen microscopy. Oxfordian sediments are characteristically more organic lean and sulphur poor than their ?Volgian - Kimmeridgian counterparts; they also contain a more significant terrestrially derived organic matter component. Lipid fractions in the remainder of the succession are typically dominated by the products of marine algal phytoplankton and bacteria. Detailed investigation of the distribution of biological marker compounds indicates that the nuclear demethylated hopanes are confined exclusively to sediments of ?Volgian - Kimmeridgian age in this basin. This feature, together with other more subtle, non-source specific variations may be attributed to a decrease in the dissolved oxygen content of the water column throughout the Upper Jurassic. All maturity measurements concur that in this region the Upper Jurassic sediments are insufficiently mature to have generated or expelled significant volumes of petroleum at any stage throughout their geological history. A range of maturities is nevertheless recognized which permits a ranking of the individual wells, and indicates that shales overlying the Beatrice reservoir have experienced the greatest thermal stress. A marked discrepancy in the maturities of contemporaneous sediments from two groups of wells presently at the same burial depth, has also been identified. This is largely consistent with the observed variation in geothermal gradients and the distribution of Kimmeridgian sands across the basin. Kinetic modelling using the biological marker maturity parameters has established that in the west 1km of sediment has been removed following Tertiary inversion of the basin. Results indicate that in the east, a figure of 700-800m is more appropriate. Potential source rock lithologies from the margins of the basin have also been examined. These included the Jurassic sediments in the Helmsdale outlier and Middle Devonian lacustrine laminites from Caithness. A Middle Devonian component to the Beatrice crude is strongly suggested by the similarity of their steroid alkane distributions.