Literature review of Devonian source rocks and Devonian-sourced hydrocarbons in the Orcadian Basin

This report summarises existing work on the Devonian source rocks and migrated oils in the Orcadian Basin. It aims to clarify the current literature with regards to the contribution of Devonian source rocks to proven hydrocarbon accumulations within the basin by providing a comprehensive summary of;  the quality and maturity of the lacustrine source rocks;  the evidence for Devonian oils in hydrocarbon fields, discoveries, shows and stains;  perceived risks for Devonian-sourced plays. Good quality oil-prone lacustrine source rocks are present in the Devonian both on- and offshore, and are believed to have co-sourced the three Inner Moray Firth oil fields – Beatrice, Lybster and Jacky. Thus an understanding of the distribution and burial history of the Devonian source rocks could be important in the search for new hydrocarbon discoveries in the region. The complex pre-, syn- and post-rift tectonic history has given rise to significant local variability in both the thickness of the Devonian section and the present-day thermal maturity. Offshore, determination of the extent of the Middle Devonian Orcadian Lake, the main source rock interval, is constrained by only a limited number of well penetrations of the Devonian interval centred on Quadrants 12 & 13, although an oil-correlation study by Robertson Research International (2001) suggested that Devonian source rocks may be developed as far east as Quadrant 15 and may therefore be more widespread than previously thought. Studies have shown that Devonian-sourced oils have some key characteristics meaning they can confidently be differentiated from Jurassic-sourced oils. In particular, the presence of carotane and gammacerane (which can both be indicative of hyper-saline depositional environments), the absence of C28-bisnorhopane (commonly found within Jurassic sourced oils), the relatively light carbon isotope ratios and depletion of C27 steranes are distinguishing features when contrasted to Upper Jurassic to Middle Jurassic sourced hydrocarbons. Basin modelling studies suggest that onshore, the Devonian source rock experienced early hydrocarbon generation, reaching mid-late oil window thermal maturity by the Late Palaeozoic. Offshore, the preferred model is for Mesozoic generation with the onset of oil generation from the Devonian lacustrine source rocks in the Beatrice Field kitchen area in the Early Cretaceous, negating the need to invoke long-range migration from mature Jurassic mudstones to charge the Beatrice structure. This report was completed before the Palaeozoic Project organic geochemistry work on the Orcadian area (Vane et al., 2016), to inform the scope of work on that study. As such, it does not integrate the results of the Palaeozoic organic geochemistry study. This report contains citations of donated proprietary and commercial reports.