The Witch Ground Graben is a product of Mid-and Late Cimmerian extensional tectonism. The principal orientation of normal faulting was NNE-SSW during the MidCimmerian (Bajocian to Early Kimmeridgian) and a combination of NW-SE and E-W during the Late Cimmerian (Late Kimmeridgian to Barremian). Block rotation on the dominant Late Cimmerian trends, combined with reactivation of older structural grains formed the fault block traps in the area.Two major depositional systems form the reservoirs of the late Jurassic and early Cretaceous of the Witch Ground Graben, namely, the deltaic and shallow marine systems of the Sgiath and Piper Formations and the deeper water turbiditic sands of the Kimmeridge Clay and Valhall Formations. The transition between the systems is highly diachronous. The turbiditic sandstones are often enclosed within or onlap sealing strata, thus providing a stratigraphic element to many of the traps in the area.A chronostratigraphic framework was established and used as the basis for understanding firstly, the timing of the tectonic episodes responsible for moulding the Witch Ground Graben and secondly, the importance of these events in determining rcscrk, oir sandstone distribution and trap development.

The Lower Tertiary petroleum system in the West of Shetland area has been reconstructed by integrating sequence stratigraphical interpretation, pressure data analysis and basin modelling with hydrocarbon migration pathways prediction.Twelve sequences in the Paleocene–Early Eocene section can be grouped into three sequence sets. The lower two sequence sets are dominated by deep marine turbidite fan reservoir sandstones encased within hemipelagic shales. The youngest sequence set is dominated by a series of prograding delta facies rocks. A regionally extensive shale, deposited during a marine flooding event sits at the top of the oldest sequence set. This shale is an effective regional seal and pressure barrier. A study of well pressure data and sedimentary facies variations within the oldest sequence set has led to the identification of three pressure cells.Structural modelling, heat flow mapping, vitrinite reflectance and apatite fission track analysis were used to reconstruct the thermal history West of Shetland. From this dataset, the Kimmeridge Clay Formation source-rock maturity and hydrocarbon generation history were modelled for several time periods. These results were used to generate migration pathways at the Base Tertiary unconformity which sits stratigraphically below the reservoirs in the oldest Paleocene to Eocene sequence set. The resulting model predicts an oil-prone southern province and gas-prone central province which accords with the Foinaven and Schiehallion oil fields in Quadrant 204 and the gas discoveries in Quadrants 206 and 214. The model also suggests the possibility of further oil potential in Quadrant 208.

The structures drilled to‐date in the NW Witch Ground Graben area have been assigned to two broad categories: (i) Upthrown traps with four‐way dip closure or tilted fault blocks, generally located on the franks of the graben: and (ii) Downthrown traps with closure against a major fault, located downthrown on the main graben border fault systems. Many of the downthrown traps are in fact combination traps, having upthrown fault‐seal and/or stratigraphic elements. A study of trap settings in the NW Witch Ground Graben has led to the following conclusions: 1. The success rate for encountering hydrocarbons in upthrown traps and downthrown traps is very similar. 2. The common belief that the footwall seal is the most critical element of a downthrown trap is questionable. 3. Modern seismic data reveals that a high proponion of unsuccessful exploratory wells were not located on a closed structure. The proportion is similar for upthrown and downthrown trap targets alike. 4. There is a lesser chance of the reservoir being absent, or there being no migration route to the trap, in downthrown structures, compared with upthrown ones. 5. Carboniferous sediments can be a footwall seal for a downthrown trap in one location, and a reservoir for hydrocarbons in another. 6. The structural evolution of the NW Witch Ground Graben, within a framework of three intersecting fault trends, has been ideal for the development of downthrown traps. The distribution of a hydrocarbon source rock, the timing of hydrocarbon migration into these structures, and the suitable distribution of sealing strata adjacent to the border fault of the structure, have also been critical.

Occurrences of oil within the Bencliff Grit at Osmington Mills were studied through an integration of organic geochemistry and a consideration of the geological setting. Oil-stained sandstones dominate the cliff outcrop with localized regions of particularly concentrated oil impregnation. A second ‘live’ seep of oil occurs where the Bencliff Grit beds pass below high tide level at Bran Point. Organic geochemical analyses showed both oils to be at least moderately biodegraded, with the oils in the cliff outcrop showing enrichment in polar constituents compared with the active seep. Multivariate statistical analysis of the molecular composition identified an enrichment in diasterane biomarkers in the oils of the live seep; this difference is ascribed to source and/or maturity differences. The oil within the outcrop is considered to represent the residual staining of an unroofed oil field, whilst the live seepage at Bran Point represents a migration pathway towards the eroded anticline.

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