Exploration in the West of Shetlands Basin spans nearly 40 years however, despite all the interest commercial successes have often been limited, although fields such as, Clair, Victory, Foinaven, Schiehallion and Suilven exist. A success and failure analysis by Loizou et al. (2006) of 109 failed exploration wells identifi ed the main risks as poor trap defi nition, reservoir presence and quality, seal presence and effectiveness and source rocks and charge. Understanding these basics, along with a thorough analysis of the subsurface geology (both lithology and structure), is required if more discoveries are to be made.
With a view to reducing these uncertainties in the West of Shetlands, this paper discusses the analysis of pressure data from 223 exploration and appraisal wells. A series of overpressure maps were generated across eight stratigraphic horizons: the Post-Palaeocene, Palaeocene, Cretaceous, Jurassic, Triassic, Carboniferous, Devonian and Pre-Devonian Crystalline Basement. Regional reservoir connectivity, vertical and lateral pressure communications and seal effectiveness e.g. Kettla Tuff, are recognized on these maps.
Conclusions drawn on the measured reservoir pressures show that the majority of wells in the West of Shetlands region are near-normally or normally pressured. The data also show evidence of a complex stratigraphic and structural isolation of some reservoirs, such that highly overpressured reservoirs that are encased by overpressured low permeability shales are present, with up to 4000 psi of overpressure. Structural isolation can also be achieved by sealing lineaments such as the Corona Lineament, one of a series of NW/SE trending features that compartmentalizes the basin into a series of sub-areas, at all stratigraphic levels. The existence of both normally pressured and overpressured high permeability sands at similar depths in the same basin implies complex basin plumbing.
Where high permeability sand units are connected to the sea-fl oor (either by stratigraphy or faulting), preferential drainage of fluids can occur along them from regions of overpressure in the deeper parts of the basin to regions of lower overpressure on the basin margins. The fl uid fl ow is known as lateral drainage and suggests the presence of hydrodynamic aquifers, in which systematic changes in overpressures are witnessed. Identification of lateral drainage is important as the mechanism can create tilted contacts which can consequently led to under- and over-estimates of expected reserves.
With the results of overpressure mapping, a clear picture of basin plumbing appears, one that has profound implication for exploration, development and production in this basin.