Abstract
Piper field discovery well, 15/17-1a, drilled through 192 ft (52 m) of oil-bearing Upper Jurassic sandstone on December 22, 1972. Subsequent appraisal wells delineated approximately 8,600 productive acres with an estimated 1.55 billion bbl of 36° API gravity low-sulfur oil in place and 650-900 million bbl recoverable.
Reservoir sandstones are Oxfordian and early Kimmeridgian in age, of marine origin, and unconformably overlie a nonmarine Middle Jurassic sedimentary sequence. The gross-reservoir thickness averages 250 ft (76 m) in the field area and is comprised of several individual sandstone bodies 40-70 ft (12-21 m) thick. Within individual sandstone bodies the grain size grades either upward or downward from very fine sandstone or siltstone to coarse-grained sandstone. The sandstones generally are well sorted, highly bioturbated, friable, and have excellent porosity and permeability. Individual sandstone bodies record local regressions or transgressions. Regressive sands, accreting seaward as foreset beds, were generally thicker than transgressive sands.
Isopachs of Triassic through Cretaceous units suggest that the Piper structure grew intermittently before, during, and after deposition of the reservoir sands. Post-Albian-pre-Toronian/Coniacian faulting and erosion and rejuvenated faulting during pre-Maestrichtian Late Cretaceous time formed a series of eroded and tilted fault blocks. The domal structure mapped at the base of the Tertiary is not evident in the younger horizons because of the masking effect of middle Tertiary deltaic deposits. Regional subsidence continued throughout the Tertiary and formed the present North Sea basin.
