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Gas Field Studies |
Arco British Ltd., London
Poroperm Laboratories, Chester
Block 49/28 lies on the northeastern side of the Sole Pit Inversion. The Leman Sandstone Formation increases in thickness from 280 ft to 702 ft southeast wards across the block. The section can be subdivided by facies and by the recognition of major bounding surfaces between aeolian sequences. Transverse dunes up to 100 ft in height have been identified. Early secondary porosity development accompanied by growth of mixed layer clays, feldspar, dolomite and silica cements with minor anhydrite was followed by burial related illitisation of clay minerals and late kaolinite authigenesis.
The most important control on reservoir quality in the block is the development of illite, which reduces permeability. The extent of fibrous illite growth was controlled by pre-Cretaceous burial. Late kaolinite, related to the migration of pore waters accompanying gas, is restricted to gas bearing structures. The relative timing of authigenic mineral growth and the structural history of the area can be combined to pinpoint the timing of gas accumulation as Middle to Late Jurassic.
