Current understanding ot' the origin of illite cements within the pore systems of sandstone rcscrvoii-s is at best speculative whilst knowledge of thc control of illitc morpholoy is, t o all practical purposes. non-existent. This is unfortunate considering the widcspread occurrencc o ! illitc and the ollen disastrous decrease in reservoir performance associated with its presence (up to 200-fold decreases in permeability have been recorded from aeolian sandstone.; o f the U . K . southern North Sen for example). An advanced knowledge of the origin of this clay mineral is thus of considerable importance to the futurc of the hydrocarbon exploration and production industry. This piper presents an asscxxmcnt 01 sandstone illite morphotypes and reviews the mechanisms which have been proposed for their l o r nation. K I -Y WORDS lllite Morphotypes Diagenesis

Summary 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.

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A polycrystalline grain of granitic composition with compound feldspar and quartz overgrowths is described from the continental Helsby Sandstone Formation (Triassic; Shemood Sandstone Group) of the Cheshire Basin, northwest England. Overgrown phases, as seen by thin section and scanning electron microscopy, are in crystallographic continuity with respective components of the host grain and have mutual relationships which indicate the quartz to be the later diagenetic phase. Energy dispersive spectrometry (EDS) of the grain surface and of the earlier formed overgrowths indicates the feldspar to have a potassium aluminosficate composition which is in accordance with the adulariahabit crystal form of the authigenic phase.

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