No abstract

No abstract

A combination of sedimentary dip, structural dip and borehole orientation resulted in a core recovered from the Auk Field being cut sub-parallel to aeolian slipface deposits. Individual 2 inch (5 cm) thick grainflow deposits may be followed for up to 16 ft (4.88 m) along core. A conventional core-plug permeability data-set indicated the importance of a deleterious effect of fine-grained ‘pin-stripe’ laminae which separate individual grainflows. Using a probe-permeameter, over 1000 permeability readings were taken from 27 ft (8.23 m) of the core, mostly on a 1 inch (2.5 cm) spaced grid, in order to study small-scale variations and controls of permeability. The readings were taken from five grainflow laminae and associated wind-ripple deposits in a single slipface deposit, and from wind-ripple laminae from an interdune setting. Individual grainflow laminae sometimes show coarsening-upward patterns of grain-size variation which can be ascribed to dispersive forces acting during sedimentation. In terms of permeability distribution, individual grainflows may be distinct from their immediate neighbours. Geostatistical analyses indicate permeability correlation lengths across grain-flow laminae equal to lamina spacings, and a within grainflow correlation length of approximately 1.5 ft (46 cm). Similar tests within millimetre-scale laminated wind-ripple deposits indicate appreciably smaller correlation distances. These results can be translated into effective sampling strategies of one reading per grainflow deposit for a traverse perpendicular to sedimentary dip, and one reading every 6–12 inch (15cm–30cm) for a traverse along grainflow lamination. Wind-ripple laminae cannot feasibly be sampled in a truly representative fashion by standard core-plugs. Petrographic analyses indicate that the principle microscopic controls upon permeability are grain-sorting, the development of authigenic cements and the detrital plus authigenic clay content.

A selection of Brent Group reservoir sequences from the Murchison field were chosen to study the nature of permeability variation within the Brent Group, with particular emphasis on the Etive Formation, which forms one of the main reservoirs in the field. Preliminary core examination of potential permeability barriers, as identified using wireline log response, has indicated that permeability heterogeneities can be placed into a five‐fold hierarchy. Permeability heterogeneities resulting from depositional or syndepositional processes comprise the first four orders of heterogeneity and include formations, facies transitions, bedding surfaces and laminations. Permeability heterogeneities which are the product of diagenetic processes are classified as fifth‐order heteorgeneities. Two major types of permeability barrier have been identified in core and on wireline logs within the Etive Formation. They include permeability heterogeneities associated with a transition between the two major facies associations (tidal channel and beach/foreshore sandstones); and heterogeneities associated with the development of inactive channel‐fill sandstones and siltstones within the tidal‐channel facies association. A detailed sedimentological model has been developed which demonstrates the complexity of reservoir zonation in the Etive Formation within the Murchison field.