The Rannoch and Etive Formations, Lower Brent Group, UK North Sea, consist mainly of shoreface, coastal barrier and strandplain sandstones with complex reservoir architectures. A sequence stratigraphic framework for these strata allows improved sub-regional correlation and paleogeographic reconstruction, but reservoir compartmentalisation is largely controlled by subtle facies variations below the scale of parasequences and sequences.

An idealised Rannoch/Etive parasequence contains an upward facies gradation through: (1) thoroughly bioturbated, interlaminated siltstones and sandstones (representing offshore transition zone deposits), (2) non-amalgamated storm event sandstone beds (distal lower shoreface), (3) amalgamated storm event beds (proximal lower shoreface), (4) massive, flat-laminated and/or cross-bedded sandstone (upper shoreface), and (5) weakly laminated and partly destratified sandstone (barrier/strandplain). Such idealised facies successions are not observed. Instead, bed amalgamation, bioturbation and mica content vary within individual facies units. Also, isolated beds of one facies type occur in thickened units of another facies type. These variations are present at a bedset (5-20') scale and they strongly influence vertical permeability distribution and reservoir compartmentalisation, especially where enhanced by early calcite cements.

Similar bedset-scale facies variations and architectures have been studied at outcrop in the shallow marine Kenilworth Member, Blackhawk Formation in the Book Cliffs, Utah, resulting in templates for reservoir characterisation and modelling in the Rannoch/Etive interval. Reservoir architectures based on these analogue templates are compared with sequence stratigraphic and conventional lithostratigraphic interpretations of the Rannoch and Etive Formations.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas