(1) ExxonMobil, WC2 2EB London, United Kingdom
ABSTRACT: Reservoir Quality Controls in the Deep Fulmar Formation, Central North Sea
Data from thirty wells in the Oxfordian Fulmar Formation of the Central North Sea were analyzed to assess reservoir-quality controls and develop predictive models. Results indicate that porosity and permeability are most likely to be preserved at depth in facies dominated by clean, well-sorted, rigid-grain sands. With burial, these sands follow a normal compactional trend documented in similar sands world-wide, in which intergranular volume stabilizes at relatively shallow depths. Reservoir quality may be further degraded by late cements, most commonly quartz and fibrous illite. In the Fulmar, these cements increase systematically with depth, except in certain wells where they appear to be inhibited.
Observations suggest that late cements are inhibited and reservoir quality preserved in the deep Fulmar by chert grain coatings formed from dissolved siliceous sponge spicules. Highest reservoir quality occurs in sands with originally abundant spicules that formed more continuous chert rims. Hydrocarbon pore fluids appear not to have significantly inhibited late cements.
Fulmar spicules are most common in intervals dominated by the relatively distal reaches of transgressive systems tracts, where clear water and limited volumes of detrital fines favored sponge growth and spicule abundance in sediments. For this reason, the originally spicule-rich retrogradational Upper Fulmar has better porosity and permeability at depth than the aggradational to progradational Lower Fulmar, in which a lower spicule content led to poor chert-rim development and extensive late cementation.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado