Ann M.E. Marchand¹, R. Stuart Haszeldine¹, Anthony E. Fallick²
(1) University of Edinburgh, Edinburgh, Scotland
(2) Scottish Universities Research and Reactor Centre, East Kilbride, Scotland

Abstract: Oil Charge Preserves Porosity in Deepwater Clastics

Deepwater Brae Formation sandstones interbed with Kimmeridge Clay source rocks on the western margin of the Viking Graben (North Sea). These sandstones form a cluster of producing 300Mbbl fields : Brae, Miller and Kingfisher. Results from basin modelling show that Miller charged before Kingfisher. This enables us to examine depositionally identical sandstones, where cementation has been retarded by oil charge arriving at different times in the geological past.

Quartz cement in the Miller Field increases from 3 to 15% and porosity decreases from 25 to 12% from crest (3,980m) to water zone (< 4,090m). In the adjacent Kingfisher Field (3,985m) a similar pattern of porosity decrease and quartz cement increase is seen. Compared to Miller, porosities are lower (13 to 6%) and quartz cement abundances higher (7 to 17%) due to later oil charge. Quartz cement oxygen isotopes and fluid inclusion salinities increase with depth in both fields reflecting porefluid evolution.

CL petrography of quartz cement shows that the number of quartz growth zones increases with depth towards the reservoir water zones. Petrographic and conventional oxygen isotope results show that quartz cementation occurred gradually from evolving porewaters during oil filling. We anticipate that SIMS ion probe analyses of the quartz growth zones will show increasing oxygen isotope values from the inner towards the outer cement zones. Predictions of aquifer support quality depend on recognition of the rapid increases in quartz cement and resultant rapid porosity loss down through the reservoir intervals in these fields.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana