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1 Koninklijke/Shell Exploratie en Produktie Laboratorium, Volmerlaan 6, 2280 AB Rijswijk, Netherlands
2 Conoco (UK) Ltd, Park House 116 Park Street, London W1Y 4NN, UK
3 Shell Training Centre, Leeuwenhorst, Noordwijkerhout, Netherlands
4 Petroleum Development Oman, PO Box 81, Muscat, Oman
5 SURRC, East Kilbride, Glasgow, UK
6 CSIRO, Mineral Resources Labs, 51, Delhi Road, North Ryde, New South Wales, Australia
Diagenetic models and the results of laboratory experiments on cores from producing Brent Group fields (Cormorant and Tern) have been combined to aid prediction of reservoir properties in a deeply buried prospect (Pelican).
In Cormorant and Tern illite particles are thin (20–40 Å), lath-shaped and liable to damage during drying before air permeability measurement, leading to erroneously high permeabilities. The magnitude of this drying effect (Kair conventional drying/Kair critical point drying) varies with illite morphology and the timing of hydrocarbon emplacement. In Tern pore-filling illites occur above and below the oil—water contact, and there is no significant difference in drying effect. In Cormorant, the drying effect is higher below the oil—water contact where a diversity of late diagenetic pore-filling illite morphologies occur.
Injectivity tests on cores from Cormorant indicate that sea water injection will not lead to impairment of reservoir quality as a result of clay dispersal or other rock-related phenomena. Impairment occurred when flooding cores with produced water and was avoided by introducing divalent cations, saline brines or approximating in situ equilibrium conditions (pH 6.4). This equilibrium point is strongly influenced by the surface chemistry of feldspar and illite, rather than the more abundant kaolinite and quartz. Burial diagenetic replacement of kaolinite by illite will increase this equilibrium towards pH 7 but should not cause impairment during sea water injection into deeper reservoirs.
Pelican contains abundant pore-filling illite. K-Ar age dates (44–25 Ma) suggest illite growth started after burial to over 8000 ft and palaeoburial temperatures of over 80–90°C. Compared with nearby reservoirs, variations in the nature and abundance of illite result from illitisation being arrested by hydrocarbon charge at different times. Optimal reservoir and aquifer properties occur in shallower structures charged from more deeply buried source rocks. Good reservoir properties occur with charge after short-lived illite growth, although later burial may reduce aquifer properties. The more prolonged illite growth experienced by deeply buried reservoirs such as Pelican, charged late in the Tertiary, has resulted in poor reservoir properties and poor aquifer permeabilities may be expected.
