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Geological Society, London, Special Publications; 1995; v. 86; p. 141-157;
DOI: 10.1144/GSL.SP.1995.086.01.11
© 1995 Geological Society of London

Case Studies

The filling and emptying of the Ula Oilfield: fluid inclusion constraints

Norman H. Oxtoby1, Alan W. Mitchell2,3 & Jon G. Gluyas1

1 BP Exploration Operating Company Ltd, BPX Technology Provision, Chertsey Road, Sunbury on Thames, Middlesex. TW16 7LN, UK
2 BP Norway UA, Forusbeen 35, 4033 Forus, PO Box 197, Stavanger, Norway
3 BP Exploration, Blackhill Road, Holton Heath Trading Park, Poole, Dorset. BH16 6LU, UK

The Norwegian sector Ula Field contains 430 million barrels of oil reserves trapped in a dip-closed Upper Jurassic sandstone reservoir and has multiple oil-water contacts (OWCs). Historically, the depth to the contacts has been the subject of some debate and development drilling has now proven the existence of at least six. The precise nature of some of these contacts remains uncertain because all were discovered after production start-up and determination of their origin is critical to future field development. The relationship of oil inclusion abundances to OWCs and rock properties (porosity, permeability) has been evaluated (a) to gain an insight into the filling history and mechanisms, and (b) to attempt to define pre-production OWCs. Oil inclusion abundances decrease with depth and towards present OWCs in Ula, but oil inclusions also occur below them. They indicate that Ula filled from the crest down, and that the filling was synchronous with cementation to at least 3500 metres subsea. Below this depth, cementation appears to have reduced permeability to such an extent prior to charge that the resulting filling geometry was irregular. Local abundance peaks within the oil column correlate with the tops of coarsening-upward cycles. Rare oil inclusions below the oil legs may represent either migration pathways updip from the source or invasion downdip from the main oil leg along higher permeability zones. Oil inclusions persist to greater depths in the east and south-east of the field which indicates that oil charged the field from these directions. A number of models explaining the variability of the OWCs have been ruled out by this work, thereby reducing the uncertainty in field development planning.





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