Summary

Production from the Cormorant IV field (East Shetland Basin, offshore UK) has shown that the reservoir has many lateral fluid compartments. RSA data indicate vertical fluid layering at present day. We investigated the fluid history of these compartments during burial, by studying the diagenetic evolution of the sandstones.

If distinct porefluids can be detected from different parts of the field at any one time in the geological past, then compartments existed within the field at that time. If the porefluids within the reservoir were uniform throughout the field at any time, then no compartments were present. Palaeofluid compositions were elucidated by examination and isotopic analysis of the diagenetic mineralogy. By examination of minerals from different stages of burial, we can deduce the connectivity of the reservoir through time.

The porefluid evolution of wells from different present-day compartments in Cormorant IV is the same. From this, we conclude that the porewater evolution within Cormorant IV has been uniform, despite the present day compartmentalisation.

The organic geochemistry of the petroleum within the reservoir indicates the possible presence of only a single sealing fault, which may isolate the NE quadrant from the remainder of the field. The vast majority of the present-day `sealing' faults did not act as barriers to oil migration.

It seems probable that the faults within the Cormorant IV reservoir are sealing only off a production time scale but are leaking slowly, such that they are not barriers to porefluid flow over geological timescales.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah