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Fault Seal Failure - An Explanation for Geological Depletion of Reservoir Pressure

Swarbrick, Richard E.; O'Connor, Stephen
Ikon GeoPressure, Durham, United Kingdom.

Pressure cells or compartments are a feature of many overpressured basins (e.g. Malay, Anadarko, North Sea and mid-Norway Basins. These compartments are controlled in most cases by faults, some large enough to be mapped with confidence using seismic data, others below seismic resolution but their location inferred from regional maps and interpretation of well pressure data. Depletion of reservoirs due to production can create or enhance pressure differences across faults, and there are examples of pressure "break through" identified in data from production wells to indicate the loss of integrity of faults locally. What is less well documented is "break through" during the geological past, in which faults lose seal integrity and allow sudden reduction in overpressure on one side as fluids move into an adjacent cell.

This paper documents two examples of geological fault seal failure: (1) Central North Sea; (2) Halten Terrace, mid-Norway. In the Central North Sea example a basin-bounding fault failed to allow hydrocarbons and water to migrate westwards out of the basin with associated reduction of overpressure in the easterly pressure compartment. The overpressure distribution in multiple well locations across the easterly compartment systematically increase away from the zone of fault failure, which also corresponds to the shallowest reservoir depths, and hence the point along the fault most likely to fail. Further there are indications of a palaeo-oil accumulation at that position as well as palaeo-temperature data from fluid inclusions which document trap failure and it's likely timing within the last 2 million years. The Halten Terrace example illustrates failure along a complex fault system allowing a portion of the main hydrocarbon-bearing reservoirs on the eastern side of the Terrace to reduce their pressures to near-normal, in contrast to the high pressure compartments on the western side.

 

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012