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ABSTRACT: Tectonic stresses facilitating natural fracturing in overpressured reservoirs - Inferences from numerical modelling

Beekman, Fred, and Tore Skar , Faculty of Earth Sciences, Vrije Universiteit Amsterdam, 1081 HV Amsterdam, Netherlands

The Halten Terrace, a hydrocarbon habitat on the mid-Norwegian continental margin, is characterised by spatial variations in subsurface fluid pressures. The main oil and gas discoveries are encountered in Jurassic reservoirs with low fluid pressure, whereas highly overpressured reservoirs have in general been proven to be devoid of hydrocarbons. The relationship between high overpressure and water-filled reservoirs suggests that caprock leakage is an important factor for the lack of hydrocarbons in the high fluid pressure domain. In these domains, overpressures and fluid movements are regulated by the stress regime (including tectonic stresses) and by the retention capacity of the caprock. Leak-off tests indicate that in the high pressure domains of the Halten Terrace the fluid pressure approach the minimum horizontal stress, and therefore the formations are close to fracturing.

Using the finite element method we investigate if far-field tectonic compression of the Halten Terrace may lead to a local reduction of the minimum horizontal stress as a function of structural position, and thereby may facilitate natural fracturing. The models show that both stress and strain vary laterally within the reservoir and cap rock, in particular across fault steps. In areas where the minimum principal stress is smallest, high fluid overpressure may result in natural fracturing.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia