ABSTRACT: The role of in-situ stress and overpressure on hydrocarbon exploration on the Halten Terrace, offshore Mid-Norway

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

Abnormal fluid pressures are encountered in Jurassic reservoirs on the Halten Terrace, offshore mid-Norway. A comparison of pressure-depth plots from different regions reveals the existence of multiple pressure compartments. Pressure compartments bounded by faults can best explain the fluid pressure distribution. Fluid pressures on the western part of the terrace approach the minimum horizontal stress as determined from in-situ stress data. Reservoirs in this highly overpressured region are mainly devoid of hydrocarbons. It is reasonable to believe that ineffective seals are the cause of exploration failure where water-filled reservoirs occur in what seem to be well defined closures, and where the reservoirs are also in direct contact with mature, organic-rich source rocks.

The mid-Norwegian margin is in a compressive state of stress and the role of tectonic stresses must therefore be considered in the evaluation of seal integrity. We have approached this problem by applying a geomechanical model in order to assess the effect of regional intraplate compression on seal integrity.

Our model predicts spatial variations in minimum horizontal stress in the caprock sequence. The predictions indicate that structurally controlled variations in the minimum horizontal stress may be a factor to consider regarding the preservation potential of hydrocarbons in deeply buried reservoirs where the retention capacity is low due to high fluid pressure. Secondly, the model predicts stress and strain localization in the vicinity of a tight fault zone that holds a fluid pressure difference of 30 MPa. We interpret this to be the result of the flexural response of fault controlled pre-existing basin geometry to regional tectonic compression.

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