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ABSTRACT: Effect of Hydrodynamics on Seals in Stratigraphic or Unconformity Type Traps: Case Histories - Updip vs Downdip Flow

Reid, Hugh W.1 and Norbert M. Hannon2
(1) Hugh W. Reid & Associates Ltd, Calgary, AB 
(2) Canadian Hydrodynamics Ltd, Calgary, AB

The ability of stratigraphic pinchout or unconformity traps to retain hydrocarbons is a function of the balance between the buoyancy (capillary) pressure of the hydrocarbon column and the lowest entry pressure of the lateral updip seal. Such traps are particularly susceptible to gradual "leakage" of hydrocarbons over time.

This is because the barrier facies is frequently not a uniform lithology such as shale with high entry pressure. Instead, the shale often contains thin silty laminations of lower entry pressure. The entire trap is therefore controlled by the "leakiest" stringer in the updip barrier.

Hydrodynamics can either increase or decrease the leakage potential of such traps. Updip flow is an additive force to the buoyancy pressure and contributes to leakage, conversely downdip flow enhances weak barriers.

In eastern and central Alberta every Viking Formation (Cretaceous) gas and oil field from Provost to Westlock is located in an area of downdip flow. By contrast in western Alberta there are several Viking oilfields located in an area of updip flow but these are in every case trapped against major lithologic barriers with high entry pressures, (e.g. Ferrier, Garrington & Leafland Fields).

In Saskatchewan regional updip flow occurs in all horizons and no large stratigraphic accumulations occur. The only known exception is the Mississippian Midale Formation where a local reversal in flow direction causes downdip flow at the subcrop edge below the unconformity zone. This aids in trapping the huge Weyburn/Steelman oil fields with 600 ft oil columns.

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