Abstract

An integrated stratigraphic and hydrodynamic study of deeply buried Mesozoic units in more than 2000 wells located within the Alberta basin illustrates the relation between reservoir architecture and pressures, contrasts the nature of regional fluid systems within select units of economic importance, and tests published models of fluid behavior in the Alberta basin. We evaluated these aspects for two Mesozoic intervals (Jurassic Fernie Formation and Lower Cretaceous Viking Formation) and, for comparative purposes, a Paleozoic unit (Devonian Nisku Formation).

The Nisku Formation is a normally pressured unit where fluid pressures increase systematically with depth in areally extensive carbonates and overpressured zones are restricted to locally developed shale basins. Gas columns have associated downdip aquifers, and water becomes increasingly fresh to the west, closer to regional recharge. Fernie and Viking units pass westward from underpressured, gas-saturated units downdip of aquifers to normally and overpressured zones, and pressures in excess of hydrostatic occur at depths below about 2600 m (8530 ft). Fernie and Viking units have contrasting styles of reservoir distribution. The former are productive from blanket units, whereas Viking production is overwhelmingly obtained from lenticular channel fills. The westward change from subnormal to supranormal pressures in Mesozoic units is associated with the lowest heat flow values and highest geothermal gradients in Mesozoic units of west-central Alberta. In the Viking, the highest pressures are associated with an increase in porosity due to grain dissolution.

This article provides several implications for understanding the nature and distribution of hydrocarbons in the Alberta deep basin. Although most studies of Mesozoic units of the Alberta deep basin have emphasized the underpressured nature of reservoirs, a pressure continuum occurs that links shallow, underpressured units with deeper, supranormally pressured zones. The maintenance of supranormal pressures at depth is due to effective vertical and lateral seals located in a zone of low heat flow but high geothermal gradient. Porosity does not progressively decline with depth in all Mesozoic units. Sandstone units containing large numbers of rock fragments and feldspars may display an increase in porosity values with depth where there are high temperatures and pressures. Highly porous sandstones may form reservoirs that have high gas deliverabilities and reserves; therefore, many lithofeldspathic Paleozoic and Mesozoic sandstones in the deepest parts of the Alberta basin may be prospective for petroleum. Generalized models of pressure distribution and maintenance that do not account for reservoir architecture and composition may overlook those apparently anomalous situations that have significant economic potential.

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