Gravity‐induced cross‐formational flow of formation fluids, red earth region, Alberta, Canada: Analysis, patterns, and evolution

Abstract

Three zones of different generations of formation‐fluid flow systems were identified from analyses of the potentiometric surface, hypsographic distribution of fresh water heads, pressure‐depth relations, water table elevations, and dynamic pressure increments observed in five extensive water‐bearing units in a 20,400‐mi2 (52,840 km2) geologically mature area in northern Alberta. In each zone, fluids move in gravity‐induced flow systems maintained by cross‐formational energy transfer and subject to past or present boundary conditions. The force fields and associated flow systems in the basal zone of Middle Devonian aquifers were generated by and adjusted to the topography of the Pliocene continental surface. However, subsequent to the erosional exposure of the sub‐Cretaceous unconformity about Pleistocene times, the drainage of the middle zone disrupted the supply of energy from the land surface to the Pliocene flow systems and changed them into slowly decaying relicts of regional fluid dynamics tending toward hydrostatic equilibrium. The same event resulted in the formation of the Pleistocene flow systems of the middle zone which straddles the sub‐Cretaceous unconformity and comprises Paleozoic and basal Cretaceous strata. It is recharged mainly in the area's major hill ranges through the overlying thick succession of Cretaceous clastic rocks which include major aquitards also, and partly through reef ‘chimneys’ from the underlying basal zone. Fluids from the middle zone discharge by ascending cross‐formational flow in the major valleys and also by flow along the bedding planes through the unconformity's outcrops at low elevations beyond the boundaries of the area. These fluids are therefore at relatively low potential, which is adjusted to present boundary conditions. The upper hydrodynamic zone is perched on top of the principal Lower Cretaceous aquitard of the area and includes Cretaceous and Quaternary aquifers. Its flow systems are generated by local differences of the Holocene (present) topography and extend only a few hundred feet in depth. Downward leakage from this zone supplies recharge to the middle hydrodynamic zone. Solution of the diffusion equation for transient fluid potentials suggests a halflife time of approximately 0.7 m.y. for the excess hydraulic heads in the basal zone, indicating pre‐Pleistocene origin for the observed flow distributions in that zone and thus supporting conclusions derived from the steady state patterns of formation‐fluid pressures.