Post-Palaeocene evolution of regional groundwater flow systems and their relation to petroleum accumulations, Taber Area, southern Alberta, Canada

Abstract

Summary Three regionally extensive fluid dynamic systems have been mapped in a 23 500 km 2 portion of the Western Canada Sedimentary Basin in south-eastern Alberta. The Modern Land Surface System reaches depths of 300 m and is well adjusted to the modern relief. The subjacent Erosional Rebound System occupies most of the 600 m thick predominantly shale Colorado Aquitard including its often gas-bearing sand members. It is characterized by extensive regions of sub-hydrostatic pressures. Below the Colorado Aquitard is the Cypress Plain System which has strong upward-directed forces and mild but definite lateral components. The lateral components oppose the modern relief and are oriented from the centre of the study area toward its western and eastern boundaries where most of the known oil fields are also located. The three dynamic systems are interpreted to be transient stages of sequentially superimposed gravitational and dilatational force fields. The gravitational fields depend on topography and the dilatational field results from elastic rebound of the Colorado Aquitard in response to erosional removal of 700 m of overburden during Pliocene and Pleistocene times. The flow pattern that existed during the time of the Cypress Plain was generated after Palaeocene (58 Ma) or Eocene (36 Ma) times by a palaeo-relief dominated by the NE-SW-trending Bow Island Topographic High. It was composed of and maintained as cross-formational gravity-flow systems throughout at least the Miocene (i.e. for more than 30 m.y.). Oil migrated to and was entrapped in the discharge regions of these systems. Subsequently, dilation of the Colorado Aquitard created the hydraulic sink in the Erosional Rebound System and induced gas to exsolve and accumulate in the sands. Based on a one-dimensional solution of the diffusion equation, periods of 10 m.y. and 0.1 m.y. respectively were calculated to be required for pore pressures below and above the Colorado Aquitard to adjust to changes in the land surface. Compared with inferred ages and durations of palaeo-topographies, these times agree with the hypothesized evolutionary history of flow systems. There was sufficient time for flow in the Cypress Plain System to adjust to the post-Eocene relief but not enough time has yet elapsed for it to re-adjust to the Modern Land Surface which was initiated during middle Pleistocene (1 Ma) times. In contrast, the Modern Land Surface System has had 10 times the 0.1 m.y. required for it to adjust to the modern relief, and thus its apparent steady state is expected.