Chapter from: M 66:  Hydrocarbon Migration And Its Near-Surface Expression
Edited By 
Dietmar Schumacher and Michael A. Abrams

Authors:
B. J. Rostron and J. Tóth

Geochemistry, Generation, Migration

Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.   All Rights Reserved.
 

Hydrocarbon plumes emanating from breached reservoirs alter the subsurface environment through which they pass. The detection of these plumes and their alteration effects form the basis for most surface geochemical exploration programs. The mechanics of plume generation and migration, however, remain poorly understood in a quantitative sense, which can lead to reduced exploration success. A two-part study incorporating numerical simulations and field mapping was conducted to better understand the generation and subsequent migration of hydrocarbon plumes in the subsurface.

Numerical simulations of oil and water flow show that plume generation is controlled by the hydraulic properties of the system: the driving or "leaking factors" (oil/water density contrast and regional hydraulic gradient) and the resisting or "sealing factors" (entry capillary pressure and intrinsic permeability). Field mapping delineated a plume of saline water (>100 g/L dissolved solids) in the Mannville Group aquifer that appears to result from the mixing of vertically migrating saline Devonian waters with the more dilute Mannville waters. Saline water and oil leak upward out of Leduc pinnacle reefs, through the overlying Ireton aquitard, and into the Nisku aquifer. Numerous smaller oil plumes coalesce in the Nisku aquifer and continue to migrate vertically up into the Mannville Group aquifer.

The results have four implications for hydrocarbon exploration: (1) the formation of saline plumes above pinnacle reefs is controlled by the hydraulic properties of the flow domain; (2) geochemical exploration for Devonian pinnacle reefs has to be conducted at the Mannville Group level; (3) there will be little, if any, surface expression of Devonian hydrocarbon plumes in west-central Alberta; and (4) the saline plume may reach the surface in other areas of the basin where ascending fluid flow occurs and the Mannville Group is closer to the surface.