Entrapment of hydrocarbon accumulations in the deepest region of the Alberta sedimentary basin is linked to certain principles of subsurface fluid flow behavior in generally tight sandstones, and to active hydrocarbon generation in adjacent source beds. Similar geologic conditions and associated deep-basin-type hydrocarbon accumulations no doubt exist in the deeper portions of other sedimentary basins.

Conventional concepts of subsurface hydrocarbon accumulation do not apply to the deep basin form of entrapment, yet both mechanisms conform to certain important physical principles of fluid flow behavior. The conventional entrapment idea, based on many case histories and sound physical principles, entails downdip hydrocarbon-over-water contacts, initial reservoir pressures greater than formation water pressures at the same position (the so-called "capillary displacement pressures"), updip reservoir seals, and accumulations that are essentially in static equilibrium.

Deep-basin concepts of hydrocarbon entrapment, on the other hand, are opposed to conventional ideas. Hydrocarbon-bearing sands in deep basin regions grade laterally updip into permeable water-bearing sands without reservoir barriers to segregate the fluids. Water-over-hydrocarbon contacts appear at the updip limits of the accumulations and generally are absent downdip. Original hydrocarbon accumulation pressures are usually less than projected formation-water pressures at common depth points. Deep-basin accumulations fed by active downdip source rocks may be in a dynamic state of slow, updip hydrocarbon migration. Hydrocarbons lost across the updip water/gas contact are replaced by new hydrocarbon influx from downdip source rocks. Prolonged hydrocarbon flux through deep-basin reservoi s may result in exceptionally low residual water saturations and favorable hydrocarbon relative permeabilities in tight sandstones.

These unusual physical principles of the Alberta deep-basin hydrocarbon accumulations are illustrated by Elmworth field examples and by physical fluid flow models.

End_of_Article - Last_Page 480------------