Page 194 - Geology and Geochemistry of Oil and Gas

PRIMARY ACCUMULATION AND FREE PHASE MIGRATION 163

Fig. 9.2. Profile of the Acheson-Homeglen-Rimbey-Leduc reef trend (after Horsefield, 1962; Hnatyuk
and Martinelli, 1967; courtesy of Pet. Soc. of CIM).

The migrating gas, however, will continue to be trapped in trap A during stage 1.
Traps C and D contain only water. In stage 2, trap A is completely filled with gas
and all the oil has been displaced to trap B, which is completely filled with oil. Oil is
starting to spill out to trap C. Trap D still contains only water. In stage 3, trap A
remained a gas reservoir and both oil and gas are bypassing it. Trap B now has a gas
cap and oil is bypassing it; oil is also expelled as the gas cap expanded. Trap C is now
filled with oil but still has no gas cap. Trap D is filled with water.
Gussow (1968) cited the oil and gas accumulations in the Leduc reefs on the
Acheson-Homeglen-Rimbey trend, Alberta, Canada, as a general illustration of this
process (Fig. 9.2). Homeglen-Rimbey is an oil reservoir with a very large gas cap.
Westerose South, immediately updip, is all gas. Farther up the trend, Westerose,
Bonnie Glen D-38, and Bonnie Glen D-3A are oil pools with free gas cap. Next come
Wizard Lake and Glen Park, oil pools without gas caps, and finally Leduc-
Woodbend, Yekau Lake, and Acheson, which have gas caps. The presence of gas
caps in the last three reservoirs, even though they are updip from the two pools that
do not have gas caps, can be explained by the large difference in depth of burial
between the two areas, which is approximately equal to 1000 ft. The lower pressure
in the shallower reservoirs allowed some of the gas to come out of solution and form
a gas cap.