Page 119 - Synthetic Fuels Handbook

FUELS FROM TAR SAND BITUMEN 105

(e.g., those in Canada) or lie directly on the ancient basement (e.g., as in Venezuela, West
Africa, and Madagascar).
A feature of major significance in at least five of the major areas is the presence of
a regional cap (usually a widespread transgressive marine shale). Formations of this
type occur in the Colorado Group in western Canada, in the Freites formation in eastern
Venezuela, or in the Jurassic formation in Melville Island. The cap plays an essential role
in restraining vertical fluid escape from the basin thereby forcing any fluids laterally into
the paleo-delta itself. Thus, the subsurface fluids were channeled into narrow outlets at the
edge of the basin.
The potential reserves of hydrocarbon liquids (available through conversion of the bitu-
men to synthetic crude oil) that occur in tar sand deposits have been variously estimated on
a world basis to be in excess of 3 trillion barrels. However, the issue is whether or not these
reserves can be recovered and conversed to synthetic crude oil. Geographic and geologic
feature may well put many of these resources beyond the capabilities of current recovery
technology requiring new approaches to recovery and conversion.

4.1.1 Canada
In Canada, the Athabasca deposit along with the neighbouring Wabasca, and Peace River
deposits have been estimated to contain approximately 2 trillion barrels of bitumen.
The town of McMurray, about 240 miles north east of Edmonton, Alberta lies at the
eastern margin of the largest accumulation in the world that is, in effect, three major accu-
mulations within the Lower Cretaceous deposits. The McMurray-Wabasca reservoirs are
found toward the base of the formation and are characteristically cross-bedded coarse
grit and gritty sandstone that are unconsolidated or cemented by tar; fine-to-medium
grained sandstone and silt occur higher in the sequence. Bluesky-Gething and Grand
Rapids reservoirs are composed of sub-angular quartz and well-rounded chert grains. The
McMurray-Wabasca tar sand deposit dips at between 5 and 25 ft/mile (1.5 and 8 m/mile)
to the southwest. The Bluesky-Gething sands overlie several unconformities between the
Mississippian and Jurassic deposits.
In the context of the Athabasca deposit, inconsistencies arise presumably because of the
lack of mobility of the bitumen at formation temperature [approximately 4°C (39°F)]. For
example, the proportion of bitumen in the tar sand increases with depth within the forma-
tion. Furthermore, the proportion of the nonvolatile asphaltenes or the nonvolatile asphaltic
fraction (asphaltenes plus resins) in the bitumen also increases with depth within the forma-
tion that leads to reduced yields of distillate from the bitumen obtained from deeper parts
of the formation. In keeping with the concept of higher proportions of asphaltic fraction
(asphaltenes plus resins), variations (horizontal and vertical) in bitumen properties have
been noted previously, as have variations in sulfur, nitrogen, and metals content.
Obviously, the richer tar sand deposits occur toward the base of the formation. However,
the bitumen is generally of poorer quality than the bitumen obtained from near the top of
the deposit insofar as the proportions of nonvolatile coke-forming constituents (asphaltenes
plus resins) are higher (with increased proportions of nitrogen, sulfur, and metals) near the
base of the formation.

4.1.2 United States

The major tar sand deposits of the United States occur within and around the periphery of
the Uinta Basin, Utah. These include the Sunnyside, Tar Sand Triangle, Peor (PR) Springs,
Asphalt Ridge, and sundry other deposits (Table 4.1). Asphalt Ridge lies on the northeastern