Page 118 - Synthetic Fuels Handbook

104 CHAPTER FOUR

Use of the term organic sediments is more correct and is preferred. The inclusion of coal
and oil shale kerogen in the category of hydrocarbon resources is due to the fact that these
two natural resources will produce hydrocarbons by thermal decomposition (high-temperature
processing). Therefore, if either coal and/or oil shale kerogen is to be included in the term
hydrocarbon resources, it is more appropriate that they be classed as hydrocarbon-producing
resources under the general classification of organic sediments. Thus, tar sand bitumen
stands apart from petroleum and heavy oil not only from the method of recovery but also
from the means by which hydrocarbons are produced.
It is incorrect to refer to tar sand bitumen as tar or pitch. In many parts of the world
the term bitumen is used as the name for road asphalt. Although the word tar is somewhat
descriptive of the black bituminous material, it is best to avoid its use with respect to natural
materials. More correctly, the name tar is usually applied to the heavy product remaining
after the destructive distillation of coal or other organic matter. Pitch is the distillation
residue of the various types of tar.
Physical methods of fractionation of tar sand bitumen can also produce the four generic
fractions: saturates, aromatics, resins, and asphaltenes. However, for tar sand bitumen, the
fractionation produces shows that bitumen contains high proportions of asphaltenes and
resins, even in amounts up to 50 percent w/w (or higher) of the bitumen with much lower
proportions of saturates and aromatics than petroleum or heavy oil. In addition, the pres-
ence of ash-forming metallic constituents, including such organometallic compounds as
those of vanadium and nickel, is also a distinguishing feature of bitumen.

4.1 OCCURRENCE AND RESERVES

Tar sand deposits are widely distributed throughout the world in a variety of countries.
The various tar sand deposits have been described as belonging to two types: (a) materi-
als that are found in stratigraphic traps, and (b) deposits that are located in structural traps.
There are, inevitably, gradations, and combinations of these two types of deposits and a
broad pattern of deposit entrapment are believed to exist.
The distinction between a structural trap (the usually description of a petroleum res-
ervoir) and a stratigraphic trap is often not clear. For example, an anticlinal trap may be
related to an underlying buried limestone reef. Beds of sandstone may wedge out against
an anticline because of depositional variations or intermittent erosion intervals. Salt domes,
formed by flow of salt at substantial depths, also have created numerous traps that are both
a structural trap and a stratigraphic trap.
In general terms, the entrapment characteristics for the very large tar sand deposits
all involve a combination of stratigraphic and structural traps. Entrapment character-
istics for the very large tar sands all involve a combination of stratigraphic-structural
traps. There are no very large tar sand accumulations having more than 4 billion barrels
in place either in purely structural or in purely stratigraphic traps. In a regional sense,
structure is an important aspect since all of the very large deposits occur on gently slop-
ing homoclines.
Furthermore, the tar sand deposits of the world have been described as belonging to two
types. These are (a) in situ deposits resulting from breaching and exposure of an existing
geologic trap and (b) migrated deposits resulting from accumulation of migrating material
at outcrop. However, there are inevitable gradations and combinations of these two types
of deposits. The deposits have been laid down over a variety of geologic periods and in
different entrapments and a broad pattern of deposit entrapment is believed to exist
since all deposits occur along the rim of major sedimentary basins and near the edge of
Precambrian shields. The deposits either transgress an ancient relief at the edge of the shield