Geology Reference
In-Depth Information
also evident in the saturated hydrocarbon fingerprints of the carbonized coal (Figure 11.4.5B, C) and coal-tar samples
(Figure 11.4.5D, E). The coal-tar residue from vent 5 alone exhibits a distinctive
-C
20
normal alkane profile
that is analogous to a middle range petroleum distillate, like diesel fuel oil (Figure 11.4.5.E).
n
-C
10
to
n
The high-volatile bituminous coal reference sample exhibits normal hydrocarbons in the
-C
36
range with a
slight odd-carbon preference (Figure 11.4.6A) that is typical of bituminous coals of this rank (Stout and Emsbo-
Mattingly, 2008). As evident in many other bituminous coals, the proportion of pristane is significantly more
abundant than
n
-C
9
to
n
-C
17
and phytane. The coal distillates generated under controlled heating conditions at 500 and
1000°C contain a more complete homologous series of
n
-C
36
normal alkanes with no odd-carbon
preference or pristane dominance (Figure 11.4.6B). Interestingly, the coal tar generated at 500°C exhibits a bimodal
normal alkane profile with a depression in the
n
-C
9
to
n
n
-C
22
and
n
-C
23
range (Figure 11.4.6C). The heavier hydrocarbons
(
-C
36
) exhibit no odd-carbon preference and likely represent the residual coal-tar distillate that remains
after the lighter hydrocarbons cracked into gas-phase fragments.
n
-C
23
to
n
n
The distillate range hydrocarbons (
-C
23
) likely represent more recently formed normal alkanes distilled from the
carbonized coal bed. The coal tar generated at 1000°C also exhibits middle distillate and residual range normal alkanes
(Figure 11.4.6D). However, the former is strongly skewed toward an abundance of lower molecular weight hydrocarbons
(approximately
-C
9
to
n
n
n
-C
11
). These lower molecular weight hydrocarbons may form due to the cracking and
hydrogenation of higher molecular weight alkanes or liberation from the increasingly carbonized coal matrix.
-C
9
to
Coke is an extreme example of devolatilized coal. In addition to pyrogenic two-ring PAHs (Figure 11.4.4E), it
contains light molecular weight normal alkanes (Figure 11.4.6E). The presence of lighter normal alkanes in the
absence of acyclic isoprenoid hydrocarbons suggests that these light hydrocarbons have been cleaved from larger
molecular weight hydrocarbons or from the coal matrix. Despite relatively higher vapor pressures, these light
hydrocarbons are often the last semivolatile compounds to volatilize out of heavily carbonized coal or coke. This
apparent contradiction makes sense if these light hydrocarbons continuously crack off larger, more thermally stable
coal constituents after the initial assemblage of semivolatile hydrocarbons are devolatilized out of the coal matrix. The
coke residue also contains long-chain normal alkanes that exhibit a very strong to exclusive even-carbon preference.
The origin of these hydrocarbons is suspicious (contamination or misidentification) and under investigation.
Biomarker Stability
G
eochemical biomarkers maintain source-specific molecular signatures over geological time and resist environ-
mental weathering very well. Does this recalcitrance extend to thermal exposure over relatively short time frames,
that is, coal fires? Inspection of the distributions of pentacyclic triterpanes in unburned coal versus those in its
carbonized equivalent and coal-tar residues in this section (Figure 11.4.7) argue that biomarkers are mostly stable
with subtle difference under the heating conditions of coal fires; however, complete destruction is theoretically
possible under extreme conditions.
The Breathitt high-volatile bituminous coal (Figure 11.4.7A) is dominated by NH followed by Tm and H and
exhibits a distribution of triterpanes typical of most bituminous coals (Stout and Emsbo-Mattingly, 2008). The
higher proportion of geologically stable
stereoisomers among the homohopanes is
consistent with the bituminous coals, but not less mature lignite or subbituminous coals that have yet to reach
equilibrium conditions (Figure 11.3.4). As noted above, there are several subtle, but significant differences between
the Breathitt coal and the carbonized coal residues (Figure 11.4.7B, C). The proportions of bisnorhopane (BNH)/
Tm, M/H31-S, Tm/NH, and H/NH are all slightly lower in the native coal compared to the carbonized coal
residues. These triterpane pattern differences apparently reflect subtle changes attributable to thermal exposure.
Importantly, the triterpane pattern among carbonized coals (Figure 11.4.7B, C) and coal tars (Figure 11.4.7D, E) is
virtually identical. This compositional similarity demonstrates a high degree of source signature stability imparted
to both the carbonized coal residues and to the coal tars produced during the coal-fire process.
S
versus biologically active
R
The pattern of triterpane biomarkers in the high volatile bituminous reference coal is dominated by H with lower
concentrations of NH and H31-S, with still lower concentrations of Tm, M, and the rest of the homohopanes
(Figure 11.4.8A). This pattern is quite different from the Breathitt coal, but it shares the absence of tricyclic
triterpanes and very low concentrations of Ts (compare Figures 11.4.7A and 11.4.8A). The absence of tricyclic
triterpanes in coals is not unexpected since these compounds are typically derived from marine algal precursors, not
