Geology Reference
In-Depth Information
the increasing abundance of normal alkanes eluting between
-C
36
). Other
features reflect the nature of the organic matter from which the coal formed. For example, many coals exhibit
different ratios of normal alkanes (e.g.,
n
-octane (
n
-C
8
) and
n
-hexatricosane (
n
-C
17
) relative to acyclic isoprenoid hydrocarbons (e.
g., pristane; a.k.a., Pr). Coal frequently contains low proportions of normal alkanes relative to isoprenoid
hydrocarbons (
n
-heptadecane; a.k.a.,
n
0.5), largely due to the oxidizing conditions of formation that tend to favor formation
of pristane from the phytol side chain of chlorophyll. In petroleum, that is largely derived from marine organic
matter under more reducing condition, higher ratios of
n
-C
17
/Pr
<
n
-C
17
/Pr are common. In coal that is largely derived from
terrigenous organic matter, the ratio of
-C
17
/Pr often reflects the genetic composition of the coal; that is, dominant
isoprenoid hydrocarbons formed from chlorophyll mixed with heavy normal alkanes (
n
n
-C
25
to
n
-C
37
) from leaf
waxes. The isoprenoid enrichment is particularly evident in ratios such as
n
-C
17
/Pr and Pr/Ph that frequently differ
by a factor of 4 over broad geographic ranges (Figure 11.3.3B
-
D).
Geochemical Biomarkers
G
eochemical biomarkers include many families of related compounds derived from ancient biomolecules. The
triterpane biomarkers are one family of geochemical biomarkers that exhibits a high degree of formation fidelity;
that is, the pattern of triterpane biomarkers is frequently specific to a particular contiguous coal bed or petroleum
reservoir. The unique triterpane patterns reflect the specific mixture of organic biomass deposited in a region and,
for some thermally sensitive stereoisomers, the thermal history of the organic matter after deposition. The triterpane
biomarkers are detectable in virtually all fossil fuels, but they are particularly abundant in high and medium volatile
bituminous coals (Stout and Emsbo-Mattingly, 2008). Interesting differences appear among many subbituminous
and high-volatile bituminous coals reflecting differences in the original biomass and thermal histories of these
coals. For example, stereoisomers, like H31-R is more abundant than H31-S (Figure 11.3.4A), which reflects a
prominence of the less stable biologically produced H31-R stereoisomer over the more stable, thermally produced
H31-S stereoisomer, indicating a lower rank coal that has not yet experienced the thermal stress necessary to form
high-volatile bituminous coal with a thermally stable (equilibrated) ratio of H31-S/H31-R greater than 1 typical of
higher rank coals (Figure 11.3.4B).
As the coal rank increases, signs of increasing maturity are often evident. For example, the ratio of H32-S/H32-R
shifts from less than 1 to greater than 1 as the coal in the Uinta Region matures from high-volatile bituminous C to
B (compare Figure 11.3.4B, C). Coals from different formations often exhibit very distinct triterpane patterns that
can reflect distinct microbial inputs. For example, norhopane (NH) is less abundant than Tm or hopane in the Uinta
Region coals (Figure 11.3.4B, C), but it is the dominant triterpane biomarker in the Breathitt Formation of
Kentucky (Figure 11.3.4D). In summary, triterpane biomarkers exhibit features with a high degree of sensitivity
reflective of the genetic properties (e.g., types of organic matter and thermal history) that can help distinguish coal
by both type and rank.
