Geology Reference
In-Depth Information
Experimental Studies of Coalification
Vitrinite reflectance (
R
o
%) is the percentage of light reflected from the surface of polished vitrinite. It is
a standard measurement used to classify organic rocks, and standard values are associated with various ranks of
coal (Table 1.1.2).
R
v max
%,
According to Cooper (1996), the time
temperature index (TTI method), based on both laboratory studies and
observations of reflectance and temperature in drill holes, assumes that vitrinite reflectance doubles for every 10°C
increase in temperature. The reflectance of vitrinite is affected by the chemical reorganization of aromatic groups
associated with the liberation of reactants (namely, O and H). From these experiments, Burnham and Sweeney
(1989) have derived the following equation relating changes in vitrinite reflectance to changes in major element
concentration:
-
−
:
H
C
O
C
R
0
%
¼
12 exp
−
3
:
3
In artificial maturation experiments on one humic coal, samples were heated isothermally for 24 h at temperatures
between 200 and 800°C (Han et al., 2001). Based on vitrinite reflectance, the sample maturity increased from high-
volatile bituminous (
R
vmax
= 5.02%)
.
The process is characterized by two fast reaction
phases and two slow reaction phases. The first fast phase occurs at 0.9% vitrinite reflectance and is associated with
the removal of attached fragments from aromatic rings. The second fast phase coincides with the late anthracite
increase in reflectance. The increase in maturity was attributed to the temperature dependent breakage of C
R
vmax
= 0.77%) to anthracite (
-
C
bonds and the generation of gaseous hydrocarbons.
In another laboratory experiment, the change in vitrinite reflectance was determined over a pressure range of 0.5
-
20.0 kbar at temperatures between 200 and 350°C (Dalla Torre et al., 1997). The results indicated that applied
pressure suppressed increases in vitrinite reflectance.
Coal Composition
T
he composition of coal can be described by its components, by the elemental concentrations, and by macroscopic
and microscopic composition.
Table 1.1.2
Coals by rank and vitrinite reflectance (% R
0
).
Coal rank
Range
*
Random
†
Maximum
†
Minimum
‡
Class
Group
Anthracite
Meta-anthracite
Anthracite
6.55
7.00
2.50
Semianthracite
2.65
2.83
1.92
Bituminous
Low volatile
1.85
1.97
1.51
Medium volatile
1.00
-
1.40
1.49
1.58
1.12
High volatile
0.3
-
1.00
0. 5
Subbituminous
<
0.40
-
0.6
0.60
0.63
0.42
Lignite
0.40
0.42
*
Pearson Coal Petrography.
†
White. (2002).
‡
Smith et al. (1994).
