Geology Reference
In-Depth Information
Fig. 11.21.
Area-depth interpretation of the Deerlick Creek coalbed methane field.
a
Cross section per-
pendicular to fault strike showing the calculated lower detachment at the base of the section. The
dashed
line
near the base of the section is the reference level. Units
1
-
8
are coal cycles.
b
Lost-area diagram for
the Franklin Hill and Strip Mine grabens. S
olid lines
are the least-squares best fits;
V.E.:
vertical exag-
geration. (Modified from Groshong 1994, after Wang 1994)
input values. The growth sequence includes depositional thickening which is greater
than the structural thinning, giving a net thickness increase in each growth unit. The
structural thinning (Table 11.1) can be recognized from the area balance (Eq. 11.27) in
spite of the net thickness increase due to deposition.
A typical field application to an extensional structure is illustrated by a pair of
grabens from the Pennsylvanian coal measures of the Black Warrior basin of Ala-
bama. The cross section (Fig. 11.21a) is based on multiple horizons from closely spaced
wells. Is this a valid cross section and how deep do the faults extend? The lost area is
measured for each coal cycle boundary. The positions of the cycle boundaries at their
footwall cutoffs coincide with regional dip across the area, from which it is inferred
that footwall uplift is negligible. Lines joining the footwall cutoffs are therefore cho-
sen as the regionals. The lost area for a given cycle boundary is the area bordered by
the position of the boundary in the graben, the faults, and the regional for that cycle.
The reference level was chosen to be at the base of the well control and parallel to
regional dip. The distance from each regional to the reference level is measured in the
center of each graben.
