Geology Reference
In-Depth Information
Fig. 11.12.
Representative segment of a
bed deformed by sub-resolu-
tion strain.
a
Undeformed.
b
Extension on small normal
faults.
c
Contraction on small
thrust faults.
d
Low-resolution
observation of
b
.
e
Low-reso-
lution observation of
c
4-6%, and perhaps closer to 1-2% in very brittle lithologies such as dolomite or
in extensional settings (Groshong 1988). If the deformation is brittle, additional
strain within the layers will commonly be accommodated by small-scale faulting
(Fig. 11.12b,c). Strain measured at the scale of the oil field or a seismic line may appear
homogeneous (Fig. 11.12d,e) but represent the combined effects of sub-resolution
faults and/or folds.
11.4
Area-Balance Methods
Area balance methods include the restoration of irregularly shaped deformed regions,
area-based depth-to-detachment calculations and the area-depth relationship. These
methods do not depend on specific kinematic models and in that sense represent the
most general approaches to validation, restoration, and prediction. All the structures
discussed in Sects. 11.4.1-11.4.3 are locally balanced (Fig. 11.5b). Regionally balanced
structures (Fig. 11.5c,d) will be treated in Sect. 11.4.4.
11.4.1
Area Restoration
Area restoration is used for structures in which deformation has produced significant
changes in the original bed lengths and thicknesses. The technique is based on the
area of the deformed-state cross section (Fig. 11.13a). It is assumed that the area has
remained constant:
A
0
=
t
0
L
0
,
(11.17)
where
A
0
= original area,
t
0
= original bed thickness, and
L
0
= original bed length. The
area between the pin lines is measured and then divided by either the original bed
thickness or the original bed length, whichever is better known, and Eq. 11.17 solved
for the unknown dimension. The original bed length might be known from that of an
adjacent key bed that has not changed thickness (Mitra and Namson 1989) or the origi-
nal thickness might be known from a location outside the deformed region. The shape
