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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Rigorous Computer Processing of
Multiple Non-Vertical Intersecting
Faults among Multiple Surfaces
By
pMaping-Contouring System (MCS) treats faulted
systems as what they are: Sets of three-dimensional blocks
containing geological markers (formation tops) which once
were continuous surfaces. The boundaries of these fault
blocks are contourable surfaces and are the
fault
surfaces.
MCS processes faulted systems in three steps designed to honor continuity of shape across faults.
- MCS first moves the
fault
blocks to their pre-faulted positions together with the contained formation tops.
- Then, having restored the "Continuous surface" attribute to the geologic markers, MCS performs all the "Stacking" (discussed later) and interpolations needed to obtain a smooth map or cross-section.
- The third step is to rebreak, i.e. reverse the first step
and return the
fault
blocks and their contents to their faulted positions, and to display as contour maps or cross-sections.
To accomplish these steps MCS needs certain data and a set of instructions:
To MCS, fault
vertical displacement or vertical separation
is just another mathematical surface which can vary
over the mapped area and can be contoured. When
displacements are positive throughout, MCS automatically
processes a normal
fault
. When displacements are negative,
MCS processes a reverse
fault
. Scissors faults result from
displacements which change signs within the mapped area.
Areas with zero displacement show where a
fault
ceased to
exist. Some growth faults can be modeled with displacements
which show great variation across the mapped area.
The user of MCS should (usually) first analyze the fault
system by making contour maps or cross-sections of all the
faults in order to:
- Test the faults for reasonableness, e.g., do observed
fault
cuts that have been assigned to the same
fault
result in a picture that makes sense?
- Infer and/or ascertain the hierarchy of the faults, i.e., when two faults meet in space, which one survives? Which one is, therefore, older?
- Infer and/or ascertain which faults form boundaries
of a
fault
block.
The analysis easily lends itself to "what if' games, i.e., the testing of various hypotheses.
The instructions MCS needs to perform its task are a
set of RESTORE commands, each of which describes a fault
block and instructs MCS to move it to its pre-faulted
position. The RESTORE commands takes us "back in
time," i.e. the first RESTORE reverses the most recent
faulting event, the last RESTORE reverses the oldest event.
The sequence and makeup of the RESTORE instructions
is derived from an analysis of the fault
and any a priori
knowledge of the area.
Consider a system with two antithetic faults, Fault
'A'
and
Fault
'B', both of which are shown in Figure 1. Since
Fault
'B' dies against
Fault
'A',
Fault
'B' is presumed to be
younger.
Fault
'A' is presumed to be the older
fault
, and the
system can be divided into three blocks: 1) Above 'B' and
above 'A', 2) Below 'B' and above 'A', and 3) Below 'A'.
In keeping with the principle that RESTORE commands
are taking us "back in time,'' we must restore first the block
which was displaced during fault
event 'B', i.e., the block
which lies above 'B' and above 'A', hence:
RESTORE (Above) 'B' Above 'A'
Next we must restore the block moved during fault
event 'A' which is the block above
Fault
'A' and which,
incidentally, contains
Fault
'B':
RESTORE (Above) 'A'
When MCS has finished complying with these commands
it would have, in effect, accomplished a vertical,
sequential, three-dimensional palinspastic reconstruction of
the fault
system. Now that the faulted system has been
restored to its pre-faulting configuration, the structural
surfaces are continuous and can be stacked.
Multi-Surface Stacking:
In areas of conformable geology, adjacent formations resemble each other. MCS uses the principle of conformable geology in its multi-surface stacking process. MCS
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first calculates isopachs (differences in value between adjacent surfaces) wherever possible. These isopachs are interpolated or extrapolated over the entire map area for all isopach intervals. Calculated isopach values are added or subtracted from known datums in order to reconstruct a complete set of Z values at all data points. This "stacking" proceeds downward first and upward second.
As a result of fault
restoration and multi-surface
stacking there is a continuity of shapes (geologic features)
across faults, and MCS does reasonable contouring in
fault
blocks which have no well or seismic control.
Final Step in Fault
Processing
The final step in MCS fault
processing is to re-break
and move geologic surfaces to their true (post-faulted)
positions. This step is the mathematical inverse of restoration.
MCS generates
fault
traces as the intersection
between structural surfaces and faults. Rigorous displacement
or separation balance is achieved at all
fault
intersections.
Figures 2 and 3 show contour maps for the 8500-
Ft. and 9200-Ft. sands.
Figure 1. Cross section showing the time-sequence of two antithetic faults.
Figure 2. Contour map for the 8500 Ft. sands.
Figure 3. Contour map for the 9200 Ft. sands.
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