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The AAPG/Datapages Combined Publications Database

Houston Geological Society Bulletin


Houston Geological Society Bulletin, Volume 34, No. 8, April 1992. Pages 12-13.

Abstract: Rigorous Computer Processing of Multiple Non-Vertical Intersecting Faults among Multiple Surfaces


Richard B. Banks

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 Previous HitfaultNext Hit blocks are contourable surfaces and are the Previous HitfaultNext Hit surfaces.

MCS processes faulted systems in three steps designed to honor continuity of shape across faults.

  1. MCS first moves the Previous HitfaultNext Hit blocks to their pre-faulted positions together with the contained formation tops.
  2. 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.
  3. The third step is to rebreak, i.e. reverse the first step and return the Previous HitfaultNext Hit 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, Previous HitfaultNext Hit 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 Previous HitfaultNext Hit. When displacements are negative, MCS processes a reverse Previous HitfaultNext Hit. Scissors faults result from displacements which change signs within the mapped area. Areas with zero displacement show where a Previous HitfaultNext Hit 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 Previous HitfaultNext Hit system by making contour maps or cross-sections of all the faults in order to:

  1. Test the faults for reasonableness, e.g., do observed Previous HitfaultNext Hit cuts that have been assigned to the same Previous HitfaultNext Hit result in a picture that makes sense?
  2. 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?
  3. Infer and/or ascertain which faults form boundaries of a Previous HitfaultNext Hit 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 Previous HitfaultNext Hit 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 Previous HitfaultNext Hit and any a priori knowledge of the area.

Consider a system with two antithetic faults, Previous HitFaultNext Hit 'A' and Previous HitFaultNext Hit 'B', both of which are shown in Figure 1. Since Previous HitFaultNext Hit 'B' dies against Previous HitFaultNext Hit 'A', Previous HitFaultNext Hit 'B' is presumed to be younger. Previous HitFaultNext Hit 'A' is presumed to be the older Previous HitfaultNext Hit, 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 Previous HitfaultNext Hit 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 Previous HitfaultNext Hit event 'A' which is the block above Previous HitFaultNext Hit 'A' and which, incidentally, contains Previous HitFaultNext Hit '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 Previous HitfaultNext Hit 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 Previous HitfaultNext Hit restoration and multi-surface stacking there is a continuity of shapes (geologic features) across faults, and MCS does reasonable contouring in Previous HitfaultNext Hit blocks which have no well or seismic control.

Final Step in Previous HitFaultNext Hit Processing

The final step in MCS Previous HitfaultNext Hit 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 Previous HitfaultNext Hit traces as the intersection between structural surfaces and faults. Rigorous displacement or separation balance is achieved at all Previous HitfaultTop 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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