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FAULT
MAP/STRUCTURE MAP INTEGRATION: A CRITICAL TECHNIQUE FOR CORRECT
FAULT GAP/POLYGON MAPPING
By:
Joseph L. Brewton
Accurately
mapping fault gaps on a structural horizon map is critical to
determining well locations, prospect and reservoir maps, and
reserves. Integration or cross contouring a fault surface map
with a structure horizon map is the most effective method for
accurately positioning fault gaps on a faulted structure map.
Incorrectly mapping fault gaps has resulted in numerous dry holes
due to wells being drilled into fault gaps or not in optimal
positions and inaccurate reserves determinations. Structure horizon
maps that do not properly honor fault cuts from well logs and
fault segment interpretations from seismic data will have fault
gaps (or fault polygons) mapped in the wrong locations and with
the incorrect widths. As faults and horizons are three-dimensional
surfaces, where these surfaces intersect determines the position
of the fault traces that geometrically define the fault gap or
polygon. Also, this basic principle of intersecting surfaces
applies to reverse faults that have overlap rather than gap.
Fault polygons are often drawn on seismic time structure maps
by connecting the points where the geophysicist stopped picking
the horizon as it approached the fault rather than the projected
intersection of the horizon with the interpreted fault surface.
Often geologists position fault traces on structure maps by assuming
a universal fault dip of 45 degrees and ignoring the impact of
the dipping and curved horizon surfaces. These common erroneous
procedures result in faulted structure maps that have errors
that vary from minor to disastrous.
A
model faulted structure map is shown in Figure 1. It shows the
following 4 (four) elements: 1) the fault surface (dashed lines);
2) the horizon (solid lines); 3) the intersection points of the
fault surface and horizon (dots with some points interpolated);
4) the resulting fault gap for a fault with a uniform 500 feet
of missing section (or vertical separation) in well log correlations.
For a discussion of missing section, vertical separation and
throw, see Mapping Throw In Place of Vertical Separation: A Costly
Subsurface Mapping Misconception on the SCA website www.scacompanies.com
under Publications Quick Look Techniques. Note that where contours
of the same value on the fault surface and structure horizon
maps intersect an intersection point is illustrated on Figure
1 as a dot. When these dots are connected for the respective
upthrown and downthrown fault traces, these traces define the
fault gap. The width of the gap and direction of the traces vary
due to the curvature of the horizon and dip relationship of horizon
dip to fault dip. For example, near the crest of the structure
the gap width is smaller because the horizon dips in the opposite
direction that the fault dips. As the fault intersects with the
horizon on the flanks of the structure, the dip direction of
the horizon changes with respect to the fault dip. The result
is a slightly wider fault gap. Also, the fault traces curve even
though the fault itself is planar. This is due to the curvature
of the horizon that intersects the fault.
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The
basic method of generating the intersection points for the
fault gap is to overlay a partially completed horizon map
on a completed fault surface map. The approximate position
to stop the horizon contours for each fault block can be
estimated by referring to the underlying fault surface map.
Then the horizon contours can be extended to the intersection
points with the fault surface contours. Note that projections
of the horizon contours continue into the opposite fault
block along the same trajectory, as if there were no fault
present. However, the horizon contour values change by the
amount of missing section as determined from the well log
correlations, in this case by 500 feet. This honors the interpretation
that the planar fault surface has not deformed the hanging
wall block except for the fault offset. This is also the
correct method for honoring missing section when contouring
across a fault. If the hanging wall block has been deformed,
then each fault block is contoured separately using the available
data. Ideally, a 3D seismic data set would be available to
map each fault block before integrating the fault surface
map with the horizon map to determine the position of the
fault polygons. The last step is drawing the fault traces
for each fault block by connecting the intersection points
to generate the fault gap. Detailed discussions of this method
applied to seismic workstation mapping as well as manual
mapping can be found in: Tearpock and Bischke 2003 "Applied
Subsurface Geological Mapping with Structural Methods", Edition
2, published by Prentice Hall, at www.scacompanies.com.
The
importance of this technique is that a map of the fault surface
integrated or cross-contoured with a structure horizon map
creates an accurate determination of the area where the horizon
or reservoir will be faulted out. There is no need to guess
based on a collection of fault cuts from well correlations
or meandering horizon terminations on a time structure ribbon
map. Furthermore, the steeper the horizon dip becomes, the
less intuitive the position of the fault traces and gap width
becomes. Inaccurate fault interpretations can result in wells
drilled in the wrong fault block or wells drilled into the
fault gap, and thus, being faulted out at the objective reservoir.
Also due to seismic data quality limitations near faults, such
as fault shadows, fault polygons drawn based on horizon picking
terminations tend to be too wide. This can result in an under
estimate of reserves for a reservoir or an attic well location
that is too far down dip. These are costly mistakes.
The
fault map/structure map integration technique assures the three-dimensional
geometric and structural validity of the faulted structure
map. It accurately determines the position and shape of the
fault traces, graphically and rigorously maps the changing
width of the fault gap (or overlap for reverse faults), and
correctly honors missing section data when projecting contours
across faults. Many reservoirs and prospects are bounded by
faults. Accurately determining where those faults are located
can be critical to a successful well design, the economic evaluation
of a discovery, or the determination of remaining reserves
in an existing field. |
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