Previous "Geophysical Corner" articles by geophysicist
Dale Bird discussed conventional uses of aeromagnetics (May and June,
1997). This article, however, deals with the new and strikingly different
uses of aeromagnetics that are emerging.
These new uses are based on a better
understanding of basement geology and how it affects the overlying sedimentary
section.
First, it is necessary to understand
the structural nature of basement, and here I refer to Precambrian metamorphic
basement that comprises the major shield areas of the world, such as the
Canadian Shield, South American Shield, African Shield, Baltic Shield,
etc.
Shields are simply the outcropping
areas of cratons, or continents, and similar metamorphic terranes are
located under all cratonic sedimentary basins.
Figure 1
is a Landsat photograph that covers a portion of the Canadian Shield (NW
part) and a portion of the adjacent Ontario sedimentary basin (SE part),
where the shield is overlapped by oil-bearing lower Paleozoic strata.
The highly fractured nature of outcropping
basement is obvious, but the sedimentary rocks in the basin hide this
fracture pattern from view. The basement fractures are reactivated at
later times during, or after, deposition of the sedimentary section and
create structures and/or sedimentary facies that become oil and gas traps
and reservoirs. Thus, the mapping of the fracture pattern under the sedimentary
section is of great importance in hydrocarbon exploration.
How can this best be accomplished?
Neither seismic nor gravity methods
can map the basement fracture pattern, although both can map part of it.
Subsurface data cannot map the basement in any detail due to the limited
number of basement intercepts in most basins.
Only magnetics can map the covered
basement fault block pattern. Why can magnetics do this? It is because
of the rock type changes (resulting in magnetic susceptibility changes)
that occur across the basement faults.
Figure 2
is a detailed surface geology map of a 30 x 30 mile (50 x 50 km) block
of outcropping basement in Wisconsin. The basement faults (actually shear
zones) are shown, and the rock type changes across them are obvious.
Shear zones in the basement are one,
two or three kilometers in width, are characterized by crushed and broken
rock across the entire width, and are usually steeply dipping. It is these
pre-existing zones of weakness that relieve stresses resulting from later
tectonic events and/or sedimentary loading.
Stresses are not generally relieved
by newly formed faults at ±30° to maximum compressive stress, at least
not in the last two billion years.
Other characteristics of importance:
- The distance between shear zones is usually two,
three or four miles (3-7kilometers).
- They fall into sets of parallel structures.
- Three or four different sets of shear zones may
occur.
The resulting melange of basement blocks
is called the "basement fault block pattern."
How does one go about mapping the basement
fault block pattern with magnetics?
Figure 3a
shows a total intensity aeromagnetic survey on the north flank of the
Anadarko Basin, Oklahoma, flown with one-mile spaced east-west flight
lines. The sedimentary section is 10,000-12,000 feet thick; the aircraft
was approximately 1,000 feet above mean terrain.
This map is dominated by a single magnetic
high on the west and an elongated low on the east, 16 miles away. It obviously
is not mapping the two- to four-mile wide basement blocks. To bring out
the individual blocks it is necessary to residualize the data or to calculate
second derivatives. Either will suffice, although some computational techniques
are better than others.
(I prefer profile residuals calculated
along flight lines where possible, as shown in figure
3b.)
Each of the magnetic highs and lows
represents a separate basement block, and the faults (shear zones) occur
on the intervening boundaries, or gradients. These faults have been marked
with shear zone symbols (red) in figure 3c;
the fault block pattern by itself appears in figure
3d (blue).
A detailed subsurface map of this area
yielded only two faults cutting the sedimentary section (shown in red
in figure 3d). They occur precisely along
or very close to the mapped basement faults as hypothesized.
This is but one example of the correlation
of basement shear zones with mapped faults. Since 1982 we have generated
hundreds of such cases of correlating faults.
Also shown in red in figure 3d is a
structural high mapped using subsurface data at West Campbell oil field
that falls between basement shear zones. As basement shear zones generally
erode low, we hypothesize that the West Campbell high is coincident with
a basement topographic high and was formed by differential compaction.
Compaction anticlines represent another type of basement control important
in exploration.
Figure 3
is useful in demonstrating another point: Both subsurface faults shown
have magnetic lows on their upthrown sides.
If structure were the only factor in
determining magnetic amplitudes, magnetic highs would occur on the upthrown
sides. However, the lithology of the basement is the primary influence
on magnetic maps, and structure is a secondary, and sometimes insignificant,
factor.
Anticlinal Oil Fields Over Basement Faults
Figure 4
shows the relationship of an asymmetric fold (Ponca City field, Kay County,
Oklahoma, with production to 1993, >12 million barrels) to an underlying
basement fault, shown with shear zone symbols. Pennsylvanian compression
reactivated the fault, forcing the west side up along a west-dipping fault
plane to give rise to the overlying fold in Paleozoic strata (see lower
part of figure 4).
The fault is "blind," as it does not
break through to the level of the folded strata.
Figure 5
shows a fault that has broken through to the level of the folded strata
in Sage Creek anticline in the Wind River Basin, Wyoming, resulting in
a thrust-fold structure. The location of the thrust at basement level
is shown by shear zone symbols.
This is but one of a chain of several
folds in the western Wind River Basin that correlates closely with mapped
basement faults. It has a strike length of 70 miles and involves seven
separate faults. Four faults are northwest-trending, parallel to the Casper
Arch thrust; three are cross-faults that successively offset the northwest-trending
faults to the north.
We have developed over two dozen examples
of asymmetric folds related to basement faults such as those shown in
figures 4 and 5.
The cause and effect relationship can be clearly seen.
The conclusion is apparent that pre-existing
basement faults are reactivated to give rise to the fold-forming faults
in the overlying sedimentary section.
Next month:
A discussion of "purely stratigraphic" traps that occur over basement
faults.