By PARKER GAY
The Geophysical Corner is a regular column in the EXPLORER, produced cooperatively by the AAPG Geophysical Integration and SEG Interpretation committees, and edited by M. Ray Thomasson. This month's column is titled "Geologic Structure of Basement, and Techniques for Mapping Basement."
Basement Mapping Highly Crucial
Click on figure to see details
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 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.
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.