Changes

There is a fairly reliable way to determine the direction of throw of certain basement faults from magnetic maps. Faults that vertically offset basement or other magnetic sources generally show abrupt amplitude changes of magnetic anomalies, both the highs and lows. In the figure below, a series of four northeast-trending magnetic anomalies on the west (two highs, two lows) abruptly loses amplitude along a northwest-trending line (A–A′) that crosscuts them. The high and low magnetic trends can be identified easily on both sides of this obvious down-to-the-east fault. The four anomalies disappear altogether along another northwest-trending line farther east (B–B′). This may be a strike-slip fault, which is not common in this area, or another down-to-the-east fault that has down-dropped the four anomalies beneath the level of detection—the preferred interpretation.

There is a fairly reliable way to determine the direction of throw of certain basement faults from magnetic maps. Faults that vertically offset basement or other magnetic sources generally show abrupt amplitude changes of magnetic anomalies, both the highs and lows. In the figure below, a series of four northeast-trending magnetic anomalies on the west (two highs, two lows) abruptly loses amplitude along a northwest-trending line (A–A′) that crosscuts them. The high and low magnetic trends can be identified easily on both sides of this obvious down-to-the-east fault. The four anomalies disappear altogether along another northwest-trending line farther east (B–B′). This may be a strike-slip fault, which is not common in this area, or another down-to-the-east fault that has down-dropped the four anomalies beneath the level of detection—the preferred interpretation.

[[file:using-magnetics-in-petroleum-exploration_fig14-9.png|thumb|{{figure number|14-9}}The figure below shows a residual aeromagnetic map of an area on the north shelf of the Anadarko basin in Oklahoma where the sedimentary section is approximately 3.6 km (12,000 ft) thick and basement lies about 3.8 km (12,500 ft) beneath flight level. The residual magnetic contours (a) are shown at a 2-nT interval. The interpreted shear zones are traced along the linear gradients separating the residual magnetic highs and lows and along truncation lines of anomalies. On the right figure (b), two faults located from subsurface mapping are shown, labeled U/D. The evidence for their existence is seen in subsurface mapping. Both are located exactly along the interpreted basement shear zones, or block boundaries, as represented by gradients on the magnetic map. Note, however, that most of the interpreted basement shear zones in this area have no corresponding overlying faults. These zones were never reactivated—at least not sufficiently enough to be detected by the existing subsurface data.]]

[[file:using-magnetics-in-petroleum-exploration_fig14-9.png|thumb|{{figure number|14-9}}.]]

The figure (right) shows a residual aeromagnetic map of an area on the north shelf of the Anadarko basin in Oklahoma where the sedimentary section is approximately 3.6 km (12,000 ft) thick and basement lies about 3.8 km (12,500 ft) beneath flight level. The residual magnetic contours (a) are shown at a 2-nT interval. The interpreted shear zones are traced along the linear gradients separating the residual magnetic highs and lows and along truncation lines of anomalies. On the right figure (b), two faults located from subsurface mapping are shown, labeled U/D. The evidence for their existence is seen in subsurface mapping. Both are located exactly along the interpreted basement shear zones, or block boundaries, as represented by gradients on the magnetic map. Note, however, that most of the interpreted basement shear zones in this area have no corresponding overlying faults. These zones were never reactivated—at least not sufficiently enough to be detected by the existing subsurface data.

==Oil field example==

==Oil field example==

In Figure 14-10, also note the structural high apparent in Devonian strata about [[length::800 m]] (2500 ft) above basement in the West Campbell field, conveniently nestled between block boundaries. Block boundaries, i.e., shear zones, generally erode low, so it follows that the interiors of blocks must, in many cases, correspond to basement topographic highs. West Campbell field appears to be a case in point and is most likely underlain by such a basement topographic prominence, although there are no wells to basement here to document it. The culmination of structural closure nearer the north end of the block rather than at its center is probably due to the south dip of basement in this area.

In Figure 14-10, also note the structural high apparent in Devonian strata about [[length::800 m]] (2500 ft) above basement in the West Campbell field, conveniently nestled between block boundaries. Block boundaries, i.e., shear zones, generally erode low, so it follows that the interiors of blocks must, in many cases, correspond to basement topographic highs. West Campbell field appears to be a case in point and is most likely underlain by such a basement topographic prominence, although there are no wells to basement here to document it. The culmination of structural closure nearer the north end of the block rather than at its center is probably due to the south dip of basement in this area.