Changes

==Interpreting depth to basement==

==Interpreting depth to basement==

It is futile to attempt to define accurately the vertical dimension, Z, of adjacent source bodies with [[magnetics]] because of the inherent ambiguity of potential field methods in determining Z (see, e.g. .<ref name=ch14r8>Skeels, D. C., 1947, Ambiguity in gravity interpretation: Geophysics, vol. 12, p. 43–56., 10., 1190/1., 1437295</ref> Furthermore, seismic and subsurface methods measure depth so much more accurately than magnetics that it is unwise to try to compete with these excellent techniques. This is not to say, however, that we should not use magnetics to estimate the approximate thickness of the sedimentary section in a new basin, i.e., in determining whether it is 2, 5, or [[length::10 km]] thick, for example, to a usual accuracy of about ±15% under favorable conditions.

It is futile to attempt to define accurately the vertical dimension, Z, of adjacent source bodies with [[magnetics]] because of the inherent ambiguity of potential field methods in determining Z.<ref name=ch14r8>Skeels, D. C., 1947, Ambiguity in gravity interpretation: Geophysics, vol. 12, p. 43–56., 10., 1190/1., 1437295</ref> Furthermore, seismic and subsurface methods measure depth so much more accurately than magnetics that it is unwise to try to compete with these excellent techniques. This is not to say, however, that we should not use magnetics to estimate the approximate thickness of the sedimentary section in a new basin, i.e., in determining whether it is 2, 5, or [[length::10 km]] thick, for example, to a usual accuracy of about ±15% under favorable conditions.

==Interpreting fault throw==

==Interpreting fault throw==

[[file:using-magnetics-in-petroleum-exploration_fig14-9.png|300px|thumb|{{figure number|2}}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.]]

[[file:using-magnetics-in-petroleum-exploration_fig14-9.png|300px|thumb|{{figure number|2}}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.]]

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 [[:file:using-magnetics-in-petroleum-exploration_fig14-9.png|Figure 2]], 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 [[:file:using-magnetics-in-petroleum-exploration_fig14-9.png|Figure 2]], 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.

==Interpreting shear zones==

==Interpreting shear zones==

==Oil field example==

==Oil field example==

In [[:file:using-magnetics-in-petroleum-exploration_fig14-10.png|Figure 3]], 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 [[:file:using-magnetics-in-petroleum-exploration_fig14-10.png|Figure 3]], 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.

==Interpreting fault location==

==Interpreting fault location==

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Using magnetics in petroleum exploration]]

[[Category:Using magnetics in petroleum exploration]]