Changes

In [[:file:dipmeters_fig3.png|Figure 3]], a raw dipmeter curve sharply distinguishes between sandstone and shale layers thinner than [[length::1 in.]] By establishing a cutoff line as shown, everything to the right can be identified as sand and everything to the left as shale. A reliable measurement of net sand thickness is thus provided, as well as the bulk volume fraction of shale in laminated form for input into laminated shaly sand saturation equations.

In [[:file:dipmeters_fig3.png|Figure 3]], a raw dipmeter curve sharply distinguishes between sandstone and shale layers thinner than [[length::1 in.]] By establishing a cutoff line as shown, everything to the right can be identified as sand and everything to the left as shale. A reliable measurement of net sand thickness is thus provided, as well as the bulk volume fraction of shale in laminated form for input into laminated shaly sand saturation equations.

==Dip computation==

==Dip computation==

In the dip determination phase, one has a choice between a geometric solution and a stochastic approach. While a geometric solution works well in a uniquely determined (three-point) situation, it becomes cumbersome in an overdetermined condition such as that found with four-arm and six-arm tools. In these cases, a stochastic or global mapping approach is more effective in that it uses the redundancy to advantage in minimizing errors. Actually, this approach acts like another stage of stacking filtering. It has the added advantage of neatly solving for orientation of the tool in space.

In the dip determination phase, one has a choice between a geometric solution and a stochastic approach. While a geometric solution works well in a uniquely determined (three-point) situation, it becomes cumbersome in an overdetermined condition such as that found with four-arm and six-arm tools. In these cases, a stochastic or global mapping approach is more effective in that it uses the redundancy to advantage in minimizing errors. Actually, this approach acts like another stage of stacking filtering. It has the added advantage of neatly solving for orientation of the tool in space.

==Dipmeter presentations==

==Dipmeter presentations==

The most common presentation of dipmeter data is the ''arrow'' or ''tadpole plot'', which is a clever two-dimensional representation of a three-dimensional quantity. In this plot, the base of the arrow is positioned at the depth of the midpoint of the correlation interval, and the distance from the left-hand margin to the base of the arrow is proportional to the true dip angle as calibrated by the scale shown on the heading. The shaft of the arrow points in the downdip direction with true north being straight up the page. [[:file:dipmeters_fig4.png|Figure 4]] is a standard arrow plot that also carries a correlation gamma ray curve and maximum and minimum caliper values. On the right-hand side is a representation of the inclination angle and direction of the tool, which will usually be similar to the deviation of the borehole. A number of other computer generated presentations have been introduced. Many are useful in special situations, but none is a replacement for the arrow plot.

The most common presentation of dipmeter data is the ''arrow'' or ''tadpole plot'', which is a clever two-dimensional representation of a three-dimensional quantity. In this plot, the base of the arrow is positioned at the depth of the midpoint of the correlation interval, and the distance from the left-hand margin to the base of the arrow is proportional to the true dip angle as calibrated by the scale shown on the heading. The shaft of the arrow points in the downdip direction with true north being straight up the page. [[:file:dipmeters_fig4.png|Figure 4]] is a standard arrow plot that also carries a correlation gamma ray curve and maximum and minimum caliper values. On the right-hand side is a representation of the inclination angle and direction of the tool, which will usually be similar to the deviation of the borehole. A number of other computer generated presentations have been introduced. Many are useful in special situations, but none is a replacement for the arrow plot.

==Applications of dipmeters==

==Applications of dipmeters==

===Structural applications===

===Structural applications===

Superficially, the determination of structural dip from a dipmeter seems simple and straightforward. In practice, it may be tricky. Difficulties arise from questions of scale and perspective. ''Structural dip'' by definition is the dip of recognizable lithological unit boundaries in the general vicinity of the borehole. However, a sharp change in lithology, such as from a shale to a sandstone, is the signature of a catastrophic event in geological history. Therefore, such a contact is liable to be highly irregular over the extremely short section exposed by the borehole. On an outcrop, a field geologist would astutely measure the dip of an eyeball average of the contact. The stringent confines of the borehole offer no such luxury of perspective, and the section of the contact exposed is liable to be highly unrepresentative of the average structural dip. Outcrop perspectives cannot be extrapolated to the borehole, so a different approach is needed.

Superficially, the determination of structural dip from a dipmeter seems simple and straightforward. In practice, it may be tricky. Difficulties arise from questions of scale and perspective. ''Structural dip'' by definition is the dip of recognizable lithological unit boundaries in the general vicinity of the borehole. However, a sharp change in lithology, such as from a shale to a sandstone, is the signature of a catastrophic event in geological history. Therefore, such a contact is liable to be highly irregular over the extremely short section exposed by the borehole. On an outcrop, a field geologist would astutely measure the dip of an eyeball average of the contact. The stringent confines of the borehole offer no such luxury of perspective, and the section of the contact exposed is liable to be highly unrepresentative of the average structural dip. Outcrop perspectives cannot be extrapolated to the borehole, so a different approach is needed.

Given computation approaches tailored for structural applications, structural dip can then be defined by looking for a consistent trend on the arrow plot. The most repetitive dip should be the structural dip.

Given computation approaches tailored for structural applications, structural dip can then be defined by looking for a consistent trend on the arrow plot. The most repetitive dip should be the structural dip.

The interpretation of structural anomalies is best accomplished by comparison to a set of models—the simpler the model, the better. A simple model, such as the one shown in Figure 5 for a normal fault with drag, is adequate to describe the geometry of such a fault. Figure 6 shows a more complicated arrow plot of low angle dips, reducing to a minimum then increasing to a high angle with the azimuth changing continuously with the dip angle. This is the signature of a tilted plunging anticline. A cross-sectional sketch of the anticline can be produced using the rule of interchangability of perspectives in which the horizontal geometry is interpretable from the vertical pattern of dip. (The application of dipmeter data to solving structural problems is covered in “[[Evaluating structurally complex reservoirs]]”.)

[[file:dipmeters_fig7.png|left|thumb|{{figure number|7}}Field example of a detailed dip computation through a sequence of interrupted meandering stream point bars.]]

[[file:dipmeters_fig5.png|thumb|{{figure number|5}}Simple dip model for the description of a normal fault with drag.]]

The interpretation of structural anomalies is best accomplished by comparison to a set of models—the simpler the model, the better. A simple model, such as the one shown in [[:file:dipmeters_fig5.png|Figure 5]] for a normal fault with drag, is adequate to describe the geometry of such a fault. [[:file:dipmeters_fig6.png|Figure 6] shows a more complicated arrow plot of low angle dips, reducing to a minimum then increasing to a high angle with the azimuth changing continuously with the dip angle. This is the signature of a tilted plunging anticline. A cross-sectional sketch of the anticline can be produced using the rule of interchangability of perspectives in which the horizontal geometry is interpretable from the vertical pattern of dip. (The application of dipmeter data to solving structural problems is covered in “[[Evaluating structurally complex reservoirs]]”.)

 

[[file:dipmeters_fig6.png|thumb|{{figure number|6}}Model of a tilted plunging anticline as it would appear on an arrow plot.]]

===Stratigraphic applications===

===Stratigraphic applications===

For stratigraphic applications, comparisons to a set of models is also a valid interpretation approach, but in this case, very simple models do not work well. Sedimentary geology is too complex. In fact, hand-drawn models may not be valid at all. It is preferable to use independently verified field examples, such as the one shown in Figure 7. In this example, the sandy sediments are made up of a sequence of interrupted, truncated, and stacked point bars deposited by meandering stream activity. Few complete fining-upward point bar cycles (gravels to sands to shales) are present. Instead, erosional cuts and festoon cross-bedding indicate the start of a new cycle that may or may not be interrupted before the deposition of low angle planar current bedding. (Additional characteristics of meandering streams are covered in  “[[Lithofacies and environmental analysis of clastic depositional systems]]”.) In the general case, a catalog of field examples of dip plots from various known depositional environments is a valuable aid to stratigraphic dipmeter interpretation.

For stratigraphic applications, comparisons to a set of models is also a valid interpretation approach, but in this case, very simple models do not work well. Sedimentary geology is too complex. In fact, hand-drawn models may not be valid at all. It is preferable to use independently verified field examples, such as the one shown in [[:file:dipmeters_fig7.png|Figure 7]]. In this example, the sandy sediments are made up of a sequence of interrupted, truncated, and stacked point bars deposited by meandering stream activity. Few complete fining-upward point bar cycles (gravels to sands to shales) are present. Instead, erosional cuts and festoon cross-bedding indicate the start of a new cycle that may or may not be interrupted before the deposition of low angle planar current bedding. (Additional characteristics of meandering streams are covered in  “[[Lithofacies and environmental analysis of clastic depositional systems]]”.) In the general case, a catalog of field examples of dip plots from various known depositional environments is a valuable aid to stratigraphic dipmeter interpretation.

 

==See also==

==See also==