GEO ExPro

Beyond Tadpoles, Beyond SCAT

Complete dip and image analysis improves structural interpretations and reduces risk.
This article appeared in Vol. 7, No. 4 - 2010

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The tadpole heads indicate measured depths and dip angles while the tails indicate azimuths. Clearly, a trend line can be fitted to the dip angles, but not to the azimuths. Thus, tadpole plots are not suited for 3D statistical analysis of dip data. Is your seismic fair or poor? Are your structures complex? Are your reservoirs sub-thrust, sub-basalt, sub-salt, or pre-salt? Are you working in areas where seismic is not permitted? Are you working with low-relief structures where dip and azimuth are difficult to determine seismically? Are you dealing with sub-seismic structures?

Structural interpretations in these situations are often poorly constrained, increasing the risk of costly errors, so additional structural control is gained from dip-meters and borehole images. The highly effective but underutilized Statistical Curvature Analysis Technique (SCAT) allows the extraction of additional structural control from dip data than the traditional tadpole plot analysis, but the GEODES dip analysis software from Computational Geology, an independent consultancy specializing in detailed, complete dip analysis, has been designed to reduce risk even further.

Borehole images allow one to distinguish structural from depositional dip and pick and orient faults more accurately than dipmeter logs. However, in a real sense, image logs are just dipmeter logs on steroids: they both effectively just find the dip only at the borehole location. But the information actually wanted is about the overall geological structure being drilled, its orientation, and whether that orientation changes within the wellbore. Most importantly, a prediction of the structure away from the borehole is needed.

This software squeezes even more geological structure control from dip data. It markedly improves structural interpretations by correcting them and adding important structural detail, especially where seismic is of poor quality. After all, dip data provide “ground-truth” structural control, whereas even modern seismic is a still a remote-sensing tool.

What's wrong with tadpole plots?

Dip vs. depth (D) plot (left), shows that dip changes with depth, indicating that this interval is folded. The D plot also suggests that there might be a dip reversal between 13,000 and 14,500 ft, where dip is at a minimum, but without identifying the trend and plunge of the fold or picking the precise location of the dip reversal.Azimuth vs. depth (A) plot (centre) shows azimuth changing with depth, indicating that the fold has non-zero plunge, but it is difficult to infer from this the precise fold trend and plunge. The data form a U-shaped curve on the Dip Versus Azimuth (DVA) plot (top right) indicating a plunging fold, and a straight line on the tangent plot (bottom right) indicating a cylindrical fold. The dip and azimuth at the bottom of the U-shaped curve give the plunge and trend of the fold, as do the amount and direction by which the trend line on the tangent plot is offset from the centre. These plots indicate that the fold plunges ~5º toward 293 (N67W) – far more information than available from the tadpole plot. A key step in structural dipmeter analysis is to estimate true structural dip at a number of points along the borehole from machine- or hand-picked dips, which show considerable scatter due to measurement and correlation errors, and to depositional and similar effects. If dip were a scalar, we could find true structural dip at any depth by fitting a trend line to the data and using the trend-line value at that depth. Dip, however, is a vector. For example, the vectorial average of 5of north dip and 5o of south dip is not 5o, as the tadpole plot might suggest, but 0o.

Although the classic tadpole plot is a very clever way to show both dip and azimuth on a single plot, dip vectors cannot be analyzed properly on tadpole plots. While it is possible to fit a trend line to the dip angle of the tadpole heads, it is not possible to fit one to the azimuth tails. So in order to fit trend lines to dip vectors, one must resolve them into two components and plot the components separately against depth.

The benefits of SCAT

The GEODES software incorporates SCAT, which comprises several different plots of the dip or image data and a number of structural principles for analyzing and interpreting them. The software then produces several different plots of the data and a number of structural principles for analyzing and interpreting them. Although SCAT has been around for 30 years, it is still not a well-known method and therefore grossly underutilized, giving companies that adopt SCAT an important competitive advantage.

SCAT works by taking both azimuth and dip angle into account and making two separate plots, a Dip vs. Depth plot (D-plot) and an Azimuth vs. Depth plot (A-Plot). Additionally, it resolves dip vectors into apparent dips parallel (Longitudinal) and perpendicular (Transverse) to geological structure and makes separate depth plots - the L-plot and the T-plot - of these components.

Finally, SCAT fits trend lines to the T- and L-plots, to show how the apparent dips in the T and L directions change with depth. By examining the geometry of these trend lines, you can pick and orient important structural surfaces, such as crest, trough, inflection and plunge-reversal surfaces, and structural discontinuities like faults (including ones difficult to see on seismic), log correlations, and tadpole plots, and transfer this critical control to T and L structure sections. It is often difficult or impossible to pick these important surfaces on tadpole plots. The trend lines can be sampled every few feet or meters and these samples posted on the sections as apparent-dip barbs along the borehole.

All this assumes, of course, that you can determine the T and L directions. This is easy with SCAT, but it is often difficult using tadpole plots; in fact, if a well is drilled near the crest of a plunging fold, tadpole-plot analysis can miss the structural trend by 90°!

To determine structural orientation, SCAT uses a polar plot of the tangent of dip and a rectangular Dip Versus Azimuth (DVA) plot. Dip data from different structural settings (e.g., homoclinal dip, non-plunging fold, plunging fold) show distinctly different patterns on DVA plots,, while dips from cylindrical and conical folds show different patterns on tangent plots. Moreover, the position of a pattern on DVA and tangent plots indicates the dip and azimuth in planar domains, or the trend and plunge of folded structures.

Traditional stereographic and equal-area projections can also indicate structural orientation, but are more difficult to interpret than DVA and tangent plots. Additionally, traditional projections and numerical techniques do not distinguish double from single plunging folds, whereas DVA and L-plots do so easily.

Beyond SCAT



Dimitri Massaras is a Structural/Petroleum Geologist with Computational Geology where he uses the GEODES software to develop prospects worldwide. He has over 32 years of experience developing prospects, first with Gulf Oil Company and later with Petrofina and Pennzoil. Jim Morse is a Structural/Petroleum Geologist and founder and President of Computational Geology. He was instrumental in the development of the GEODES software and has over 25 years of worldwide experience interpreting dip data, first at Amoco and later with Computational Geology. T-section and structure map on the P13 top (shown in red), computed from the T and L sections. The “TTT” symbols on the map indicate the line of section. Tadpoles on the structure map indicate the azimuths; the lengths of the tadpoles are proportional to the dip angles. While GEODES incorporates SCAT techniques and methods, it goes beyond it by recognizing and dealing with false and missing machine-picked data. It can filter data according to quality, lithology, or trends in the dips themselves, pick structural domain boundaries (i.e. surfaces across which structural orientation changes abruptly), automatically fit trend lines to the T- and L-plots, automatically pick and orient important structural surfaces, such as fold crests (which are often difficult to pick accurately on seismic and tadpole plots), and remove structural dip in order to analyze the depositional ‘under-print’. Structural dip is removed continuously, ensuring accurate results even in zones where it changes rapidly.

In wells that cross domain boundaries, DVA, tangent, stereographic, and equal-area plots show complex, composite patterns that are difficult or impossible to interpret. Traditional numerical techniques yield a single, compromise orientation that doesn't accurately represent the orientation of any of the domains. GEODES includes a powerful graphical tool for picking domain boundaries so that users can accurately determine structural orientation domain by domain.

Analysis of the dips within fault zones places important constraints on fault orientations and is a critical complement to borehole-image analysis, which provides the orientations of faults only at the borehole. Owing to local irregularities, these can be quite different from the overall or ‘average’

The software provides innovative plots showing how the azimuth frequency distribution varies with depth. It helps see through the scatter in the data, generates a structurally smoothed dip log, and semi-automatically converts it to local structure sections. For each horizon, a structure map extending many meters from the borehole can be created and exported to 3D modeling packages as xyz grids, thereby ensuring that the geological and reservoir field models are constrained by all the structural controls that have been extracted from the dip data.

Geological modeling packages such as EarthVision can then ‘stitch’ the patches together in a way that honors the seismic between the patches. The end result is better structural maps, sections, and 3D models, reducing the risk of costly errors.

GEODES software predicts and projects geological structure away from the borehole by examining how dip changes and varies along the borehole. The farther up and down you look, the farther away from the borehole you can predict, so the more data available, the better. Therefore, dipmeter or image data should be acquired over as much of a borehole as possible. The common practice of collecting dip data over just the reservoir section is ‘penny wise and pound foolish’!

In the early days, geoscientists honored tops. With the advent of dipmeters and borehole images, geoscientists honored dips as well as tops. Now they can honor curvature as well as dips and tops, by making their field-wide maps, models, and seismic interpretations tie with the local sections and maps they've constructed with GEODES. Given all this additional control, why would anyone want to go back to contouring just tops or just tops and dips? Especially today, when companies are drilling very deep, very expensive wells in complex structures, where seismic is often poor and risk reduction paramount.

And this can all be done with existing data. Having spent a small fortune acquiring and correlating borehole images and dipmeter curves, why not make the most of them? Why not squeeze the maximum amount of structural control from your dip data?

 

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