Quick Look Techniques

A Method For Predicting Thick Sand Intervals

Did you ever drill a well that did not contain sand or encountered unexpectedly thin sands?

If so, then the following technique may help. The seismic technique presented here can be used to predict the presence of thick sands. We have learned that a few groups used a qualitative version of this method for years in prospecting, but the work seemed to have vanished during the boom of the late 70's.

Most geoscientists know that during deposition shale sections have a higher initial porosity than sand sections, and, upon burial, shales compact more than sands. If a growth normal fault is present within a sand and shale section, then, upon burial, the fault will have a low angle of dip in the shale section and a higher angle of dip in the less compacted sand section as shown in Figure 1. Thus, a growth fault cutting through a shale interval and into a sand interval will steepen its dip within the sand interval. This change in fault angle can be seen in Growth Fault 1 on the depth corrected seismic section shown in Figure 2. This change in fault shape produces the antilistric or "sand indicator" fault bend (Figs. 1 and 2).

Fig. 1

Fig. 2

This technique can be used in a general, qualitative manner to obtain an indication of the presence of sand. A quantitative method for estimating gross sand percentage through a thin alternating sand-shale sequence is presented in Tearpock and Bischke (1991). Our work with the method suggests that synthetic fault dips can be used to predict sand percentages at shallow depth using vintage seismic data. The resolution of the method is primarily dependent on the ability of the interpreter to pick changes in fault dip on the seismic sections, and secondarily on the velocity versus depth functions required to depth correct the fault traces. At deeper depths, where synthetic faults sole or flatten out, crestal antithetic faults can be used to estimate sand percentage.

Many interpreters depict faults on seismic sections as smoothly curved surfaces, typically listric in shape. Most growth normal faults are typically not continuously listric in shape. Instead the faults change dip with depth. A fault may start out listric, then go antilistric (steepening downward), go listric again, etc. Many geoscientists have not correctly interpreted these changing fault dips. On seismic sections, this is partly so because most seismic sections are vertically exaggerated. These vertically extended sections have the effect of straightening out fault surfaces, therefore masking changes in fault dip. The interpretation work must be approached in a carefully observant manner, looking for these subtle changes in dip. Therefore, it is very useful to depth correct the sections in order to analyze fault geometry and to apply this technique. This can be simply done by digitizing and depth correcting a fault trace under study.

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