In an unstable or growth tectonic environment (Fig. 1 B), the correlations are displaced by a larger vertical distance (Δd). In the case of a growth environment, the resulting Δd/d curve will have a higher or steeper slope than the one for a stable environment. Typically, only two (2) dip wells are required to establish this relationship. To employ the technique, measure the vertical distance between relative correlation points and generate the Δd/d plot.

Δd/d diagrams, which show changes in the expansion rate, are very sensitive to changing structural and sedimentary growth patterns. A discontinuity on a Δd/d curve indicates missing sections due to a fault or an unconformity. An erratic change in slope may indicate a miscorrelation. The slope of the curve can be used to determine high and low growth intervals and when the growth or deformation started and stopped.

We have found after examining several thousands of these plots from different areas of the world, that the slopes on the Δd/d graphs are nearly linear or roughly monotonic. This is the result of uniform or nearly uniform growth over long sections of sediment. This uniform growth simplifies the interpretation of the plots.

E x a m p l e

Recently, we were working on a field study of a salt dome field that had excellent 3 D seismic and well control except along the flanks of the
salt dome. Wells drilled into salt on the flank of the dome encountered thin sand intervals that were difficult to correlate to sand and shale intervals in off structure wells. Complicating the problem was that the seismic data lost coherency due to high bed dips near the face of the salt. The 3 D geophysicist, assigned to the working group, was able to tie the sand tops in the off structure wells to the coherent seismic data. The coherent data were picked up to where the beds turned up near the flank of the dome and the data became incoherent. The coherent portions of the data were then tied to the wells on the flanks of the salt dome by projecting the dip of the coherent data to the sand tops in these wells that either penetrated salt or were on top of salt. A problem arose late in the project when the 3 D geophysicist noticed that, after turning on all of the horizons, that two (2) horizons crossed, which is impossible.

A number of questions arise that are significant to cost effective petroleum exploration. What do you do in a situation when you cannot correlate your well log data with certainty and the seismic data are incoherent? Also, could we have saved valuable time by modifying our methods? How do we resolve our problems? Are the well logs miscorrelated or did we encounter an unrecognized fault?

When confronted with any problem that could involve miscorrelation, we suggest that all wells be correlated to a type well that has the most complete stratigraphic section. In this case, our type log is off structure Well No. 4 (Fig. 2).

In our problem example, Well No. 4 is correlated to problem Well No. 1 and Wells No. 2 and 3 that were drilled from the same platform as problem Well No. 1, The depths for the tops of each sand are entered into a spreadsheet and Ad is calculated between Wells No. 1, 2 and 3 versus Well No. 4, Δd values are plotted on the y axis of a graph, and are plotted relative to the structurally higher depths of the sands tops (Δd), which are plotted on the x axis of the graph.

Figures 3, 4 and 5 are Δd/d plots for Wells No. 2 and 3 plotted against type Well No. 4. Each point on Figures 3, 4, and 5 represent a sand top correlated to type Well No. 4. Wells No. 2 and 3, which were drilled from the same platform as Well No, 1, exhibit Δd/d plots that show gradually increasing monotonic growth with increasing depth (Fig. 3 and 4).

Figure 5 suggests two (2) possible interpretations that could solve our structural problem. The first interpretation is that sand correlation top No, 20 is on an 800 ft normal fault that cuts Well No. 1 below top No, 1 1. This interpretation is very unlikely as this 800 ft fault is absent in Wells No. 2 and 3, which were drilled from the same platform as Well No, 1. The second interpretation is that sand correlation top No. 20, which is the last sand above the salt, is not sand correlation top No. 20 but a higher sand in Well No. 1, How much higher is this sand? The plot tells you this distance is about 800 ft. higher than the No. 20 sand top should be in Well No. 2.

The Δd/d technique can help resolve seemingly intractable problems, and can save both time and money. We recommend that Δd/d be applied early in a project to check the correlations before mapping is initiated.

Richard E. BiSchke, PhD