To demonstrate how the method works consider two wells in a stable (pre-growth) tectonic environment (Figure 1A). If sedimentary sections (parasequences or shale breaks) are correlated between these two vertical wells, then, the vertical distance (Δd) between these correlation points is small (Figure 1 A). This change in depth (Δd) is then plotted against its correlative subsea depth, d₁,, the depth in the structurally higher well (Figure 1A). In a stable tectonic environment, if two wells are situated on different structural levels (i.e. not on strike), then the slope on the Δd versus d plot is small or approximately flat.

In an unstable or growth tectonic environment (Figure 1 B), the correlations are displaced by a larger vertical distance (Δd). Typically only two wells are required to establish this relationship, although the wells must be on different structural levels. To employ the technique, measure the vertical distance between relative correlation points and generate the Δd/d plot. 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.

The resulting Δd/d diagrams, which show changes in the sedimentation rate, are very sensitive to changing structural and sedimentary growth patterns. Discontinuities or changes in slope on the Δd/d curves indicate faulting or hard to locate sequence boundaries, while erratic changes in slope may indicate miscorrelations.  The slopes of the curves can be used to determine high and low growth intervals, and can determine when the growth or deformation started and stopped. Steep slopes indicate higher growth rates, which may have implications concerning oil migration and entrapment. Negative slopes on these diagrams indicate the existence of condensed or reduced sections.

EXAMPLE

Today, because geoscience staffs have been reduced and specific tasks must be done, time is an important factor. It makes little sense spending days distinguishing a fault from an unconformity with standard techniques if a potential problem can be resolved within several hours using a new method.

In order to demonstrate an application of the method, examine the logs shown in Figure 2. These wells are located across the crest of a rollover. During the mapping process it was concluded that the "Initial structural interpretations were difficult because of limited subsurface control, poor seismic data, an unconformity at the Het. Horizon, coupled with a compensating thickening of the overlying Discorbis Interval" (Nauman, 1989, Typical Oil and Gas Fields of Southwestern Louisiana, v.3, p.17).

After correlating the logs, and entering the correlation depths in each well for the various correlation points, we rapidly generated the Δd/d plot shown in Figure 3 (Δd is plotted on the Y axis and d is plotted on the X axis). The horizons can be compared to Nauman's (1989) final cross section shown in Figure 4. The growth plot is interpreted as follows:

1. The sedimentary growth was high between the Discorbis 10 to Horizon 2 interval.
2. The sedimentary growth changed rapidly and was low between the deeper Het. and Marg. Vag. intervals.
3. A probable unconformity exists between Horizon 2 and the Het. horizon. Although correlation to another well is required to prove the unconformity above the Het. Horizon, the rapid change in growth across the discontinuity is immediately suggestive of an unconformity.

A comparison of the unconformity in Figure 3 with the Horizon 2 to Het. interval in Figure 4 shows that the Δd/d method locates the unconformity in the Amerada Dow-Norman No. 2 well.

In conclusion this method can help distinguish unconformities from faults using well logs. After locating the unconformity in a good data area where it is subtle, one could easily correlate the unconformity to the problem missing section. In this manner, it can be determined if the missing section in other wells is an unconformity or a possible fault.

Similarly, the method can be shown to apply to seismic data. On seismic sections the method can identify sequence boundaries where they do not exhibit baselap, and can locate subtle stratigraphic traps.

The strengths of the technique are that the process is rapid, accurate and can be available in the beginning of a field study to help with the interpretation.