Exploring the Ten Habits: Habit 6a
Bob Shoup and Dan Tearpock
SCA receives many queries about the “The Ten Habits of Highly Successful Oil Finders“. Since applying the best practices that are the foundation of the 10 Habits will help you, or your company, reduce the number of dry holes you drill, SCA has written a column elaborating each of the Habits. Here is the column for Habit 6a.
Successful oil finders know which methods, tools, and techniques are needed to define and understand the subsurface.
Resource plays bring many new challenges to interpreters. One of those challenges is ensuring the validity of the correlations of the hundreds, or even thousands of previously interpreted well logs. Re-correlating the wells would be a very time intensive undertaking. One could construct a series of cross sections, but this too would be very time intensive.
There is one method, however, that an interpreter can employ to rather quickly validate previous correlations to ensure their validity; the Δd/d method developed by Bischke (1994).
The Δd/d method uses a spreadsheet to compare the change in depth for a formation (Δd) against the depth of that formation (d) in a reference well. The interpreter inputs the depth of a horizon, or a series of horizons, from well logs, mud logs, or scout tickets into a spreadsheet. The spreadsheet can then be used to calculate the difference in depth (Δd) for that formation compared back to the reference well (Table 1). It is similar to subtracting multiple horizon grids from a common horizon grid, creating multiple interval isochore maps from a 3D survey. Anomalies on the maps can be interpreted as miscorrelations, faults, unconformities, or changes in expansion rate. Any structural element you can interpret from logs or seismic correlations can be observed on single Δd/d plots and multiple plots referenced to a common reference well or 3D map horizon. The multiple well method is known as MBPA (Multiple Bischke Plot Analysis).
Table 1: Δd for a series of Frio markers in the UN TAT7 well measured against the reference well TOC 382#1
With wells that are properly correlated, a plot of Δd/d will show all points falling on a line (Figure 1). Changes in slope or breaks in the line can indicate faults or unconformities. When there is a miss-correlation, points will plot off of the line (Horizon 27, Figure 2).
Figure 1: Δd/d plot for the wells shown in Table 1.
Figure 2: Δd/d plot for wells #1 and 5. Note correlation marker 27 is miss-correlated
So when you have a lot of previously correlated wells, or mud log or scout ticket data, enter the tops into a spreadsheet, which is an industry best practice in its own right, then calculate and plot Δd. It will help you validate the existing correlations relatively quickly.
Bishke, R. E., 1994, interpreting sedimentary growth structures from well log and seismic data (with examples): American Association of Petroleum Geologists Bulletin, v. 7, p 873 – 892
Editor’s Note: To learn more about Δd/d and other tools, methods, and techniques to help ensure accurate subsurface maps, register for SCA’s signature course Applied Subsurface Geologic Mapping. Visit scacompanies.com to learn more about SCA’s training Program and other services, or to read more of the 10 Habits of Highly Successful Oil Finders.