Shaping Logs for Quantitative Analysis
Calculators and computers will process data exactly as it is input. Therefore, it is essential to edit or shape logs at abrupt bed boundaries. Otherwise, unreasonable answers will result in the boundary area. The example below illustrates a typical coal bed with rounded corners at the bed boundary.

All hydrocarbon zones show a similar bed boundary effect on the resistivity log, resulting in strange water saturation values near the bed boundaries. Software is now available to perform resistivity inverse modeling that can sharpen the bed boundaries. Modern array induction logs and laterologs have smaller bed boundary anomalies than older logs.

Shaping log curves at bed boundaries

An obvious lag is seen in the density and neutron data at the top of the coal zone. The shaded areas indicate how the log should be shaped, so that the computer analysis program will not give false shows of hydrocarbon at the top of the zone. Sonic and resistivity logs demand similar edits.

If necessary, trim the tops and bottoms of other bed boundaries, including interfaces between clean sand and shale, and limestone - dolomite - anhydrite - salt sequences. Depth adjustments should be made concurrently.

Thin beds may also require curve shaping. Illustrated below are two thin beds on the shallow resistivity curve at the base of each clean sand. The deep resistivity curves show only a subtle hint of these beds and do not indicate their true value. Resistivity modeling software or a sharp red pencil will fix this.

Thin Bed Recognition on Shallow Resistivity Log

The analyst can ignore the problem in this example, since the zones are thin and do not contribute or detract from the reservoir, but you may need to do significant work or run specialized thin-bed logs in other cases. 

A second approach is to apply thin bed corrections, available from service company charts. This work is usually done by hand as it is more difficult to write suitable computer programs. Such programs scan up and down one curve searching for thin beds and apply reasonable corrections to another curve.

A third and more pragmatic approach is to shape the deep resistivity curve to match the shallow curve. Depending on the anticipated hydrocarbon content, the curve amplitude can be amended higher, lower or equal to the shallow curve. Do not try to extend this concept to match the resolution of micro resistivity logs. Some service companies can provide this service on some newer logs.


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