Spectral Gamma as an Aid to Correlation and Zonation

Peter Martin Lucas
Fugro Robertson Limited, Llandudno, United Kingdom

Spectral gamma data processing at Fugro Robertson has been developed to aid correlation, reservoir subdivision/zonation, especially where there is restricted biostratigraphic recovery.

Spectral gamma data is a measure of the proportions of uranium, potassium and thorium, that make up the total gamma ray count. Lithology controls the main abundances of these elements, however, variations in depositional climate, hinterland, water chemistry, biological productivity, etc., often control the background variability. It is these, often subtle, longer wavelength variations that the GAMMASTRAT^TM processing aims to highlight. The techniques focus on comparing predicted with actual log values to create derivative curves relatively unaffected by lithology. These curves are integrated with other wireline log and biostratigraphic data, etc. for interpretation.

The GAMMASTRAT^TM system utilises wireline data, without the need for sampling cuttings, etc. and scores over such analyses in that there is no depth uncertainty, no cavings/mud-cycling or other contamination issues, nor time required to transport or process any materials. It also involves a normalisation process to compensate where radioactive muds are used. One by-product is a refined Vshale curve relatively unaffected by radioactive grains in sandstones.

Using the long wavelength derivative curves generated by GAMMASTRAT^TM correlation on a 10 metre (seismic) scale for both exploration and broad field correlation has been possible. The associated shorter wavelength processing often allows correlation on a finer scale (metre plus) and the relatively precise resolution of unit boundaries. The techniques has so far proved valuable in a wide range of depositional settings.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005