R. M. Bateman
Petrophysical Data Consultants, Houston, TX
V. R. Hepp
Amoco International Oil Co.
Houston, TX
Introduction
In matters of well-to-well correlation and reservoir volume calculation, logs are necessary instruments. In the simple case where the wells are vertical and the bedding is horizontal, correlations can be made directly between neighboring wells, and reservoir volume is calculated by multiplying reservoir thickness, directly derived from the logs, by reservoir area, delimited by other means.
However, this simple case is exceptional. For one thing, most reservoirs exist as the result of some structural event or accident, implying some formation dip at least at the reservoir periphery. For another, most wells deviate to some extent from vertical, intentionally or not. As long as dips and deviations do not exceed a few degrees, the simple vertical-horizontal case is approximated closely enough not to need corrections. But when deviations and dips exceed about ten degrees, corrections are needed because apparent formation thicknesses measured on logs are greater than true stratigraphic thicknesses by different amounts in different wells. This adds to the difficulty of well-to-well log correlation. Also, if wells are deviated from vertical, and if formations have substantial dip, apparent thicknesses differ from the vertical thicknesses needed for reservoir volume calculation, and must be corrected.
To achieve these corrections in a convenient manner, modern data processing affords three different computed log products: the TVD, TST and TVT plots. Their principle, their meaning and their interpretation are the object of this paper. It will be seen that proper interpretation requires considerable caution and may be quite difficult.