Thickness Measurements and Thickness Maps

Abstract

Thickness has multiple definitions, the choice of which depends on the purpose and the data available (Fig. 4.1). The true stratigraphic thickness (TST) is always the distance between the top and base of a unit measured perpendicular to the top. In a completely exposed outcrop, bed thickness can be measured directly across the bed. In a well that is perpendicular to bedding, the measured thickness in the well (MD) is the true stratigraphic thickness. Commonly, however, thicknesses must be determined from oblique traverses across beds or from wells that are not perpendicular to the bed boundaries. The measured thickness in a vertical well (or along a vertical traverse) is the true vertical thickness (TVT). The measured thickness in any other direction is here termed a slant thickness. A “thickness” measurement that is easily derived from well data is the TVD or true vertical depth thickness, and is the difference in elevation between the top and base of a unit in a well log. This “thickness” is more related to the orientation of the well, however, and for a horizontal traverse or a horizontal well, the TVD is zero. In the following sections the true stratigraphic thickness is found first and then other thickness determined from it, as needed.