Geology Reference
In-Depth Information
Thickness Measurements and Thickness Maps
4.1
Thickness of Plane Beds
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 ex-
posed outcrop, bed thickness can be measured directly across the bed. In a well that is per-
pendicular 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 “thick-
ness” 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 hori-
zontal traverse or a horizontal well, the TVD is zero. In the following sections the true strati-
graphic thickness is found first and then other thickness determined from it, as needed.
4.1.1
Universal Thickness Equation
The stratigraphic thickness can be determined from a single equation (Hobson 1942;
Charlesworth and Kilby 1981) based on the angle between the direction of the thick-
Fig. 4.1.
Vertical cross section in the
dip direction showing mea-
sures of thickness:
MD:
mea-
sured distance on well log or
traverse;
t:
true stratigraphic
thickness =
TST
;
t
v
:
true verti-
cal thickness =
TVT
;
t
s
:
slant
thickness;
TVD:
true vertical
depth thickness = difference in
z
coordinates between top and
base of unit. (Standard well-log
terminology from Robert L.
Brown of Shell Oil Co.)
