Page 150 - Well Logging and Formation Evaluation

140 Well Logging and Formation Evaluation

will often lead to logs being run that have no value. Conversely, where
the results are unexpected, additional logging having a high VOI may
also be proposed.
• The petrophysicist and production geologist will often be using well
deviation data from different sources. It is essential that these be
checked for consistency before any work on true vertical log data is
shared.
• Both the petrophysicist and the production geologist may have access
to reports and logs that are outside the domain of information shared
digitally between departments. Where there is poor communication and
lack of a proper library structure, it may often occur that neither has
access to the most complete information that can be used to improve
his models. On numerous occasions I have seen this with respect to core
data.
10.1 UNDERSTANDING GEOLOGICAL MAPS

10.1.1 Basic Concepts

Consider a three-dimensional surface, such as the top of a particular
horizon in the subsurface. If you were standing on such a surface, there
would be a direction in which the surface slopes most rapidly. Relative to
north, this direction would have an azimuth, referred to as the azimuth
of the dip direction. The angle between this direction and the horizontal
is referred to as the dip magnitude. If we were to take a horizontal line
perpendicular to this direction (called the strike line) and measure the
angle going clockwise from north to this line, we would have the strike
direction. These items are illustrated in Figure 10.1.1.
The strike lines of the surface, when combined for a specific horizon-
tal elevation, form contours. Maps of a surface are created by showing
contour lines for fixed vertical spacing. For a smooth surface, these lines
will be continuous. However, where the surface is not smooth (for
instance, where faulting occurs), the lines will be discontinuous. The
average dip magnitude may be measured from a contour map by taking
the distance between contour lines and using the formula:

tan a = ( vertical contour spacing) ( horizontal contour spacing) (10.1.1)
()
where a is the dip magnitude. Example: If the contours on a 1:50,000
map are every 200m and are spaced by 6cm, what is the dip magnitude?