Page 179 - The Geological Interpretation of Well Logs

THE DIPMETER

12.1 Generalities dipmeter —
microresistivity
eee
The log
The dipmeter log provides a continuous record of forma-
tion dip and direction of dip or *azimuth. It comes from
a two slage process: acquisition of the data followed by
7 Y correlation
data processing. The tool acquires, typically, four
7\ surface
microresistivity curves from orthogonal positions around
the borehole: the first stage. By comparing the differences

in depth between the curves across the borehole, the
computer can provide a dip and an azimuth: the second
stage (Figure 12.1).
Modem dipmeter tools have become very sophis-
ticated: they consist not only of the logging sonde for
Figure 12.1 The principte of the dipmeter. Detailed resistivity
the microresistivity curves, but also a positioning sonde
curves are measured from (typically) orthogonal positions
so that tool orientation, inclination and speed are known,
around the borehole and then correlated to give a surface
all essential to the computation of the dip and azimuth. which has dip and azimuth. It is a two-stage process (from
Moreover, the dipmeter microresistivity curves are Cameron ef al., 1993).
sampled 30 to 60 times more densely than ordinary logs
(Figure 12.4).
Principal uses
Over the past few years, dipmeter processing has
Clearly, the principal use of the dipmeter is to provide
passed from being exclusively in the hands of the logging
dips! But there is more to the meaning of dip than is
companies, to being available to all users. What previ-
implied in the preceding sentence. The dipmeter provides
ously required a mainframe or workstation can now be
data for two rather different domains: structural geology
done satisfactorily on a personal computer, even a laptop.
and sedimentary geology (Table 12.1). In structural
This has changed attitudes and practices: processing is
geology it provides information on structural dip,
now an interpretation tool. A tool, moreover, using inter-
unconformities, faults and folds. Structural dips from the
active software.
dipmeter can be compared to dips on seismic sections
or used in log correlation. In sedimentary geology, the
*Azimuth is used lo mean the direction, reJative to ue north, of dip
or any other orientation measurement, dipmeter can provide facies information, bedform
Table 12.1 Uses of the dipmeter log.

Discipline Feature Comments

General Hole position gives continuous deviation and TVD
Hole size and shape gives shape with orientation
Breakouts derive stress field orientation

Sedimentary Geology Sedimentary structure (bedform) orientation fram foresets, dunes, HCS, ete
Palacocurrent analysis using statistical analysis
Reservoir (sand-body) orientation from compaction drapes, slumps etc
Facies characterisation thin bed analysis possible

Structural Geology Structural dip seismic overlays used
Unconformities vector analysis plots used
Faults stereographic analysis used
Folds graphic analysis used
Fractures (images now mostly used)