Page 155 - Petroleum Geology

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POROSITY
01 02 03 0.4 0.5

Fig. 6-13. Porosity-transit time relationship in granular beds from eq. 6.12.

when Atma is taken as 55 ps/ft (180 ps/m) in sands and 47 or 48 ps/ft
(155 ps/m) in carbonates. Local experience may indicate other values, and
for the exponent x; and it has been used successfully to obtain porosity
profiles over incompletely-cored reservoirs, and those where core recovery
was poor (the parts lost being mainly those with larger porosity).
Since any sonic-log derived porosity is an average over the detector spac-
ing, detailed correlations with porosity determined from plugs taken from
cores is not to be expected if there is much vertical variation in porosity.
There is an undoubted gas effect in some reservoirs, but by no means all, the
transit times being abnormally long.
The relationship between transit time in mudstones (At,) and porosity,
already discussed at length in Chapter 3, does appear to be represented best
by a time-average, linear formula. This is probably due to the platy nature of
the grains and the consequent flattened geometry of the pores.

Dipmeter

The dipmeter, as its name suggests, provides a profile of dips and strikes
of the strata in the borehole. The principles are best illustrated by the old
tool, which was a centralized sonde with three electrodes (resistivity) at 120"
to each other in a plane normal to the axis of the sonde, held in pads to the
wall of the hole. If the borehole is perfectly perpendicular, any dipping layer
with characteristic resistivity will be detected by each electrode at slightly
different depths, the amount of the difference being a function of the bore-
hole diameter and the orientation of the sonde relative to the direction and
amount of dip. By means of a small gyrocompass, the orientation of a par-
ticular electrode is recorded; and this allows computation of the apparent