Page 156 - Petroleum Geology

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dip and strike of the bed in the borehole. By means of a similar device that
records the angle of the borehole from the vertical, and the azimuth of such
deviations from the vertical, the true dip of the bed can be computed.
The modern tool works on the same principle, but has four arms so that
dips can be obtained when the hole geometry is irregular. The data are re-
corded for computer processing.
This is the geologists' rather than the engineers' device, but it must be
remembered that it is a bulky tool with greater than normal risk of sticking
and loss in the hole. While this risk is normally quite acceptable, there may
be occasions when it is not. For example, a 4 km hole that penetrated mud-
stone for the last kilometre might well present risks that are out of all pro-
portion to the value of the data that might be obtained if the dipmeter
were to be run from bottom. In such a case, if the engineer in charge re-
quested it, it would be reasonable to run the dipmeter from 500 m or so
above bottom.
To the student who has been encouraged to draw sections with beds of
regular geometry and equal thickness, it may come as a shock to find evi-
dence that both dip and strike may change significantly in a borehole with-
out unconformities. Some changes are due to faults, others to unconfor-
mities, and to internal features such as cross-bedding. There may be parts
where the vertical sequence of dips suggests the environment of deposition
of the sediments with some clarity.

TEMPERATURE

Ideally we should know the true temperature distribution in the subsur-
face both at the time of logging and when thermal equilibrium has been re-
established. We need to know the temperatures for quantitative interpreta-
tion of resistivity logs, and for the proper evaluation of reservoir perfor-
mance (because temperature affects fluid viscosities and volumes). Geologists
and geochemists also need to know temperatures so that they can assess the
thermal role in petroleum generation and the local prospects of petroleum
accumulation.
First, it must be emphasized that the Temperature log is not the log to
use. The commonest use of this log is to locate the top of cement behind
casing. The setting of cement is an exothermic reaction, so the location of
setting cement behind casing can be detected by its elevated temperature".
The sonde can take a maximum-reading thermometer, and the maximum
reading on each run is recorded on the log heading. This bottom-hole tem-

* At least one published temperature profile was a temperature survey to locate top of
cement in the well.