Page 18 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

1.8 Well-log interpretation and uses identify on the logs, down to the nearest metre, he is hard
put to find, even tentatively, at outcrop.
The accepted user of the well log is the petrophysicist.
In the following pages it is intended to relate the out-
His interest is strictly quantitative. From the logs, a
crop more closely to the wireline, geophysical well log.
petrophysicist will calculate porosity, water saturation,
Logs can and should be interpreted in terms meaningful
moveable hydrocarbons, hydrocarbon density and so on,
at outcrop. They contain as much information, even
all the factors related to quanUfying the amount of hydro-
sometimes more, than the outcrop, but can be studied
carbons in a reservoir for estimates of reserves. The
conveniently on a small desktop or computer screen.
Society of Professional Well Log Analysts (SPWLA), the
principal society of log interpreters, is mainly composed
1.9 This book —- content and aims
of petrophysicists.
Reservoir rocks, however, comprise perhaps only 15% Table 1.2 shows the logs considered in this book and their
of a typical well, and of this 15% only a small percentage principal applications, both geological and geophysical.
actually contains hydrocarbons. The petrophysicist is The applications have been divided into qualitative,
therefore not interested in 85% or more of the well logs semi-quantitative and strictly quantitative. Seismic and
recorded. The exploration geologist, in contrast, should petrophysical applications are generally, by necessity,
be interested in 100% of well logs, as the amount of geo- quantitative or semi-quantitative: geological applications,
logical information they contain is enormous. by default, usually qualitative. This should not be. A jog
The geophysical measurements made during logging sample set of over 100,000 values for a typical well of
are sensitive, accurate and characteristic of the formation 2000 m represents an enormous quantitative database.
logged. However, to those familiar with the aspect of Statistical, quasi-quantitative and of course purely quanti-
rocks as seen at outcrop, the geophysical] signatures of tative methods applied to this digital log database bring
this selfsame rock in the subsurface are impossible to precision to geological interpretation. So this book is con-
imagine. To an experienced geological] analyst of well cerned with qualitative and, wherever possible, the more
logs, the reverse is true. A formation that he can instantly quantitative methods of geological log interpretation.