Page 78 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

for this reason that the simple gamma ray log has been to derive shale volume. Qualitatively, in its simple fom, it
called the ‘shale Jog’, although modern thinking shows can be used to correlate, to suggest facies and sequences
that it is quite insufficient to equate gamma ray emission and, of course, to identify lithology (shalyness). The spec-
with shale occurrence. Not all shales are radioactive, and tral gamma ray can be used additionally to derive a
all that is radioactive is not necessarily shale (Figure 7.1) quantitative radioactive mineral volume and a more
— see Section 7.6. accurate shale volume. Qualitatively it can indicate dom-
inant clay mineral] types, give indications of depositional
Principal uses
environment, indicate fractures and help to localize
The gamma ray log is still principally used quantitatively
source rocks (Table 7.1a,5).
Table 7.1(a) Principal uses of the gamma ray log.

Discipline Used for Knowing

Quantitative Petrophysics Shale volume (¥s/) gamma ray (max)
gamma ray (min)

Qualitative Geology Shale (shaliness) gamma ray (max)
gamma ray (min)

Lithology typical radioactivity
values

Mineral identification Mineral radioactivity

Sedimentology Facies Clay/grain size relationship
Sequence Stratigraphy Parasequence & condensed Clay/grain size & organic
sequence identification matter/radioactivity relationships

Stratigraphy correlation -

Unconformity identification

Table 7.1(b) Principal uses of the spectral gamma ray log.

Discipline Used for Knowing

Quantitative Petrophysics Shale volume (V,) Th (max), Th (min)
for pure shale

“ Radioactive mineral V., (Th), K (max),
volume K (min) for shale

Semi-quantitative Geology Dominant clay Th, K, U content of individual
-and qualitative material clay minerals

Detrital clay Radioactive content of individual
mineral suite clay minerals

Sedimentology & Condensed section Normal U and Th content
Sequence recognition from or TH/U ratio of shales
Stratigraphy excess uranium

Climatic changes? Th/K ratio changes in shale

Reservoir geology Fracture detection Uranium contribution to radioactivity

Geochemistry Marine source rock Uranium content of organic matter
identification

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