Page 163 - The Geological Interpretation of Well Logs

- LITHOLOGY RECONSTRUCTION FROM LOGS -

A * *
° ® 7 MMA RAY API 100 . 3 oO, CUTTINGS %|
, 1 1 L , 7 ie 7 1 a = zx oO

Se er is ce an 400 a2! 3° | 20406080
2400m t oe at ee ston

gamma ray

drill rate

2425
‘smeared’ bed
boundaries on
cuttings log and
‘delayed’ arrival
of new lithology

2450

2475 PhLearo
T
Fr 2460

ca | 2480

# = fig Gata as in Fig. 11.1 (L) = Log data
Figure 11.2 Comparison between rig-derived lithology interpretation and subsequent log interpretation. Note the effect of the
arrival of a new lithology on the cuttings percentages. The rig bed boundaries are only ‘good guess.’
11.3 Lithology from cores — direct
the lithology interpreted from the wireline logs. Indeed,
physical sampling such comparisons are used frequently through this
book: they are essential to a proper understanding of the
Cores may be cut during drilling, when a continuous,
capabilities of wireline logs. Cores provide the geologist
cylindrical sample of the formation is recovered, or they
with the only record of rea] subsurface Sithology.
may be taken after drilling, when small, punctual samples
Cores do in fact need interpretation and processing
may be taken from the borehole wall.
before they can be compared to logs. The principal prob-
Cores cut during drilling lem is one of depth. Cores are cut during drilling so that
During drilling and before logging, when a complete their depth limits are calculated by adding al] the lengths
record of lithology is required (for example in a reser- of drill string together. Mistakes often occur, and .
voir), a continuous sample is taken by coring. The drill bit frequently these depths do not agree with the depths
is replaced by a core barrel. The retrieved core, depend- shown on the well logs. The logs are taken as the refer-
ing on the preceding hole size, will be a cylinder of rock ence: for detail the reference may be just one log,
2-15 centimetres in diameter and up to 60 metres long frequently the sonic or the density log. The drill depths
(Blackbourn, 1990). It is a direct physical sample of the for a core must therefore be adjusted to log depths. The
formation. changes are usually about +5m but may be as much as
Being a real physical sample of the formation lithology, *+15m. Moreover, because of recovery problems, the
a core appears to need no interpretation. In fact the reverse depth changes between contiguous cores are frequently
is tue; cores should be used as a reference to calibrate the different. When working with the more detailed logs,
logs from a lithological point of view and to compare with such as the dipmeter (Chapter 12) or the image logs

153