Page 161 - The Geological Interpretation of Well Logs

11

LITHOLOGY RECONSTRUCTION

FROM LOGS

11.1 Introduction 11.2 Lithology from drill
data — the mud log
There are two independent sources of lithology data
available from oil wells, one set of data coming directly The mud log and the way in which it is made is described
from the drilling and one set from the wireline logging. briefly so that the data it represents can be used properly
The drilling data consist of cuttings, cores and all in log interpretation. Drill-derived data and log-derived
the recorded drilling parameters (and, of course, the data often appear to be in conflict. Which can be
MWD/LWD logs, although not discussed here). The believed? This book describes how logs can be used and
logging data consist of the wireline, geophysical log suite this section describes how drilling data can be used, their
and sidewall cores. For a reliable lithological reconstnic- reliability, and some pitfalls in their use.
tion, both sets of data are essential. As a result of the great The mud log (a misnomer that has somehow stuck) is
sophistication of wireline logs, the drilling data are often the geologist’s record of the drilling of a well. Before
forgotten. This should never be the case since the only wireline logging was invented, it was the only record that
continuous sample of formation lithology comes from existed. On this log is recorded the lithology, the drilling
drill cuttings. rate, bit changes, gas record, calcimetry, dates and events
This chapter describes the methods for interpreting (Figure 11.1). The lithology is based on an examination
lithology by the manual method using log and dnl data, of cuttings — small chips broken off the formation as the
semi-automatic methods using mainly logs and fully drill advances. They are washed away in the stream of
automatic methods using only a computer and logs.

@] CUTTINGS z= SAMPLE TOTAL GAS MUO
DRILLING RATE miném rd % r Re LTH . , & *% sits
DESCRIPTION
2346 10-20 4040 1 8] 2040 a 05161 2040 60 80 | CASING
SH gy. bl, frm-hd.
v. carb, mic. non-cale. mud wt,
1,40
SST Ii. gy. fi-me
Sub-ang. mod. sted.
matrix card. por.
SH as above
SST a.a.
SLST Ii. gy. carb.
fasier drilling mic, non-calc.
vy rate in sands
2 i1olal gas
So
N
~ SST a.a. ad. org, from ‘degassing’
.
~
r mat. drilling mud
ow
calcimetry analysis
of cuttings.
SH gy-li. bro. firm
i.e. 10% dolomite
raig of drill
sub-fiss. sl. silty
advance (i.e. 25 min DOL. tces. mod. yel. \ \

for 1m advance)
22/1/2004
nd. mic. xin.
i
t
|
cuttings analysis
I
Le. 80% shale 20% sand
72
interpreted
I
7o°E
Nihology
\
NB #20
Figure 11.1 A typical mud log. The log is the well-site record of lithology (cuttings) and some driJling parameters. In this example
cutting samples were taken: every 2—3m.
151]