Page 164 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

(Chapter 13), minor depth miscalibrations are seen with- should be a stylized representation of a core, an implied
in single cores. These can be due either to the core itself, interpretation having been made. This sedimentological
where there may be losses in friable or broken zones or to detail remains on a 1:200 scale log even though there has
the tool, where minor sticking and cable stretch can cause been a 200% decrease in scale. For general work this
depth errors. A perfect match at fine scales is often not detail is usually lost, and at a working 1:500 scale the
achieved. only difference over a cored interval is a slightly more
Even with the core depths perfectly matched to the precise and accurate lithology (Figure 11.3) (Blackbourm,
log depth, inconsistencies between the two may arise. It 1990).
is at this point that the capabilities of the logs emerge.
The problems of bed definition (see Chapter 2) become Cores cut after drilling
clear, as do those of depth of investigation (a/so Chapter Several methods are available for core sampling once a
2) and what this implies in terms of the volume of rock hole has been drilled and logged. All of them involve cut-
sampled by a log. In extremely heterogeneous forma- ting into the borehole wall.
tions, the directional tools such as the density-log tool The most frequent method is sidewall coring. A side-
may be difficult to calibrate because of changes across wall ‘gun’ is lowered into the hole on the logging cable:
the small distance between the core and the borehole it consists of a series of hollow cylindrical ‘bullets’ 1.8
wall. These particular problems are discussed in the cm in diameter and 2.0-3.0 cm long (Figure 11.4). The
chapters on individual tools. ‘bullets’ are arranged in series along.the sidewail tool and
The level of detail at which cores can be compared to attached to it by retainer wires. The tool is run to total
logs is an important aspect of core utilization. A core- depth, depth-calibrated with a gamma ray tool on the gun,
derived sedimentological log should already bring the and then pulled up the hole. The sampling points are
natural detail of a core to a manageable level for compar- decided in advance and are based on an inspection of the
ison with the logs, at a scale of 1:200 (Figure 11.3) or if logs already min. When a sampling point is reached, the
necessary 1:50. Reservoir, sedimentological and calibra- sidewall tool is stopped exactly at the depth chosen and a
tion studies can all be kept at these scales. However, for ‘bullet’ is fired. A small directional charge shoots the
genera) stratigraphical work, the 1:500 scale completion hollow steel sidewall bullet into the formation wall which
log (see below) is used and much less detail is desirabte: it penetrates by force. As the tool is moved upwards to the
20 m of core is represented by only 4cm on the log, so next location, the retaining wires pull the bullet out of the
that considerable lithological generalization is usually formation. The sample that the sidewall gun recovers is,
necessary (Figure 11.3). Generalization is a skill learned as indicated, about 1.8 cm in diameter and up to 3 cm
through experience and it must give clear and concise, but long. This sample is a reliable indicator of lithology
accurate results. The natural lithological detai) of a core (depending on the accuracy of the depth calibration), but
is not ‘mimicked’ by a sedimentological log. The latter because of the sampling method the rock is frequently

GAMMA RAY API
CORE DATA

120

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t Maer,
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ne ye ~
20m
CORE DETAILED RECORD COMPLETION LOG
1:1 1:200 1:500
Figure 11.3 Record of cores cut while drilling. The natural detail of a core is ‘generalized’ on the sedimentological log at 1:200
scale, The sedimentological detail is generalized to lithology only at the 1:500 scale.
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