Core and core handling: Part Two

Logging core is always a combination of recording details of the core and interpreting what is seen. The scientifically minded will point out, quite correctly, that the geologist should always distinguish between the two, but in practice this can be difficult.
The reason is twofold. First, decisions have to be made from the moment logging begins: whether the depths given by the driller are correct; whether there are missing sections in the core; whether the core has been reassembled in its correct order after falling from the core barrel onto the drilling floor. These decisions depend on the geologist’s judgement, and are thus interpretative, yet logging could hardly commence without them. Second, geological terminology is very largely genetic. To some extent this is avoidable. There is no doubt, for example, that the terms ‘cross-bedding’ or ‘inclined bedding’ are preferable to ‘current bedding’. It is, however, virtually impossible to describe succinctly the characteristic pattern of, say, a Chondrites burrow system without simply naming it, and yet this is strictly interpretative. And it is difficult to describe wave ripples in any clear way without implying that the geologist considers they were deposited in an oscillating current.
Although it is difficult to avoid, the geologist should be aware of the extent to which the core is being interpreted in simply making a ‘descriptive’ log. Of course, many logs will also carry an ‘interpretation’. Indeed, there is little point in cutting a core if it is not ultimately to be interpreted. This ‘interpretation’, which might be of depositional environments in a sedimentary sequence, the
economic zones in a mineral deposit, or the inferred planes of weakness in a slope stability study, is best kept quite separate from the basic ‘description’. This rule should be followed whatever log format is chosen.
There is no generally preferred layout for a core log. There are numerous different types of core and an unlimited number of ways in which a core log can be used. Written descriptions are present on most logs. During logging it is wise to make a note of any features encountered, even if their origin or significance is uncertain at the time. The initial log must be kept clear and tidy,
but there is no harm in its being packed with information. Final drafted logs are usually kept concise. The aim of a log is to act as an informative but succinct summary of the core, rather than a geological essay. Nonetheless, it is possible with practice to convey detailed information in several short sentences, especially with careful use of abbreviations.
The order chosen for any written description will depend on the nature of the rock and the reason for logging it. A written description will apply to a certain section of the core, which is commonly defined either by a set depth interval predetermined on the logging form, or as a section of core considered by the geologist to form a unit for descriptive purposes.
Sometimes it is useful to highlight the similarity (or contrast) between the lithology being logged and that recorded elsewhere (in the same well or otherwise). This is the type of information that may be obvious to the logger at the time, but is much less clear from the log later. A simple note (‘Similar to sandstone between 102 m and 105 m’) can be invaluable.
There is no need to repeat in written form all the details that are recorded on the various graphic logs. The written description is most effective when it is a concise summary of the most significant features of the corresponding length of core, with an additional note of other details that might otherwise go unrecorded.
The geologist’s interpretation of a core is naturally of the utmost importance. However, the final interpretation should not be added until all the relevant information, including laboratory analysis, if appropriate, is available. Notwithstanding this, the geologist should give careful thought to an interpretation while the evidence of the core is at hand. On completion of logging the first task is to check all necessary data have been entered. Methodical logging will ensure that omissions are minimized.

To read the first part of this series click here

 Click to view log of a short length of core, showing various different ways of presenting data