Oil and Gas Evaluation Report : Oil & Gas Evaluation Professionals : Obsidian Energy Company : · Seismic Interpretation, Petroleum Engineering

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We will periodically feature a guest author and this post was contributed by Staffan Van Dyke*

When used as a standalone tool for Vshale, there are some known issues about gamma-ray (GR) logging to consider.  Petrophysicists frequently use the term ‘Vshale’ or ‘Vclay’ (where V stands for Volume) to help establish gamma-ray cut-off values (baseline) to determine a shale from a clean sandstone in clastic depositional environments. Typically, in most geologic settings, this works reasonably well, but of course, the tool, and the use of a single measurement, isn’t always perfect.

So, what exactly is a gamma-ray tool actually measuring? Radioactivity. Because most shales tend to contain one, or more, of the radioactive elements Potassium (K), Thorium (Th), or Uranium (U), they have higher radioactivity levels.  The higher the radioactivity measured, the ‘hotter’ the interval.  Since sandstones typically have few radioactive minerals, they are termed ‘clean.’  GR response is usually calibrated by a ‘clean’ or radioactive-free zone in the rock column being measured.  This GR response is then contrasted to more radioactive rock in the column, notably the much finer-grained shales – this then creates the ‘baseline’ by which all rock classifications are then based.

As aforementioned, this method can work very well as a lithology indicator, particularly in the Gulf of Mexico (GOM), however, there can be misinterpretations.  Kaolinite is virtually a radioactively clean and common clay.  So, if a gamma ray log was run through a purely kaolinitic clay, you would have a very ‘clean’ or zero-radioactive GR response, which could be misinterpreted as Vclay = 0%; hence, the clay would then be wrongly identified as a ‘clean’ sandstone...well, obviously, this is not the case.

On a similar note, this same phenomenon (i.e., an erroneous reading) can occur to a perfectly good reservoir-quality sandstone, as well.  Sandstones are typically characterized by sand-sized particles (very fine to very coarse), and are usually highly quartzose (i.e., non-radioactive). Theoretically though, they could be almost exclusively feldspathic (as seen in active margin settings [e.g., California]), or calcitic, or basaltic, or even hematitic.  Since the provenance of many reservoir rocks found in California oil fields were sourced from granitic uplifts, the resultant reservoir rocks contain radioactive aggregates (primarily Potassium Feldspar [K-Spar]).  Because of this, these rocks (with permeabilities in the order of Darcies) light up on the GR sonde as ‘hot’ lithologies, i.e., it would lead one to classify them as shales.  Obviously, this is not the case, either.  In order to avoid these potential misinterpretations, a thorough background geologic study of the field at hand should be carried out.

We can see that there are inherent errors associated with using the GR sonde as the sole lithology indicator in certain geologic settings, not only with Kaolinite in shales, but with K-Spar in sandstones, as two end-member examples.  This is why it is always wise to use several tools at one’s disposal for proper lithologic classification.  So, in addition to the familiar GR tool, consider using neutron-density, and possibly a Spectral Gamma Ray tool in conjunction with the Spontaneous Potential (SP) log for additional insight on the true lithologic character of the intervals being examined.