Open Hole Logging: Potential Gamma-Ray Interpretation Concerns

Staffan Kristian Van Dyke1, Jim Basick

1, and Vinay K. Sahay

2

1 Nexen Petroleum U.S.A. Inc., Dallas, TX, U.S.A. (email: staffan_vandyke@nexeninc.com)

1 Nexen Petroleum U.S.A. Inc., Dallas, TX, U.S.A. (email: jim_basick@nexeninc.com)

2 Maheshwari Mining Ltd, Ramesh Nagar, India (email: geo_vinay@yahoo.co.in)

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

with 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 (Figure 1). 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

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., and 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.

Figure 1: Example of a spectral gamma-ray log; the black curve represents the total radioactivity measured, while the

other curves represent the contribution of radioactivity from the elements: Thorium (blue curve [ppm]), green for

Potassium (green curve [%]), and red for Uranium (red curve [ppm]).

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.