HOW TO USE GAMMA SURVEYS TO EXPLORE FOR OIL AND GAS

Richard Harnett
Consulting Petroleum Geologist
Tulsa

One of the easiest and more successful techniques to detect areas of surface alteration caused by microseeping hydrocarbon gases is surface gamma ray surveying, also known as radiometrics (Fig. 1).

The technique is fast, rather simple to run, and not too difficult to interpret. The following is intended only as a primer of the steps in surface gamma ray surveying. This article is written to help an interested party get started.

GAMMA DETECTOR

The most difficult part of the survey will be to rent or borrow a suitable gamma detector. Used suitable equipment costs $5,000 or more.

Suitable here means that the equipment has a sodium iodide crystal of at least several cubic inches that is stable and has some means of giving a field strength reading. Headphones and a Geiger counter for $99 won't serve. Suitable also means that you can take it out into the field, i.e. portable.

The two most commonly available types of instruments are rate-scalers and counters. Rate-scalers are less accurate in measuring the average of gamma emissions. Because gamma emissions are random (stable only on long interval average), the meter needle of a rate-scaler is constantly in motion, even with dampening circuits.

Counters take a reading over a preset interval of time, usually 100-300 sec., which can then be divided for a very accurate counts-per-second value. Rate scalers are usually coupled to a chart recorder, and your eye then picks out the mean value. Rate-scalers can be moved while counting, possibly with meaningful results at 20-25 miles/hr.

Counters require that one stop and record, then move on. Counters' output is recorded as individual points. Some expensive counting type instruments have the capability of splitting the broad range of gamma ray energy levels into segments (windows). For the purposes of this discussion the broad "total" count is used. "Total" is usually defined as the energy levels between 0.5 and 3.0 milli-electron volts (Mev).

THE OBJECTIVE

The object of gamma surveys are unexplained values 15-30% lower than background range. Look for low values that can't be explained by visual observation.

Limestone is a low gamma emitter. So is water, such as a stock pond, a flowing creek, swamp, etc. Terrain can give problems if the surveyor is standing on a pinnacle (gamma low) or deep in a narrow roadcut (gamma high).

Man-made alterations are always suspect and obvious, but the most common manmade alterations one will encounter are the shoulders on major roads. Ponds or pinnacles let a surveyor test hat his equipment is responding, which should give assurance.

SURVEY EXECUTION

Next the surveyor lays out the planned survey. Here is where many workers fail. The area to be surveyed must be substantial, many times larger than the hoped for target. The surveyor must establish what background (barren) values are before he can find something that is anomalous.

Rate scalers can be moved while taking readings, and speed will depend on equipment. Counters must pause to make their readings. Reconnaissance surveys with counters should possibly have stations at 1/10 mile intervals. One rewarding target to look for is an untapped or unexpected extension to an existing or abandoned waterflood.

Since the surveyor is looking for acreage to lease and possibly drill, he wants the average of a swath, not the count of an individual pebble. He must carry the detector high.

At waist level the detector is about 3 ft off the ground and is getting meaningful contributions from a radius of perhaps 8-10 ft, less the shielding effect of his body. At 6 ft-the height of a pickup truck cab-the circle of investigation is 20-30 ft.

If mounted on a vehicle, the area of contribution will be skewed by the shielding of the steel. If the detector is moved over to the side, above the passenger door, then the area of contribution is a large kidney bean shape to the right of the vehicle.

In laying out a survey, consider the surface conditions. It took a 100 year drought before meaningful surveys could be run in part of the Florida Everglades. Consider if the target is pressure depleted or grossly underpressured (D and J sands of the Denver basin), where there is no microseep at the surface and consequently no alteration to measure.

Consider how dense the survey grid needs to be: a perfect section line road grid can miss viable production in intermediate to shallow areas.

INTERPRETING DATA

Once data are in hand things get interesting.

For those who used a rate-scaler with a continuous chart, the anomalies almost announce themselves. Anomalies usually have sharp edges, often detectable within 50 yd or less on the surface.

An anomaly on a rate-scaler chart will look like the cross section of a frying pan edge. On a plot constructed from counter data, there will be a notable discontinuity between stations that define the edge. Always have in mind that you are looking for values that are 15-30% lower than anticipated.

If the background (barren) seems to be running 5052 counts/sec, that means a drop of 7 1/2 counts/sec is needed to be anomalous somewhere in the 42-44 range or lower.

The important task is to find the edge of an anomaly. Anomalies do not have progressively lower values toward the center. There may be roughness to the data inside an anomaly, but no reading inside an anomaly can differentiate which location may be better (Fig. 2).

This is not the end. Hopefully an anomalous area has showed up. Now consider whether the anomaly is approximately the size of the fields to be expected. Is it plausible from your scant subsurface control? If half the world looks anomalous, you may want to reexamine what is the background (barren) range.

In mature areas, such as the U.S. Midcontinent, ma y of the undrilled anomalies found turn out to be stratigraphic traps. Gamma surveys only measure the variations, of emissions coming from the upper few inches of the altered surface.

Gamma surveys cannot reliably tell the difference between oil and gas or the depth to the microseeping reservoir, but they can tell where to consider putting additional exploration/exploitation efforts.