Paul A. Roales
P.A.R. Consulting Service
Tulsa
This article examines a large radiometric low recently found by Helmerich & Payne Inc. in Railroad Valley, New, in an attempt to determine if it is an indicator of a subsurface hydrocarbon accumulation.
It also examines other potential causes of the anomaly.
BACKGROUND
There appears to be a demonstrable correlation between radiometric lows and hydrocarbon accumulations. Many previous articles have pointed this out.
Calhoun pointed out that radiometries was the highest rated of the unconventional methods tested with an 88% accurate overall and an 83% success rate at predicting productive wells.
Exactly how the technique works is not fully understood. Kolvoord 2 has proposed a model where the area within the plume above a hydrocarbon accumulation would be depleted in potassium when seeping hydrocarbon molecules (or other compounds that are associated with oil field brine) altered the clay minerals in the soil above the hydrocarbon accumulation.
He also believes that the mobility of uranium might be enhanced through the formation of complexes in this environment and then the uranium could be leached out. Under this model, a lower than normal reading in potassium and uranium would be expected over hydrocarbon accumulations.
Gallagher 3 explained the radiometric lows as the result of several factors acting together:
- Leaching of uranium, as mentioned above;
- A decrease in near surface porosity and permeability due to secondary mineralization; and
- An increase of the low-radiation secondary, carbonates in the soil above a hydrocarbon accumulation.
H&P'S PROGRAM
Helmerich & Payne hired Gallagher Research & Development Co. Inc. to collect radiometric data and a limited number of soil gas samples over six of H&P's prospects in northern Railroad Valley, New.
The radiometric data were collected using a 256 in. crystal mounted on an all terrain vehicle that made east-west traverses approximately every one third mile across the anomalies.
Gallagher provided both profiles and contour maps of total count, potassium 40, bismuth 214, and thallium 208 in its presentation. This article will follow convention and use the total count map.
Helmerich & Payne's Railroad Stock prospect is in 11n-57e and 12n-57e, Nye County, Nev., and was originally venerated when surface geology projected from outcrops, photogeology, gravity, data, and one purchase seismic line all identified the area as prospective.
Radiometrics was then used because of its low cost and supposed direct hydrocarbon detection ability. The contour map of total radioactive count on the Railroad Stock prospect shows a number of closed lows (Fig. 1). The lows seem to correspond with structural highs from seismic and gravity data, and in at least one case the radiometric low is supported by a soil gas anomaly.
Close examining of the map reveals that the southeast portion of the area has higher overall values and the northwest portion has lower overall values. Before Helmerich & Payne could analyze this data, an explanation for this apparent change in background count was needed.
POSSIBLE EXPLANATIONS
A radiometric anomaly such as the one at Railroad Stock can be caused by many things, a few of which are listed below:
- Radioactive ore
- Rock type
- Soil type
- Absorption effect/cover effect
- Transportation of soil
- Cosmic ray's
- Weather changes
- Topography
- Radon gas dust
- Radioactive fallout
- Structure
- Hydrocarbon accumulation.
An examination of this list shows that most of these potential causes can be eliminated in this case.
URANIUM ORE
The former U.S. Atomic Energy Commission stated that a reading of two to three times background is high enough to warrant investigation for radioactive ore.
In this case, the range across the anomaly only varies from a maximum of 193 counts to a low of 150 with the mean being 169.22 (this area had the second lowest mean value of the six areas Helmerich & Payne surveyed).
The high on the southeast part of this anomaly apparently isn't due to radioactive ore.
ROCK TYPE VARIATIONS
Potential causes 2 through 5 will be discussed together since they are all interrelated.
It is well known that radiometric values will vary with rock type and/or soil type as the content of radioactive elements in the rock or soil changes.
In the case of the Railroad Stock area, an examination of the rock types in the adjacent outcrop supplies a possible explanation for the change in background values.
Moores et al. 5 provided an excellent geologic map and cross section (B-B') of this area. Prominent on this map (Fig. 1) is an igneous rock outcrop known as Railroad Stock.
It is described as a biotiterich, orthoclase-bearing quartz monzonite intruding unmetamorphosed Cambrian limestones, The monzonite has been age dated at 32.9 million years by biotite and hornblende and 32.6 million years by biotite. 5
Monzonite has a higher natural radioactivity than the limestone that surrounds it. In fact, on the log of the well in 2-11n-57e, the limestone has a gamma ray reading of about 15 API units whereas the monzonite has an average gamma ray reading of about 130 API units. Published tables present a similar picture.
The relative average radioactivity of common rocks has been shown to be shale 4-6, sandstone 1.3-4, limestone 1-3, light igneous rock (such as granite) 5-12, and dark igneous rock (such as basalt) 3-6 . 4
So the edge of this monzonite intrusive would cause a marked change in the background radioactivity. But the anomaly is located out in the valley away from the outcrop.
Perhaps the intrusive is buried in the subsurface?
In 1988 the U.S. Geological Survey published an open file report that used all the aeromagnetic data of record to delineate the granitic plutons in Nevada.
With this aeromagnetic data, the USGS projected Railroad Stock into the subsurface, and the edge of that projected granitic mass corresponds quite closely with the change in background data defined by Helmerich & Payne's survey (Fig. 2).
However, it is documented that gamma rays are absorbed by material and have limited penetration ability. It has been shown that gamma rays are completely absorbed by 1 ft of rock, 1-2 ft of soil, or 2-3 ft of water.
Since the igneous rock is covered b,.- 1,820 ft of alluvium and 50 ft of carbonate rock in the only well in this area, Pan Am 1 McDonald in 2-11n-57e, the effect of the igneous rock could not be measured at the surface.
Likewise, a weathered soil profile above the igneous rock could not be thick enough to approach the surface and cause the anomaly.
In the area, the valley fill is all detritus derived from the adjacent mountains, so it might be possible that the source of the detrital soil is different across the anomaly.
In fact, intermittent stream channels, which would likely serve as avenues for transported soil and rock fragments in the southern part of the anomaly, do originate in the igneous rock outcrop whereas those to the north originate in the Cambrian carbonate outcrops (Fig. 3).
So this could be the cause of the apparent change in background values across the area surveyed.
CHANGES THROUGH TIME
Potential causes 6 and 7, which depend upon changes in cosmic rays or weather over time, can be eliminated in this case because, although the survey took some time to complete, at its conclusion a south to north line of data was collected across the entire area.
It also showed the change in background values recorded on earlier lines, despite being collected at a later time.
TOPOGRAPHY EFFECTS
Topography can also affect back-round values.
In a gully or sharp valley, the background radioactivity may be different from the background measured on a nearby level surface. But in this case, although Broom Canyon does cross the northern low background area, the low background area is not confined to Broom Canyon but extends well beyond it.
WIND EFFECTS
Possible causes 9 and 10 are both controlled by the wind and are short term effects.
Radon gas can be released when rock is broken. Once liberated the gas can cling to dust particles and collect on exposed surfaces.
Its signature is readings that fluctuate widely and are dispersed in a few hours. This signature was not noted in the case of Railroad Stock.
Likewise, radioactive fallout from nuclear testing can cause radiometric anomalies. This is Nevada, home of federal atomic test facilities.
Fallout from above-surface atomic tests in Nevada has been detected as far away as South Dakota.
However, the effect only lasts a few days, 4 and there have been no above-ground nuclear tests recently conducted in Nevada.
STRUCTURE EFFECTS
Structure can cause a radiometric low.
Kolvoord reports that some Russian studies of radiometrics concluded that it may be a structural indicator rather than a geochemical one. Their theory is that the radiometric low seen over producing fields is caused by variation in clay mineral content in the sediments above structure.
They believe that differential compaction above the buried structure would create a higher energy environment above the structure than adjacent to it, thereby the clay "fines" would be winnowed out of sediments above the structure. The lower clay content would produce reduced potassium 40 gamma readings over the structure.
This is a second possible explanation for the low radiometric readings over the northwest portion of the surveyed area. However, this is a favorable interpretation. If a structure is present then hydrocarbons are still a possibility,.
HYDROCARBONS
The final explanation for the anomaly from the list is that the low on the northwest portion of the surveyed area represents the plume of an accumulation of hydrocarbons at depth.
The map could easily be interpreted to indicate the entire northwest portion of the surveyed area has potential for hydrocarbon production.
An examination of the radiometric map shows that there is a halo of high values on the edge of the low before the numbers fall back to background in the extreme southeast portion of the surveyed area.
This halo of high numbers surrounding the low "might be expected around the margins of a reducing plume where relatively oxidizing waters, in which uranium was more soluble, were affected by local reducing conditions"2 and is often but not always present around hydrocarbon accumulations (Kolvoord citing Gallagher). As part of this survey Helmerich & Payne had Gallagher collect 13 soil gas samples across this anomaly.
All 13 were collected in the lower value area to the northwest, but they were scattered across the area and separated by a maximum of almost 5 miles.
The lowest methane value reported was 4.4 ppm, the highest was 65.6 ppm, and the average methane value for the 13 samples was 23.8 PPM
These numbers are very promising. Some proponents of soil gas surveys claim that a methane reading of as small as 10 ppm is anomalous.
Perhaps the survey is showing the effects of a plume over a large hydrocarbon accumulation covering the entire northwest portion of the area.
CONCLUSIONS
From our examination of this anomaly, we have arrived at three possible causes for the large radiometric low Helmerich & Payne found in Railroad Valley. They are:
- A difference in the type of rock fragments present in the soil.
- A structure.
- A hydrocarbon accumulation at depth.
Two of the three explanations are favorable for hydrocarbon accumulations.
The only way to prove which explanation is correct is to drill the anomaly. Helmerich & Pane is seeking partners to do that.
ACKNOWLEDGMENTS
The author thanks Helmerich & Payne Inc. for permission to publish this article and for providing the radiometric survey. Bryan Hughes and Don Solter of Helmerich and Payne Inc. read the article and made suggestions. The author also thanks Al Gallagher, president of Gallagher Research & Development Co. Inc., Lakewood, Colo., for collecting the radiometric data and for helpful discussions.
REFERENCES
- Calhoun, Gerry G., How 12 Geochemical Methods Fared in GERT Project in Permian Basin, OGJ, May 13, 1991, pp. 62-68.
- Kolvoord, Roger, A Review of Geophysical Methods For Detection of Alteration Chimneys Associated with Hydrocarbon Accumulations, in notes from short course: Surface and Near-Surface Geochemical Methods in Petroleum Exploration, presented on June 2, 1985, at AAPG regional meeting in Denver, Colo., pp. H1-H44.
- Gallagher, Al, Radiometrics For The Petroleum Explorationist, notes from a presentation on June 5, 1985, at AAPG regional meeting in Denver, Colo.
- Wright, Robert J., Prospecting with a Counter, U.S. Atomic Energy Commission, Washington, 1954, 68 p.
- Moores, E.M. et al., Tertiary Tectonics of the White Pine-Grant Region, East-Central Nevada, and Some Regional Complications, Geological Society of America Bulletin, December 1968, pp. 1,703-1,726.
- Grauch, V.J.S, et al., Geophysical Delineation of Granitic Plutons in Nevada, U.S. Geological Survey Open File Report 88-11, 1988, 7 p.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
