Radon-Thoron Isotope Ratios:
 a new method of geochemical exploration for oil and gas

  • Reports in the literature describe the success of radiometric surveys in exploration for oil and gas. Most of these papers refer to gamma radiation, but some refer to radon. Most of the gamma radiation comes from a bismuth isotope which occurs later in the uranium decay series than radon. Radon geochemistry can be more sensitive than gamma, because it is based on alpha radiation which has a lower background count.
  • Radon:thoron ratios are more sensitive still. These ratios have proven successful in uranium exploration and should prove useful in oil and gas exploration as well. We have a formula to resolve radon from thoron giving on-the-spot results with our field instruments.
  • Like all exploration methods, radon and thoron geochemistry will benefit from a thorough understanding of the geological setting of the reservoir.
What causes these radon anomalies? Some possibilities:

Upward migrating aqueous-phase formation fluids
  • Aqueous-phase formation fluids can carry members of the uranium decay series (radium etc.), whether the radioactive elements come from an enrichment in the reservoir, or, more likely, are leached out of formations with normal background levels. These elements are then deposited near surface.
  • Near surface enrichments of radium will result in recognizable anomalies of radon and possibly gamma radiation. Radon-thoron ratios allow us to correct for local variability in soil permeability and weather conditions.
Upward migrating non aqueous-phase formation fluids
  • Radon geochemistry in soil gas and groundwater has been widely used by environmental scientists in the search for hydrocarbon contamination (NAPL or non aqueous-phase liquids). Link
    Radon has an affinity for organic liquids, and in the presence of NAPL the radon concentration in groundwater or soil gas is reduced due to partitioning into the organic NAPL phase.
  • Matolin et al. (see below) noted increased levels of radon and thoron over an oil productive zone, which they attribute to enhanced gas emanation from U- and Th-bearing minerals decomposed by hydrocarbon-generated groundwater acids ("enhanced emanation power").
Location and character of the anomaly
  • The situation is complicated. As shown above, sometimes we expect higher than normal radon levels over the reservoir, and sometimes lower.
  • The impermeable structure which traps the oil and gas into a commercial reservoir can deflect the upward migration path of these fluids, resulting in a recognizable pattern at surface, with a concentration above the field boundary, ideally in the form of a halo or part halo.
  • Another possible cause of radon anomalies over reservoir edges is the development of fractures due to differential compaction, resulting in increased permeability. This process, in the case of reef structures is described by Sikka and Shives, 2002 (see below).
  • On the other hand, in some cases we can expect migration upwards from the reservoir itself, in this case resulting in an anomaly over the reservoir.
Fractures and shale gas
  • Ghahremani (see below) hypothesized that shale gas yields will be optimized where natural fracturing of the shales is greatest, and that such fracturing will allow radon to migrate to surface causing radon anomalies in soil gas.
INSTRUMENTS
  • We measure: radon - radium - thoron - radon daughters - alpha radiation.
  • The Lucas cell is recognized as the most sensitive and reliable method for these elements.
  • Our instruments are used around the world by mining and oil companies, research labs and health physics professionals.
  • Modern, low-power, field-rugged electronics.
  • Sensitive to geochemical trace levels necessary for radon in lake water and for radon-thoron isotope ratios.
  • Can work in a tent without electricity or be carried from point to point in the field.
  • 40 readings per day. Results available immediately.
  • Can be operated by junior personnel if carefully supervised.
  • Same instruments used for radon and radium in soil, sediment, plant parts, rocks, water, soil gas, air, and snow, and for radon daughters in air.
  • Click here for pictures of instruments.
  • For instruments contact
    R.H. Morse & Associates Ltd.
    1-416-269-9979
    morse@finderschoice.com
  • Multilingual consulting and training (if required).
Robert H. Morse, Ph.D., P.Eng.
July 15, 2010
Click here for studies of radon in uranium exploration
ANNOTATED BIBLIOGRAPHY
Armstrong, F.E. and R.J. Heemstra, 1973. Radiation halos and hydrocarbon reservoirs: A review. U.S. Bureau of Mines Information Circular, 8579. Reviewed 237 papers on radiometric prospecting for oil and gas. 85% reported that relationship exists. Balance disagree (reviewed by J.G. Morse, et al., 1982).
P. O. Banks and D. T. Ghahremani, Fracture control of radon leakage, Moreland Hills, Ohio: Implications for shale gas exploration: Association of Petroleum Geochemical Explorationists Bulletin, v. 3, n. 1, p. 40-55. 1987 See paper below by Ghahremani.
Card, J.W., K. Bell, G.M. Denham, and S.R.A. Shah. "Radon decay product measurements in radiometric uranium exploration: implications for petroleum exploration." The Oil and Gas Journal 83 (June 24, 1985): 114(5). annotations later
Dyck, W. and J.R.Jonason, 2000, Geochemical Remote Sensing of the Sub-Surface, Edited by: Hale, M. © 2000 Elsevier, Chapter 11 on radon An extensive scientific discussion of the geochemistry of radon
K. Fan, T. Kuo, Y. Han, C. Chen, C. Lin and C. Lee, 2007
Radon distribution in a gasoline-contaminated aquifer
Radiation Measurements Volume 42, Issue 3, Pages 479-485 		"Abstract Naturally occurring radon-222 gas in groundwater was investigated as a partitioning tracer to detect non-aqueous phase liquid (NAPL) in a gasoline-contaminated aquifer. The radon-222 activity of groundwater decreased significantly from an average of 7.38 +/- 1.68 Bq/L measured in monitoring wells located upgradient in the uncontaminated zone to an average of 2.30 +/- 0.60 Bq/L measured in monitoring wells inside the NAPL source zone. Meanwhile, the radium-226 concentrations measured in aquifer matrix were virtually homogeneous at several locations both upgradient of and inside the NAPL source zone. Furthermore, the NAPL concentration obtained from the Radon Deficit Factor agrees reasonably with the results derived from direct sampling and chemical analysis of soil samples taken from the residual NAPL source zone. The field results of this study confirmed the general applicability of groundwater radon to detect residual NAPL source zone."
J.E. Garcia-Gonzalez, M.F. Ortega, E. Chacon, LF. Mazadiego, E. De Miguel, 2008
Crupo de Ceoquimica Ambiental, Universidad Politecnica de Madrid, E.T.S. Ingenieros de Minas, Alenza 4, 28003 Madrid, Spain
Field validation of radon monitoring as a screening methodology for NAPL-contaminated sites
Applied Geochemistry, Volume 23, Issue 9, September 2008, Pages 2753-2758 		If a non aqueous phase liquid (NAPL) is present, radon is enriched in the NAPL and depleted in the gas phase. There is less radon free to enter our sampling tool and we should find low radon values over hydrocarbon enrichments.
Based on a field study of 67 station, the authors conclude:
"...as was theoretically predicted, reductions of Rn concentration in soil air above subsurface accumulations of hydrocarbons can be deterministically differentiated from background values (the former being between 5 and 10 times lower than the latter) under real field conditions."
"Emanometry has the advantage that it can locate and determine the boundaries of free-phase plumes of contaminants even when the amount of organic vapors reaching the surface is very low or non-existent."
The paper cites a number of studies which gave similar results.
Ghahremani, D. T. 1985. Radon Prospecting for Hydrocarbon: Potential Strategy for Devonian Shale Gas in N.E. Ohio; PhD Thesis, Department of Geological Sciences, Case Western Reserve University, 259p. They hypothesized that shale gas yields will be optimized where natural fracturing of the shales is greatest, and that such fracturing will allow radon to migrate to surface causing radon anomalies in soil gas.
They measured radon in soil gas at 600 sites and found localized anomalies associated with bedrock fracturing as revealed by topographic lineaments. They found anomalously high levels of light and heavy hydrocarbons where radon levels were highest. The hydrocarbon composition is more like Devonian shale gas than of Clinton or other gases.
See note below.
SIGNIFICANCE OF RADON/HYDROCARBON SEEPS IN PETROLEUM EXPLORATION
Darioush T. Ghahreinani, TerraTech International, Inc., P.O. Box 22288, Cleveland, Ohio 44122, 1988? 		See paper above by Ghahremani.
Heemstra, R.J. and R.M. May, T.C. Weeson, J.R. Abrams, and G.A. Moore, 1979. A critical laboratory and field evaluation of selected surface prospecting techniques for locating oil and natural gas. BETC/RI-78-18. U.S. Department of Energy, January. This paper looks interesting. It discusses the upward movement or radium in solution in formation waters. I am still trying to find the paper.
R. W. Klusman and J. A. Jaacks, Environmental influences upon mercury, radon and helium concentrations in soil gases at a site near Denver, Colorado: Journal of Geochemical Exploration, v. 27, p. 259-280. 1987 annotations later
Langford, G.T., 1962. Radiation surveys and oil search. World Oil, pp114-118, April. annotations later
LeSchack, L.A., and D.R. Van Alstine, 2002, High-resolution ground-magnetic (HRGM) and radiometric surveys for hydrocarbon exploration: six case histories in Western Canada, in Surface exploration case histories: Applications of geochemistry, magnetics and remote sensing, D. Schumacher and L.A. LeSchack, eds., AAPG Studies in Geology No. 48 and SEG Geophysical References Series No. 11, p. 67-156. see http://www.cseg.ca/publications/recorder/1994/10oct/oct94-magnetic-horizontal.pdf
Li Y.L., Yuan G.-J. and Peng D., 2006. Discussion on geochemical exploration predicts the depth of oil reservoir (Chinese). Wutan Huatan Jisuan Jushu 2006 28/4 (367-371). Abstract in Geological Abstracts 2007 #2973. We only have the abstract. The authors claim that the pattern of radon values at surface can help predict the depth of an oil reservoir.
Mansker, William L.,(1989) Applied Radon Geochemistry in Oil and Gas Exploration: ABSTRACT, AAPG Bulletin Volume 73, p1166 "...uranium tends to remain in residual hydrocarbon fluids, whereas radium tends to associate with brines. Upward microseepage of brines transports and precipitates radium within reduced geochemical columns. Positive radon anomalies over reservoirs are associated with transported subsurface and near-surface radium."
States that good radon anomalies found over fields at depths greater than 6,000 feet and "...high-resolution traverses appear to sharply define reservoir boundaries."
Unfortunately, we only have the abstract, and none of these claims are referenced.
Matolin, M.; Abraham, M.; Hanak, J.; Kasparec, I.; Stranik, Z. GEOCHEMICAL AND GEOPHYSICAL ANOMALIES AT THE ZDANICE OIL- AND GASFIELD, SE CZECH REPUBLIC, Journal of Petroleum Geology, Volume 31, Number 1, January 2008 , pp. 97-108(12). We only have the abstract. It includes much of the same data, interpretation and conclusions as the next paper, plus they also analysed a number of rock samples and noted the same decrease in K, U and Th levels over the OPZ.
M. Matolín, Z. Stráník, Radioactivity of sedimentary rocks over the Ždánice hydrocarbon field
Geophysical Journal International, vol. 167, no 3, pp1491-1500, 2006 		At the Ždánice field in the Czech Republic the oil and gas reservoir is at a depth of 900 meters. The paper gives results of gamma-ray spectrometry and radon and thoron determinations at 388 stations along two profiles of 6880 and 8335 meters length. Gamma-ray spectrometry shows decreases of K, U and Th over the oil productive zone (OPZ) which the authors ascribe to "leaching of natural radioelements caused by hydrocarbon-generated groundwater acids." They also noted increased levels of radon and thoron over the OPZ, which they attribute to enhanced gas emanation from U- and Th-bearing minerals decomposed by these same groundwater acids ("enhanced emanation power").
MA Zhi-fei LIU Hong-fu ZHANG Xin-jun Analysis of The Cause of Radon Anomaly over Oil and Gas Reservior Based on Theory of Helium-Radon Clusters
MA Zhi-fei LIU Hong-fu ZHANG Xin-jun, Taiyuan University of Technology 		They report a halo-like radon anomaly over an oil and gas reservoir. The link to the left has the abstract in English. The abstract is brief and we have copied it as follows:
"The halo-like radon anomaly over oil and gas reservior can instruct the border clearly and it may provide some useful information for oil and gas exploration. This paper analysied and summaried the general understanding of its causes firstly, then used the theory of helium-radon cluster to explain the phenomenon, provided some enlightenments for basic theoretical researching of oil and gas exploration by using radon."
You can see the full text (5 pages) in Chinese at
http://www.paper.edu.cn/en/downloadpaper.php?serial_number=200804-142&type=1.
Morse, J.G., M.H. Rana and L. Morse, Radon mapping as indicators of subsurface oil and gas: Oil & Gas Journal 80(May 10, 1982) p.227-246. Their model calls for radon anomalies at surface due to dissolved radium in upward migrating formation water, and possibly radon carried by upward moving gaseous hydrocarbons. This would result in anomalously high radon values over the edge of the trap, and anomalously low values over the inside of the trap, ideally in the form of a halo.
They surveyed radon in soil gas over a portion of the Irondale field in the Denver basin, which produces oil from a depth of 7,000 feet. They used an RD-200 portable radon detector, the same type of instrument which we provide.
Their results show a well defined band of high radon values between a producing well and a dry hole, suggesting the boundary or halo effect described above.
See note below.
Radon Measurement for Oil & Gas Exploration, Pakistan Institute of Science and Technology annotations later
“soil radon gas surveying: an exploration tool for oil and natural gas”, Presented at Saudi Aramco (30 June, 1996). annotations later
Sikka, D. B. 1959. A Radiometric Survey of Redwater Oilfield, Alberta, Canada; Ph.D. Thesis, McGill University, Montreal, P.Q., p.218. annotations later
Sikka, D.B. and R.B.K. Shives, 2001. Mechanisms to Explain the Formation of Geochemical Anomalies Over Oilfields AAPG HEDBERG CONFERENCE “Near-Surface Hydrocarbon Migration: Mechanisms and Seepage Rates” SEPTEMBER 16-19, 2001, VANCOUVER, BC, CANADA annotations later
Sikka, D.B. and R.B.K. Shives, 2002. Radiometric Surveys of Redwater Oilfield, Alberta: Early Surface Exploration Case Histories Suggest Mechanisms for the Development of Hydrocarbon-related Geochemical Anomalies, in Surface exploration case histories: Applications of geochemistry, magnetics and remote sensing, D. Schumacher and L.A. LeSchack, eds., AAPG Studies in Geology No. 48 and SEG Geophysical References Series No. 11, 243-297. annotations later
Integrated radiometric prospecting for petroleum in the Kailu Basin, Inner Mongolia, China
Sun Zhongjun, Zhu Bingqiu, Du Guangtong, Yu Hui, Wang Wei and Liu Haisheng
Institute of Geophysical and Geochemical Exploration, Langfang, Hebei 102849, China
Journal of Geochemical Exploration Volume 55, Issues 1-3, December 1995, Pages 275-282 Geochemical Exploration 1993 		annotations later
Tilsley, J.E. and R.J. Nicholls, 1993. Investigation of soil gas radon as a petroleum exploration technique, Ontario Geological Survey, Open File Report No. 5876 After this web site opens, you have to go down on the left and click on "ofr5876.pdf"
Tripp, R. Maurice, 1945. Measurement of soil-air ions over the Fort Collins Anticline. Geophysics, Vol. X, p238-247 He used an ionization chamber to measure radon in soil gas and corrected the data for temperature, barometric pressure and wind condition. "Stations which were reoccupied at successive intervals over a period of several days or weeks checked very well." He tested 53 points and contoured the results to compare them with the structural contours of the anticline. Correlation of radon values with the anticline is good.
Weart R C, Heimberg G., 1981. Exploration Radiometrics: Postsurvey D rilling Results, Unconventional Methods in Exploration for Petroleum and Natural Gas .Southern Methodist Univ Press,1981, pages 116-123. I am still looking for this paper.
Wells, Arthur W., Richard W. Hammack, Garret A. Veloski, J. Rodney Diehl, Brian R. Strazisar, Henry Rauch, Thomas H. Wilson, and Curt M. White Monitoring, mitigation, and verification at sequestration sites: SEQURE technologies and the challenge for geophysical detection The Leading Edge, Oct 2006; 25: 1264 - 1270. ......deep Earth gases such as hydrocarbons and radon. The assumption is that conduits which permit hydrocarbons and radon to migrate to the surface would also be...tracers such as light hydrocarbons and radon. Light hydrocarbons (methane in particular...... annotations later
Note: It is likely that if the authors had used our formula to distinguish between radon and thoron ,
and plotted both the radon and radon/thoron ratios, results would have been better.

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