ON THE ACCEPTANCE AND REJECTION OF SURFACE GEOCHEMICAL EXPLORATION

Martin J. Davidson
M.J. Davidson Co.
Dallas

This is hardly the first time that I have written or spoken about the struggle to have this distinct microseepage technology embraced by the scientific community and the oil industry.

Each time I believed that the geoscience fraternity and the oil industry were on the verge of achieving significant and explicit acceptance of surface geochemical exploration. Each time I was wrong.

Reluctantly but without abandoning hope I am skeptical about the effect of this conference on the meaningful acceptance of surface geochemistry into the mainstream of petroleum exploration practice.

In its various forms, all of which depend upon vertical migration, surface geochemical exploration continues to be used. For both real and imagined reasons, however, it has not been supported or even sanctioned by the predominant portion of the geoscience community.

NRC REPORT

It was less than a year ago that the U.S. National Research Council, through its Committee on Applied Research Needs Related to Oil & Gas, published a seemingly inviolable report entitled "Advanced Exploratory Research Directions for Extraction and Processing of Oil & Gas."

The NRC is the principal operating agency of both the National Academy of Science and the National Academy of Engineering. Its charge is to provide services to the government, the public, and the scientific and engineering communities. The NRC is the very pinnacle of U.S. applied science.

The report was requested by the Department of Energy, which provided the financial support. The lead topic, requested to be studied by the DOE, was "novel and promising areas of research related to oil, gas, shale, and tar sands." The DOE charged the committee to study "both incremental improvements to longstanding challenges" as well as innovative, high-risk explorations of new ideas.

As it has to do with certain aspects of oil and gas exploration, it is truly a fine report. It recommends research into increasing the dominant frequency in seismic reflection data from 25 or 30 hz to 90 or 100 hz, noting that "three to four times the resolving power would be available from such data." The report notes that surface seismic reflection surveys using frequencies of 100-400 hz have already been demonstrated at shallow depths. Extending this capability from depths of a few hundred feet to several thousand feet is feasible and would pay tremendous dividends."

ALLUSION TO GEOCHEMISTRY

The section on geochemistry is worth quoting in its entirety.

"A variety of geochemical techniques for correlating sources with their products are available and could be applied to better understand the organic and inorganic evolution of sedimentary basins. For example, provenance studies of sedimentary rocks may indicate the source of the sediments, thus allowing better paleoreconstruction of the basin. Inorganic signatures could potentially be used to indicate the organic source rock for hydrocarbons in reservoirs, thus providing insight into migration processes. Research on the geochemistry of natural gas could elucidate the influence of lithology, fluid chemistry, pressure, and temperature on gas composition."

What does this distinguished report have to say about surface geochemical exploration? All of nothing.

Have you ever crashed a party or contradicted the boss, and as a result were given the "silent treatment?"

One might say that the members of the distinguished committee were totally unfamiliar with the very existence of surface geochemical exploration. It is not my place to comment on that. However, if they were aware of the existence of surface geochemical exploration, they were most certainly condemning it by a conspiracy of silence.

LOGICAL POSITIVISM

What's nice about an after-dinner speech is that, unlike a formal paper, there is enough latitude to be personal.

Logical positivism underlies modem scientific thought. Logical positivism claims that the ultimate basis of knowledge rests upon experimental verification rather than upon personal experience. Nevertheless there is nothing wrong with "personal experience," but only as a starting point.

In 1957 I was assistant to the vice-president of exploration of a family owned integrated oil company of which at the time I was a member of the family. At fancily dinners and on similar occasions the head of the family, who was a legitimate oil tycoon, would taunt me about my continued interest in surface geochemistry.

"Marty, you're just hepped on geochemistry."

If I heard that once I heard it a hundred times. However, he was a good family man - and was deeply concerned that I was going off my rocker. Accordingly he hired an eminent Texas A&M geology professor to talk surface geochemistry with me. it turned out to be an interesting conversation with a delightful man who was, in fact, a greatly knowledgeable geologist. He was also a retired, successful oil finder for a major oil company and a much esteemed past president of the American Association of Petroleum Geologists.

He made three points regarding surface geochemistry:

Leo Horvitz, a surface geochemical contractor, was the best measurer of infinitesimally small quantities of hydrocarbons (remember, this was prior to the hydrogen flame gas chromatograph) and that his own company and other majors regularly employed Horvitz as their expert witness in pollution lawsuits;
The only known source in nature of ethane and heavier hydrocarbons is oil and gas;
"But, Marty, those gases can't migrate straight up through the impervious shale and cap rock, they come to the surface through faults and fissures miles away from an accumulation. The seal of the oil or gas accumulation is impervious."

VERTICAL MIGRATION PRINCIPLE

Now 40 years later this dogma is no longer as robust as it was then, but neither is it dead. There is no broad acquiescence to the principle of vertical migration from an accumulation to the surface.

Accidentally, in the way of personal experience, my undergraduate degree was in ceramic engineering-ceramics as you know are artificial rocks. Accidentally again, my first real job was as a research engineer at the Advanced Development Laboratory of Sylvania Electric Products, where my work was centered on stopping the intrusion of gases into the evacuated, sealed glass envelopes of radio tubes.

Very small quantities of ambient gases in the atmosphere would poison the cathode and destroy radio tubes. It was not diffusion through the vitreous envelope, it was intrusion through minute cracks in the glass that could sometimes be seen, but then only under reflected, bright light and with a microscope. The intrusion of gases into the vacuum tubes occurred at room temperature and atmospheric pressure.

It took no stretch of the imagination to relate microfractures in glass to microfractures in the earth's subsurface. Nor did it take any stretch of the imagination to recognize that elevated pressure and temperature would enhance the movement of gases through microfractures.

Not until MacElvain's 1969 paper, "Mechanics of Gaseous Ascension Through a Sedimentary Column," was there a theoretically satisfactory explanation for the vertical migration of light hydrocarbons, a mechanism which Leigh Price of the U.S. Geological Survey emphasized in his 1986 paper and in subsequent papers having to do with vertical migration. Also in his 1986 paper Price concludes, "Hydrocarbon microseepage does exist and can create surface anomalies which outline hydrocarbon pools and therefore can be used as a viable exploration tool."

A GEOLOGIC HERESY

It should be noted that without vertical migration there is no subsurface geochemical exploration. But vertical migration is almost never taught to geology students in universities. With the recent publication of Prof. Ron Klusman's 1993 groundbreaking text on surface geochemical exploration, however, that may change.

Vertical migration has been and, in many quarters is today, a geologic heresy. It is the refusal on the part of many if not most geologists, to accept vertical migration, that has drastically impeded the serious funding of both theoretical research and especially of detailed, properly performed, properly financed evaluations of drilling results.

Little grant money means little public domain research which means in turn that few qualified investigators are interested or able to perform the needed research. Both in industry and in the universities it has been professionally hazardous to be a strong proponent of surface geochemical exploration.

With few exceptions, our knowledge of surface geochemical exploration is empirical in nature, and not especially authoritative at that. The need for more extensive - and more intensive - theoretical research and more authoritative empirical research, in this arena, is imperative.

I used the word arena purposely. To paraphrase the philosopher of science, Thomas Kuhn, a fundamental change in scientific thought cannot occur without controversy and conflict. Each such change in the history of science necessitated the community's rejection of one time-honored scientific theory in favor of another theory that was essentially incompatible with its predecessor.

"Competition between segments of a scientific community is the only historical process that ever actually results in the rejection of one previously accepted theory or in the adoption of another."

Resistance to change is anything but irrational. Those who would not - or who could not - adjust to change, justifiably fear the obsolescence of their physical capital, not to mention their own particular skills. Intellectual and. financial interests can thus rightfully be expected to fight, in order to maintain their own positions.

INTEGRATING TECHNIQUES

I believe that the measurement of fight hydrocarbons is the most fundamental of all the surface geochemical methods. One particular cost of inadequate theory in the use of hydrocarbon measurements is the continued inability to determine on geochemistry alone whether to drill on an anomaly of high values or in the low values center of a halo anomaly.

In the absence of thoroughly adequate theory, it is absolutely necessary to combine the use of surface geochemical exploration with subsurface geology and/or seismic. Far from obsoleting geologists and geophysicists, the integration of their skills with surface geochemistry is now and will always be essential.

One of the critical factors in the rejection of surface geochemical exploration is the myth that it stems from doodle-buggery and as such has no place in serious geoscience. It is because of this significant, rejecting barrier that I must talk some about the history of surface geochemical exploration.

PIONEERS OF GEOCHEM

The beginnings of surface hydrocarbon geochemical prospecting - the first continuously sustained vertical migration method - came out of the inner sanctum-sanctorum of German, Russian, and American science. The initial work in 1929 was performed by Gunter Laubmeyer at Germany's prestigious Max Planck Institute and was quickly substantiated by V.A. Sokolov, who was associated with the U.S.S.R. ministry of Geology.

The undisputed father of American geochemical prospecting is Dr. Ludwig W. Blau of Humble Oil Co., now Exxon. Wallace Pratt had selected Dr. Blau in the early 1930s to establish the petroleum industry's first geophysical research laboratory, which was located in Houston as part of Standard Oil Development Co.

Blau read and replicated the works of Laubmeyer and Sokolov and developed a cheaper, easier, qualitatively indicative technique. By his own admission his scientific work in geochemical prospecting was extremely simplistic, but until the day of his death he maintained that it had been extraordinarily effective.

Blau was aggressively opposed to reflection seismography. It was his contention that, at the least, some seismic highs were truly of a geochemical origin. He maintained that the vertical migration of hydrocarbons causes secondary mineralization of the strata above an accumulation which gives rise to higher velocity wave movement in the harder, mineralized strata - and that, in turn, is reflected as a seismic high.

During annual research meetings of the Standard Oil of New Jersey group in the 1930s, Blau reportedly would bang his fist on the table and proclaim, "Get rid of your seismic crews and geophysicists. All the oil that's been found could have been found using geochemistry without firing a single shot." Such statements more than slightly tended to antagonize geophysicists as well as geologists, not to mention the burgeoning seismic industry. Blau's attacks on seismic were tremendously counterproductive and did much to contravene the acceptance of surface geochemical prospecting. The opposition of many if not most seismologists continues to this day.

Two eminent geophysicists, both pioneers in the introduction of reflection seismography, pursued Blau's geochemical lead. They were Eugene Rosaire, one of the founders of Independent Exploration Co., and Eugene McDermott, one of the founders of GSI, later Texas Instruments. Within their own seismic companies both Rosaire and McDermott pursued different approaches to surface geochemical exploration. Independent Exploration Co. entered into a patent-pooling agreement with Standard Oil Development Co. and in 1935 hired Leo Horvitz, a young PhD in chemistry, who was directed to work on the measurement of hydrocarbon gases.

GSI, also under license from Standard Oil, hired its own chemist, Ed Reagor, to pursue Blau's admittedly simplistic method or some variation thereof. It was in this early period that Gene McDermott and Everett DeGolyer were said to have spent close to $1 million developing what later came to be known as the "delta carbonate" method. Prior to a 1979 disclosure of this method by William Duchscherer, it had been strictly "black box" which of itself did significant harm to the cause of surface geochemical exploration.

Because Rosaire believed that the great future of oil exploration lay in geochemistry and not in seismic, he sold his interest in Independent Exploration Co. in 1936 and, with Horvitz in his employ, started the first exclusively geochemical contract service company, Subterex. Perhaps out of necessity, Rosaire in the late 1930s oversold the capability of surface hydrocarbon geochemistry. Accounting for many of his dry holes were difficulties related to measurement, to the interpretation of results, and above all, to the failure to appreciate the full significance of surface geochemistry's inability to determine depth of the accumulation.

With World War II and the resulting falloff in exploration and with Dr. Horvitz called to work on the atom bomb, Subterex went out of business. After the war, Horvitz founded Horvitz Research Laboratories, which he continued to operate until his death in 1986.

Gene McDermott's partners in GSI wanted out of the surface geochemical exploration business in order to concentrate on seismic. In 1942, together with those associates who had been actively engaged in the GSI geochemical effort, McDermott bought GSI's geochemical laboratory and set up-an independent firm, Geochemical Surveys Inc. Geochemical Surveys continued to operate under the subsequent ownership of Bill Duchscherer until his death in 1989. It should be noted that after a 40 year hiatus, GSI in 1982 returned to surface geochemical contracting using hydrocarbon gas measurement, primarily as a means of furthering its seismic business.

GEOCHEM FAILURES, SUCCESS RATES

The literature is replete with the successes of surface geochemical exploration in its many, varied forms. Invariably those successes were achieved - as they should have been - in conjunction with other methods such as subsurface geology, seismic, core drilling, etc.

Surface geochemistry has its failures, and they, too, are many. But until such failures are critically examined and reported upon in detail, no satisfactory understanding of surface geochemistry's limits can be determined.

The American Petroleum Institute's data on new field wildcat success rates in the U.S. over the last 50 years peaked in 1979 with a success rate of 19%.

Twenty five years earlier, in 1954, Leo Horvitz published in Mining Engineering a 59% new field wildcat success rate behind his acid extraction adsorbed hydrocarbon geochemical anomalies (23 out of 39 prospects tested). Horvitz never cited those figures in subsequent papers on the grounds that he would be laughed out of the industry.

In 1983 the International Geology Review published a paper of Y.Y. Kuzmin who reported a 58% new field wildcat success rate behind surface hydrocarbon geochemical anomalies in six basins of the U.S.S.R. (26 out of 45 prospects tested).

W.H. Curry in 1984 published a 64% new field wildcat success rate (57 out of 87 prospects tested) behind ionization chamber radiometric anomalies in the Powder River basin.

In an excerpt from a report of the Scientific Council of the U.S.S.R. Academy of Science and the U.S.S.R. Ministry of Geology (published in English in 1971), V.A. Sokolov writes, "Gas surveys (that is, surface hydrocarbon geochemical surveys) were carried out in areas where the presence of oil and gas was not known. These were followed up by drilling, sometimes many years later, because many geologists mistrust gas anomalies. Today we have a great deal of information concerning the results of drilling carried out over gas anomalies located by gas surveying. There are more than 100 such cases ..."

A gas anomaly in the Bashkir Socialist Republic was drilled against the advice of the geologists and geophysicists, and a new oil field was discovered there. Surface gas anomalies in the Komi region led to the discovery of at least six new petroleum and gas fields. The success of gas surveys on the platform structures of the Komi region is about 90%. In the middle Volga region the efficiency of surface gas surveys was about 70-80%. Some 19 fields were discovered.

Behrman and Land (1992) report that of 89 acid extraction hydrocarbon anomalies in the Gulf of Mexico, 43 were eventually drilled resulting in 38 new fields for a predictive accuracy of 84%.

In the way of initial reconnaissance, airborne methods including micro-magnetics, radiometrics, radar gas sensing, and magneto-electric methods, are among those that are being used or that have been used with varying degrees of success.

WHERE NOT TO DRILL

There are a multitude of different methods of ground surveying. My own personal bias for attempting to delineate a prospect is to measure adsorbed ethane and heavier light hydrocarbons by acid extraction from soil samples with the sampling done at depths of at least 10 ft, and sampled widely over a dense grid.

Such exploration is by no means inexpensive. Neither is seismic. Many though certainly not all of the failures of hydrocarbon surface geochemistry have been occasioned by skimping on the number, depth, and spacing of samples. Both service contractors and operators are equally guilty of doing halfbaked surface geochemistry on the cheap. Poor-boy geochemistry has done more damage to surface geochemical exploration than all of its detractors put together.

Perhaps the most intriguing use of surface geochemistry is in determining where not to drill, regardless of structure. The absence of an acid-extraction light hydrocarbon anomaly, after performing a deep-sampled, detailed hydrocarbon geochemical survey over a broad area, should condemn drilling and by so doing significantly minimize wildcat dry holes.

W.B. Lewis of Standard Oil of New Jersey together with McDermott and Rosaire first reported on this phenomenon in 1940.

"Over 30 wells were drilled in areas condemned by the soil analysis method (adsorbed gas). Twenty nine of the 30 negative predictions proved to be correct. That is, drillings showed that the prospects did not have sands carrying commercial oil deposits even though several of the prospects (proved to be) structurally high."

Horvitz in 1957, reporting on a 1944 hydrocarbon geochemical survey performed on the Gulf Coast for Crown Central Petroleum Corp., wrote:

"At least 45 dry wildcat wells were drilled in the background, or negative areas. Thirty five of these dry holes were drilled after the geochemical data were obtained."

Since publication of that 1957 paper an additional 29 dry holes were drilled in those same negative areas.

Behrman and Land in their 1992 paper reported that in the same Gulf of Mexico survey area in which 89 anomalies were found there were 160 dry holes subsequently drilled in the negative non-anomalous areas.

Kuzmin, in the 1983 article noted earlier, wrote that of 19 areas not recommended for drilling and which were drilled anyway, 15 were condemned by dry holes for a dry hole prediction rate of 79%.

W.H. Curry, in his 1984 paper, wrote that of 189 wells drilled outside the radiometric anomalies 169 were dry for a dry hole prediction rate of 89%.

Let us finally put to rest the fallacious concept that specific negative predictions are worthless merely because statistically after the fact most new field wildcats are dry anyway.

DAMNING PUBLICATIONS

Scientific papers published in prestigious journals have contributed strongly to the rejection of surface geochemical exploration.

Smith and Ellis, in the lead article of the AAPG Bulletin for November 1963, attacked geochemical prospecting by claiming that ethane and heavier hydrocarbons could have grasses and other vegetation as their source. Biogenic ethane and heavier hydrocarbons would so confound the interpretation of data as to make hydrocarbon geochemical exploration totally inoperable.

Horvitz, a contractor and the strongest proponent of hydrocarbon geochemical prospecting, was offered the opportunity to publish a note in the back of a subsequent issue of the Bulletin. Rightly or wrongly, Horvitz insisted upon the publication of a full blown paper in refutation of Smith and Ellis. Nine years later, in 1972, over significant opposition, a courageous editor of the AAPG Bulletin published Horvitz' paper as a lead article. It conclusively demonstrated the error in Smith and Ellis' measurement technique. That was the last serious claim about ethane and heavier hydrocarbons being sourced from anything but petroleum.

LATERAL VS. VERTICAL MIGRATION

In 1972 McCrossan et al. of the Geological Survey of Canada published an evaluation of geochemical prospecting for petroleum in the Olds-Caroline area of Alberta by taking some 4,500 samples over producing fields and attempting to correlate the surface geochemical data with the subsurface reservoirs. The negative conclusions that were reached have provided another formidable barrier to the acceptance of hydrocarbon geochemical prospecting on land.

Using the raw data of the Canadian Geological Survey and nonparametric statistical analysis, Spalding and Davidson (1976 and 1977) completely reversed the McCrossan conclusions. It was demonstrated that the anomalous ethane points over oil fields could happen by chance only once in 200 million situations.

The Spalding and Davidson papers were submitted to the appropriate science journal and rejected. They were published, in a slightly different form, in the proceedings of both the American Institute for Decision Sciences and the Institute for Management Science.

In their review of surface geochemical methods used for oil and gas, Philp and Crisp (1982) offer the following generalization:

"Onshore prospecting using hydrocarbon gases is a difficult technique due to the likelihood of lateral gas migration, perhaps for many hundreds of kilometers. The principle reasons for this appear to be the presence of aquifers, faults, and fracture zones which influence the upward migration of gas bubbles or water containing dissolved gases. No study has yet been published which adequately takes these factors into account. It is likely that these hydrological and geological factors will prove too complex to unravel in many situations. The application of onshore prospecting should, at present, be restricted to areas where the hydrology and geology can be determined, and far more attention should be devoted to these factors."

Philp and Crisp appear, from the body of their paper, to be relying on J.E. Smith et al. 1971, in which it was calculated that, "A lateral water velocity of 1 m/year would move the vertically diffusing gases a lateral distance of 190 km." If this were so, there could be no onshore surface hydrocarbon geochemical prospecting.

Vertical migration does not necessarily refer to "plumbline vertical." There may be some shifting of an anomaly due to lateral groundwater movement, but (except under unusual circumstances) it is minor enough to be ignored when drilling in the center of any but the smallest or narrowest anomaly. Paul H. Jones (1981) observed that "the effects of moving groundwater in shallow aquifers on the geochemical plume configuration (or chimney) depend on the relative velocity flow vertically and horizontally... Where unaffected by local withdrawal from wells, (the) rate of groundwater flow in sand-bed aquifers is very slow indeed, in the range of 0.1 in./day," the same 1 m/year. At that rate, a surface geochemical anomaly having a lateral extent of 1 mile might be displaced one width in 1,736 years. Long before then, perhaps within months or years, the leading edge of the shifting anomaly would have dissipated (most likely by microbial action) and the trailing edge of the anomaly would have been replenished by vertical migration. The displacement force itself would be a function of both vertical and horizontal movement. The strong upward flow of waters overpowers the very slow lateral movement of groundwater. W.H. Roberts (1980) describes the vertical pressure gradient as 233 times the horizontal pressure gradient; and rates of flow are direct functions of pressure gradients.

It is all too often inferred that nothing that a geochemical service contractor writes need be considered as accurate or even truthful. This is a blatant and unjustified insult to many fine scientists. It should be kept constantly and firmly in mind that just about all of the advances in exploration seismology came from service contractors, and that includes 3D seismic.

THE FUTURE

What surface geochemical exploration needs most of all from government and industry is an unequivocally authoritative empirical database covering well predictions, their detailed bases, the results of drilling, and equally important, post drilling critiques. Such a database will, in turn, elicit the much needed public domain theoretical studies.

If there is to be an economically viable upstream industry in consuming nations, the time is now for the geoscience profession to stop marginalizing and trivializing surface geochemical exploration.

BIBLIOGRAPHY

Behrman, R.C., Jr., and Land, J.P., Offshore hydrocarbon surveys, Offshore Technology Conference Paper No. 6858, Houston, 1992.

Curry, W.H., III, Evaluation of surface gamma radiation surveys for petroleum exploration in the deep Powder River basin, Wyo., Unconventional Methods in Prospecting for Petroleum and Natural Gas III, Southern Methodist University, Press, 1984, pp. 25-39.

DeGolyer and MacNaughton, Twentieth Century Petroleum Statistics, Dallas, 1993, P. 29 (API data).

Duchscherer, William, Jr., Carbonate and isotope ratios from surface rocks: a geochemical guide to underlying petroleum accumulations, Unconventional Methods in Exploration for Petroleum and Natural Gas II, Southern Methodist University Press, Dallas, 1981, pp. 201-218.

Horvitz, L., Near-surface hydrocarbons and petroleum accumulations at depth, Mining Engineering, 1954, Vol. 12, pp. 3 7, and Vol. 6, pp. 1,205-09.

Horvitz, L., How geochemical analysis helps the geologist find oil, OGJ, Vol. 55, No. 45, 1957, pp. 234-242.

Horvitz, L., Vegetation and geochemical prospecting for petroleum, AAPG Bull., Vol. 56, 1972, pp. 925-940.

Horvitz, L., Geochemical exploration for petroleum, Science, Vol. 229, 1985, pp. 821-827.

Jones, Paul H., personal communication, March 1981.

Klusman, Ronald W., Soil gas and related methods for natural resource exploration, John Wiley & Sons, New York, 1993, 483 pp.

Kuhn, Thomas S., The structure of scientific revolutions, University of Chicago Press, Chicago, as noted in Davidson, M.J., State of the art and the direction of unconventional oil and gas exploration, Unconventional Methods in Exploration for Petroleum and Natural Gas III, Davidson, M.J., and Gottlieb, B.M. (eds.), Southern Methodist University Press, Dallas, 1962, pp. 4-8.

Kuzmin, Y.Y., Analysis of results and methods of increasing the effectiveness of geochemical exploration for oil and gas on the Russian plate, International Geological Review, September 1983, pp. 1,089-94.

Lewis, W.B., McDermott, E.M., and Rosaire, E.E., Geochemical methods, in Jakosky, J.J. led.), Exploration Geophysics, First Edition, Times Mirror Press, 1940.

MacElvain, R.C., Mechanics of gaseous ascension through a sedimentary column, Unconventional Methods in Exploration for Petroleum and Natural Gas, W.B. Heroy (ed.), Southern Methodist University Press, Dallas, 1969, pp. 15-28.

McCrosson, R.G., Ball, N.L., and Snowdon, L.R., An evaluation of surface geochemical prospecting for petroleum, Olds-Caroline area, Alberta, Geological Survey of Canada Paper 71-31, 1971, 101 p.

National Research Council, Advanced Exploratory Research Directions for Extraction and Processing of Oil and Gas, National Academy Press, Washington, 1993, 61 pp.

Philp, R.P., and Crisp, P.T., Surface geochemical methods used for oil and gas prospecting, a review, journal of Geochemical Exploration, Vol. 17, 1982, pp. 1-34.

Price, Leigh C., A critical overview and proposed working model of surface geochemical exploration, Unconventional Methods in Exploration for Petroleum and Natural Gas IV, Davidson, M.J. led.), Southern Methodist University Press, Dallas, 1986, pp. 245-304.

Roberts, W.H. III, Design and function of oil and gas traps, Problems in Petroleum Migration, Roberts and Cordell (eds.), AAPG Studies in Geology, No. 10, 1980, pp. 217-240.

Smith, G.H., and Ellis, M.M., Chromatographic analysis of gases from soils and vegetation as related to geochemical prospecting of petroleum, AAPG Bull., Vol. 47, 1963, pp. 1,8971,903.

Smith, J.E., et al., Migration, accumulation and retention of petroleum in the earth, in proceedings Eighth World Petroleum Congress, Applied Science Publishers, London, 1971, pp, 13-16.

Sokolov, V.A., The new methods of gas surveys, gas investigations of wells and some practical results, Geochemical Prospecting for Petroleum and Natural Gas, Toronto, Canada Institute of Mining and Metallurgy (CIM), Special Vol. No. 11, 1971, pp. 538-543.

Spalding, J.B., and Davidson, M.J., Developing a decision science methodology for use in surface geochemical prospecting for petroleum, proceedings of the Southeastern Section of the Institute of Management Science, Atlanta, Ga., October 1976, pp. 1-10.

Spalding, J.B., and Davidson, M.J., Statistics taken in vain, proceedings of the Southwest Section of the American Institute for Decision Sciences, March 1976, San Antonio, pp. 13 14.

Spalding, J.B., and Davidson, M.J., Application of decision science to petroleum exploration, Proceedings of the Southwest Regional Meeting of the American Institute for Decision Sciences, New Orleans, March 1977, pp. 1-5.