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Some unconventional geophysical methods: A reviewThe dramatic decrease in oil and gasexploration in the United States since1986 has renewed the interest of smallindependent operators in less costly, un-conventional techniques. Among theseare near-surface soil gas analysis,radiometric investigations, and magne-tometry surveys of shallow Precam-brian topography. The purpose of thisarticle is to review the results of originalresearch that I conducted in these areasin the period 1962-80 and to assess thepossible value of these methods in mod-em exploration.Near-surface soil gas analysis. Onefundamental assumption in soil sam-pling and soil-gas sampling is that thelighter hydrocarbons in an oil or gasreservoir migrate vertically to the sur-face of the earth. Oxidation and subse-quent polymerization of these migra-tory hydrocarbons may form waxy orliquid materials in the near-surface zoneof the earth. Some techniques investi-gate the association of hydrocarbon mi-gration with the accumulation of inor-ganic compounds in the immediate sub-surface.Investigation into the relationship ofnear-surface hydrocarbons to oil-and-gas producing areas started during thepioneering era of geophysical explora-tion (e.g., 1929 in Germany and 1932 inRussia). These early efforts measuredthe enriched soil gas over producingreservoirs. US explorationists becameinterested in 1936.One of the early investigators, LeoHorvitz, analyzed soil samples col-lected at depths of 8-12 ft, for methane,ethane, and heavier hydrocarbons. Heconcluded that “every good oil accumu-lation produces a hydrocarbon anomalynear the surface.”By D.O. ACREYGeophysical ConsultantAmarillo, TexasIn 1961, I learned that Rayflex Ex-ploration had formed an ExperimentalMethane Count Exploration Crew to de-velop a tool for exploration in shallowand stratigraphic producing areas. Thiscrew (six men in the field plus one in theWinnfield, Louisiana office) coveredabout four miles a day. Basic field pro-cedure was to drill a hole with a depthof 2-3 ft, seal the hole as soon as theauger came out, and take a sample of thesoil gas. Interpretation consisted of con-verting readings to methane values andcontouring those values on a map.The crew first worked over a shallowoil field and generally recorded highermethane readings over the producingarea. Problems encountered were thepresence of “marsh gas” at the surfacein some locales and faulting whichcould detour the methane’s path to thesurface.Subsequent work at Goodpine, Lou-isiana (which resulted in an oil discov-ery) used a new field procedure involv-ing a pattern of 20 holes at each station.Statistical analysis determined whichvalues were high enough to be signifi-cant. The percentage of significant val-ues, not the absolute values, were con-toured. The discovery well had 14 ft ofpay at the top of the Wilcox Formation(depth about 1900 ft). The crew wasable to work 6-10 stations per day inthis area. Spacing was 1320 ft for recon-naissance work and 660 ft in a zone ofinterest. Seven producing wells hadbeen drilled in this new field by January1962.At about this same time, I was work-ing with the late Horace Ridge11 in theinvestigation of ethane anomalies.Ridgell’s procedure of ethane intensityexploration involved correlation of thegeology of a producing area with anempirically determined diffusion fac-tor; i.e., in an area of equal geologicreference, ethane intensity readings inshale mantle were half those in sand-stone mantle, and the readings in lime-stone mantle were a third of those insandstone mantle.During the summer of 1962, whileworking as a seismic consultant to thelate W.T. Graham, I collaborated withgeologist Roy Short in a research pro-gram concerning methane intensitymeasurements in areas where seismicsurveys had revealed the geologic struc-ture. We also conducted exploratorymethane surveys over known hydrocar-bon producing areas, stratigraphic aswell as structural. Our “Graham proce-dure” was based on what had beenlearned from Ridge11 and from the Ray-flex crew (which I had observed).Field experiments began in the falland continued for three years. Duringthat period, I also corresponded withLajos Stegena of the Roland EötvösGeophysical Institute in Budapest,Hungary, who was conducting similarresearch. Stegena had published resultsof his initial work in GEOPHYSICS in196 1 and I was impressed by the simi-larity in our conclusions.By April 1964, Stegena and I hadagreed on several fundamentals inmethane intensity exploration. Themost important was our conviction(which Ridge11 also shared) that meth-ane is much slower to migrate than othergases; therefore, methane will be re-tained longer and will comprise thegreater quantity of residual gas in anygiven reservoir.Methane intensity research underGraham’s auspices came to an end inSeptember 1965 because of the initia-tion of an accelerated seismic explora-SEPTEMBER 1994 THE LEADING EDGE 953tion program. Conventional single-foldseismic reconnaissance had defined 13major prospects at depths of 5000-9000ft. Based on our comparison of the shal-low production of Goodpine, Louisianaand the deeper nonproducing area of theMunster Arch in north central Texas,these prospects were considered toodeep for accurate near-surface soil gasanalysis.Radiometric investigations. In theearly 1960s, radiometry was considereda possible method of direct detection ofhydrocarbon accumulation. This wasbased, as was methane analysis, on theupward migration principle.In that time, most research con-cluded that the ability of rock forma-tions to trap hydrocarbons is continu-ously exceeded over the life of a produc-ing field so that there is an almost con-tinuous stream of upward seepage.A common denominator was ob-served for most of the various seepagephenomena-the anomalous radioac-tivity of the producing areas. Hydrocar-bon molecules have an affinity for ra-dioactive minerals, precipitating anduniting with them. As components of ahydrocarbon reservoir migrate towardthe surface, they absorb and transformthe radioactive minerals in each layer,resulting in a reduced radioactive mea-surement at the surface. These “nega-tive” areas are often marked by a sur-rounding halo of higher radioactivereadings. Several such anomalies foundover producing fields led to the conclu-sion that a new oil exploration tool hadbeen found.In 1964-65, I conducted severalradiometric research projects for Gra-ham. Scintillator surveys were con-ducted across shallow producing


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