Some unconventional geophysical methods A review By D O ACREY Geophysical Consultant Amarillo Texas T he dramatic decrease in oil and gas exploration in the United States since 1986 has renewed the interest of small independent operators in less costly unconventional techniques Among these are near surface soil gas analysis radiometric investigations and magnetometry surveys of shallow Precambrian topography The purpose of this article is to review the results of original research that I conducted in these areas in the period 1962 80 and to assess the possible value of these methods in modem exploration N ear surface soil gas analysis One fundamental assumption in soil sampling and soil gas sampling is that the lighter hydrocarbons in an oil or gas reservoir migrate vertically to the surface of the earth Oxidation and subsequent polymerization of these migratory hydrocarbons may form waxy or liquid materials in the near surface zone of the earth Some techniques investigate the association of hydrocarbon migration with the accumulation of inorganic compounds in the immediate subsurface Investigation into the relationship of near surface hydrocarbons to oil andgas producing areas started during the pioneering era of geophysical exploration e g 1929 in Germany and 1932 in Russia These early efforts measured the enriched soil gas over producing reservoirs US explorationists became interested in 1936 One of the early investigators Leo Horvitz analyzed soil samples collected at depths of 8 12 ft for methane ethane and heavier hydrocarbons He concluded that every good oil accumulation produces a hydrocarbon anomaly near the surface In 1961 I learned that Rayflex Exploration had formed an Experimental Methane Count Exploration Crew to develop a tool for exploration in shallow and stratigraphic producing areas This crew six men in the field plus one in the Winnfield Louisiana office covered about four miles a day Basic field procedure was to drill a hole with a depth of 2 3 ft seal the hole as soon as the auger came out and take a sample of the soil gas Interpretation consisted of converting readings to methane values and contouring those values on a map The crew first worked over a shallow oil field and generally recorded higher methane readings over the producing area Problems encountered were the presence of marsh gas at the surface in some locales and faulting which could detour the methane s path to the surface Subsequent work at Goodpine Louisiana which resulted in an oil discovery used a new field procedure involving a pattern of 20 holes at each station Statistical analysis determined which values were high enough to be significant The percentage of significant values not the absolute values were contoured The discovery well had 14 ft of pay at the top of the Wilcox Formation depth about 1900 ft The crew was able to work 6 10 stations per day in this area Spacing was 1320 ft for reconnaissance work and 660 ft in a zone of interest Seven producing wells had been drilled in this new field by January 1962 At about this same time I was working with the late Horace Ridge11 in the investigation of ethane anomalies Ridgell s procedure of ethane intensity exploration involved correlation of the geology of a producing area with an empirically determined diffusion factor i e in an area of equal geologic reference ethane intensity readings in shale mantle were half those in sandstone mantle and the readings in limestone mantle were a third of those in sandstone mantle During the summer of 1962 while working as a seismic consultant to the late W T Graham I collaborated with geologist Roy Short in a research program concerning methane intensity measurements in areas where seismic surveys had revealed the geologic structure We also conducted exploratory methane surveys over known hydrocarbon producing areas stratigraphic as well as structural Our Graham procedure was based on what had been learned from Ridge11 and from the Rayflex crew which I had observed Field experiments began in the fall and continued for three years During that period I also corresponded with Lajos Stegena of the Roland E tv s Geophysical Institute in Budapest Hungary who was conducting similar research Stegena had published results of his initial work in GEOPHYSICS in 196 1 and I was impressed by the similarity in our conclusions By April 1964 Stegena and I had agreed on several fundamentals in methane intensity exploration The most important was our conviction which Ridge11 also shared that methane is much slower to migrate than other gases therefore methane will be retained longer and will comprise the greater quantity of residual gas in any given reservoir Methane intensity research under Graham s auspices came to an end in September 1965 because of the initiation of an accelerated seismic explora SEPTEMBER 1994 THE LEADING EDGE 953 tion program Conventional single fold seismic reconnaissance had defined 13 major prospects at depths of 5000 9000 ft Based on our comparison of the shallow production of Goodpine Louisiana and the deeper nonproducing area of the Munster Arch in north central Texas these prospects were considered too deep for accurate near surface soil gas analysis R adiometric investigations In the early 1960s radiometry was considered a possible method of direct detection of hydrocarbon accumulation This was based as was methane analysis on the upward migration principle In that time most research concluded that the ability of rock formations to trap hydrocarbons is continuously exceeded over the life of a producing field so that there is an almost continuous stream of upward seepage A common denominator was observed for most of the various seepage phenomena the anomalous radioactivity of the producing areas Hydrocarbon molecules have an affinity for radioactive minerals precipitating and uniting with them As components of a hydrocarbon reservoir migrate toward the surface they absorb and transform the radioactive minerals in each layer resulting in a reduced radioactive measurement at the surface These negative areas are often marked by a surrounding halo of higher radioactive readings Several such anomalies found over producing fields led to the conclusion that a new oil exploration tool had been found In 1964 65 I conducted several radiometric research projects for Graham Scintillator surveys were conducted across shallow producing areas near Bartlesville Oklahoma and over the Mobeetie Field in
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