V.T. Jones and R.J. Pirkle
Presented to the American Chemical Society, March 29 - April 3, 1981, Atlanta, Georgia


GS&T has been conducting soil gas geochemical surveys since 1974 which strongly support the association of both helium and hydrogen with tectonic activity. Examples will be given over the San Andreas Fault in California, the Duchesne Fault Zone in the Uinta Basin, and a thrust fault near Glacier National Park in Montana. The largest helium anomaly observed was 1243 ppm near Glacier National Park while a maximum value of 350 ppm was observed over the San Andreas Fault and 372 ppm over the Duchesne Fault Zone. Hydrogen values are typically between twenty to several hundred ppm. Hydrogen anomalies are also associated with petroleum deposits. Measurement of these gases can be useful in interpretation of geochemical data.

GS&T has been involved in surface geochemical prospecting since the inception of the earliest applications, around 1932 when A. J. Teplitz and J. K. Rogers first tried to extract ethane from near-surface soils (Figure 1). Great strides have been made over the past 50 years, with results culminating in the series of papers given here today. Most of our effort has been directed toward exploration for oil and gas (Jones, 1980). Some of our initial results were reported at the April 1979 annual AAPG meeting held in Houston, Texas in a paper by Jones and Drozd (1979) which indicated that the chemical compositions of the light hydrocarbons measured in the near-surface soil gases could be used to predict whether oil or gas was more likely to exist within the surveyed area. This concept has been extended to the marine environment by Williams and Mousseau, as described in the preceding paper. Incorporation of both the surface and marine derived indicator parameters will be presented in the next paper by Pazdersky, Drozd and Jones. The consistency of these hydrocarbon seep parameters leaves little doubt of the validity of surface geochemical prospecting as an adjunct to hydrocarbon exploration. This paper will address two additional soil gas indicators, helium and hydrogen, which have been found to be useful in soil gas prospecting, Figure 2, (Kartsev et al. 1959, Orchinnikov et al. 1972, Sokolov et al. 1971).
The results of this paper will be directed toward the demonstration that soil gas helium and hydrogen anomalies show a clear association with deep fault zones. Examples shown today will consist of soil gas profiles over three different types of faults. The areas chosen are the San Andreas fault which is probably the largest and most active fault in the US, the Duchesne fault zone which is a major east-west fault zone in a less active area; and the last is a very lose spaced geochemical profile over a proposed thrust fault in the Disturbed Belt of the Rockies near Glacier National Park in Montana.

In addition to demonstrating an empirical relationship between helium and hydrogen soil gas anomalies and faults, I will briefly discuss the possible sources of these gases and suggest applications for their use. For example, helium and hydrogen appear to offer considerable promise in structural mapping and prospecting for buried mineral deposits (Eremeev et al. 1972). Because of the obvious relationship to tectonics, these gases may even have promise for earthquake forecasting, although this has not yet been demonstrated (Barsukov 1979, Borodzich 1979, Geothermal Energy 1979, Geotimes 1981). In this regard GS&T donated one of our helium-hydrogen chromatographs to Cal-Tech in 1980. It was installed on their most recent earthquake monitoring station at Pacoima Dam in California.

Examinations of the published literature, coupled with extensive field applications by GS&T scientists have indicated that large volumes of diverse gases continually escape from the earth's crust into the atmosphere. Areas of especially high activity appear to be related to zones of deep tectonic fracturing and the accompanying jointing in which mineralization is sometimes located (Figure 3). Typical deep gases are CO2, N2, CH4, H2, He, Ar, Rn, Hg, SO2, COS and H2S. The major components are CO2, N2, CH4 and H2 with the remainder of this list generally found as minor or trace components. The isotope ratios of hydrogen, helium, carbon, oxygen and uranium have considerable potential for helping to define the sources of these gases.

According to published literature, the magnitudes of deep gas anomalies are governed strongly by tectonic and magmatic activity (Figure 4), thus stronger patterns are encountered in seismically active areas of late orogenic activity. Accordingly, the weaker patterns are observed in platform and shield areas that are relatively quiescent; that is in consolidated blocks of the earth's crust.

That earthquakes may possibly be preceded or accompanied by deep mobile gases appears to have first been observed in the USSR in 1966, Figure 5, (Fursov et al. 1968). Studies of this type were also undertaken in China at about the same time (King 1980) and in Japan in 1973 (Wakita et al. 1978). Chemical monitoring of earthquake activity was not widely practiced in the US where most efforts were geophysical until about 1975, when limited studies were initiated using Radon (Science News 1976). GS&T also conducted our first measurements of helium and hydrogen in 1975.

Two recent landmark publications in Science by Wakita et al. (1978, 1980), Figure 6, have indicated that both helium and hydrogen are observed in anomalous quantities along active faults. In 1978 helium was observed to be as large as 350 ppm within a nitrogen vent on the Matsushiro fault swarm. The authors proposed to call these anomalous areas "Helium Spots" because the helium leakage was not homogeneous throughout the fault zone. These unevenly distributed helium spots were reported to occupy areas of about 30 x 50 meters. Extensive experience in soil gas prospecting by GS&T indicates that all soil gas anomalies occur in such spots. This localization of seeps occurs because the migration of gases are dominated by faults and fractures, either on the macro or micro scale.

A second paper, published in October, 1980 in Science, reported 70 measurements for hydrogen in the Yamasaki fault zone. These measurements, made in 0.5 to 1 meter deep holes reported hydrogen anomalies ranging from 2 to 30,000 ppm H2 in the fault zone, with ambient background values of 0.5 ppm observed outside the influence of the fault. The authors postulated that the hydrogen was formed by the reaction between groundwater and fresh rock surfaces formed by fault movement.

Additional support for a deep H2 source has been advocated by Hebert Hawkes (1972, 1980), Figure 7, who has suggested that water at a depth of about 15 km (T~300ºC) would react with minerals containing ferrous iron to yield hydrogen. Hawkes calculations indicate that the contrast in the ratio of H2 to water between a depth of 15 km and the surface is about a billion to one. Thus if this deep water were to migrate upward from depth, then H2 gas would be continually coming out of solution as bubbles. Considerable support for this deep hydrogen mechanism can be obtained from Gold (1979), Levinson (1977) and Hunt (1980). Both Levinson and Hunt have suggested additional study on the mechanisms that could produce hydrogen. These would include hydrogen generated near the surface by hydrogen-producing bacteria, maturation of organic matter at various depths, magmatic sources and even from metamorphism of graphite-containing rocks. In addition to the possibility of various deep H2 Sources, we must also consider existing hydrocarbon reservoirs and their source rocks as a viable alternative.

Hydrogen is a common constituent of all petroleum reservoirs, although it is rarely reported at the lower levels of 0.1%, or less, because such levels are considered to be insignificant for reservoir engineering purposes. More recent analysis by GS&T have always found H2 to be present. In western Siberia, Nechayeva (1968) found H2 in 15% of all reservoir samples analyzed. As shown in Figure 8, peak hydrogen concentrations were 0.9% for gas fields, 6% for gas condensate fields, and 11% for oil fields. The highest H2 concentrations were observed in gases from organic-rich Jurassic sediments suggesting that the H2 was related to thermal decomposition of the organic matter.

Additional confirmation for the presence of hydrogen in subsurface gases was obtained at GS&T by putting a He-H2 detector on some of our wildcat wells . Figure 9 shows typical He-H2 values encountered on a gas log run by GS&T on a well drilled at the research lab. The most important point as regards this talk is to note that both helium and hydrogen occur with the three major hydrocarbon shows observed during the drilling of this well, which initially open flowed at ~1/MMCFPD. This gas contained 0.2% He and, 600 ppm H2. Helium and H2 have proved to be useful gases to measure in well logging because both gases appear together not only in reservoirs, but also in more permeable zones such as faults. An additional positive element is the fact that no contamination source exists for He and H2 in normal mud additives.

Figure 10 shows the location of soil gas surveys conducted in the US and Canada since 1972 (Jones and Drozd 1979). The dots represent areas in which geochemical surveys were conducted. This map contains about 120 surveys having more than 21,000 measurements and covering 10,000 line miles of geochemical data. Data from this base will be reported on here today.

A diagramatic representation of our soil gas sampling equipment is shown on Figure 11. The soil gas measurements are made in a shallow hole, at least 12 feet deep, which is generally auger drilled with a 3-1/2 diameter auger. As shown, a packing device which also serves as a soil gas probe is placed in this hole. The packer is inflated to isolate the bottom of the hole from the atmosphere. The soil gases are then pumped into a gas chromatograph for the determination of light hydrocarbons (methane, ethane, ethylene, propane, propylene, iso-butane, and normal-butane), helium, and hydrogen in our newer dual gas chromatographs. There is a location in the input line where one can connect an evacuated cylinder and collect a soil gas sample that can be brought back to the Laboratory for additional gas analyses. This is currently used for mass spectrogram and carbon isotope analysis of the soil gases. During the early surveys reported on here, the helium and hydrogen analysis was conducted at the GS&T labs using evacuated cylinders.

The first example is a soil gas survey over the San Andreas fault zone. Figure 12 is a geologic map of California (Jennings 1977) which shows the location of the soil gas profile. The survey consists of a regional line which follows Highway 46 from Famosa to Paso Robles. The objectives of this survey were two fold: (1) to cross the Lost Hills Oil Field and the San Andreas fault. Figure 13 is a color Landsat image on which one can clearly see the San Andreas fault. Vegetated areas appear red on the color Landsat composite. As shown in these two figures, the San Andreas fault is a giant shear zone that extends some 650 miles through Southern California and along the Coast Range of Central California. This huge fissure has been instrumental in the development of the topography all along its course, and has caused two of the largest earthquakes in California's history (1857 and 1906), plus numerous smaller earthquakes.

Like all major faults, the San Andreas is not a single break in the rock, but is a wide zone, made up of several lines of activity that are roughly parallel. It is not of a single age, but includes the remnants of ancient faults that have been quiet for countless centuries and other active breaks that form the lines of most recent activity within the zone. The discontinuous movement along the fault has given rise to a confused surface appearance consisting of old features that have been heavily eroded, plus the fresher results of movements of the past few thousand years.

Most California earthquakes originate at points about 10 miles deep, but the San Andreas fault undoubtedly extends down through the earth's crust (a distance of some 20 to 30 miles) to the East Pacific Rise, which is one of the spreading centers associated with continental drift and plate tectonics.

Within California, the San Andreas varies considerably in width. In places, it may be less than 100 yards wide and made up of but a few entangled lines of rupture. In most sections, however, it is several hundred yards to a mile or more in width and is interlaced with any number of sub-parallel fault lines. Its actual edges are indefinite because of the many old lines of activity that are now hidden under recent gravel deposits or alluvium, and because of the landsliding that has covered several miles at a stretch.

Figure 14 shows the 50 mile long traverse that was conducted along Highway 46 in 1975. As shown on this profile, the Lost Hills anticline has a direct soil gas anomaly which has a contrast of 5000 to 1 as compared to values observed east or west of the field. Production ranges from 200 to 6000' in depth and is Pleistocene to Miocene in age. Note that the west side of the basin is clearly anomalous with respect to the east side. As shown, very low hydrocarbon magnitudes are observed on the eastern side. This is particularly significant since the western side of the basin contains most of the major oil fields.

Helium and hydrogen are plotted on the uppermost profile. As illustrated, hydrogen shows a strong correlation to the hydrocarbon anomalies and thus appears to be a definite petroleum predictor. This data provides support for the contention that helium is a deep fault, or tectonic indicator, which is often independent of the oil and gas. Along this 50 mile long line there are a couple of helium anomalies associated with the western flank of the Lost Hills Field, and possibly with the cross faults. Another cluster of helium values appear to be associated with this fault on the east side. As shown, the helium anomalies do not occur randomly, but are highly localized and appear to be concentrated near basement faults. The most outstanding helium anomaly occurs directly over the San Andreas fault. Figure 15 illustrates the close-spaced geochemical profile along Highway 46 over the San Andreas fault. As shown, values as high as 430 ppm helium were observed over this deep fault. Adjacent sites ranged from 40 to 98 ppm within a distance of 60 to 120 ft of the mapped location of the fault. Hydrogen is 50 ppm on the fault and methane is 3.2 ppm. All three gases would be expected on a deep crustal fault.

Figure 16 shows the location of known active faults as of 1973 as compiled by C. W. Jennings (1975). All Quaternary faults which have shown evidence of movement during the past two million years are orange. Faults which have moved during the past 200 years are shown in pink, along with the dates of their last movements. At Cholame, where this profile is located, the fault moved in 1857, 1906, 1922, and most recently in 1966.

Figure 17 shows two pictures taken immediately after this last earthquake on July 22 when the offset was only two inches, and again 12 days later, on August 4th, when the highway offset has increased to five inches (Earthquake Country 1976).

Figure 18 is a picture of our GS&T technician operating the portable auger drill which was used for this survey. This picture was taken on Gold Hill, looking north. The coverings shown are a USGS tiltmeter station.

A second traverse 5 miles north of Highway 46, on Gold Hill, has a very similar signature as the first profile. Helium values at Gold Hill ranged from 30 to 70 ppm, Figure 19. Sites 181 and 182 are located about 1/2 mile to the south of the profile on Gold Hill. This data, leaves little doubt that helium and hydrogen are associated with the San Andreas fault. Although CH4 appears to correlate with helium on the fault, it is interesting to note that there were no heavier hydrocarbons observed directly over the fault trace.

The second example was chosen to provide helium and hydrogen observations over a less active, but deep basement fault. Figure 20 is a Landsat image of the Uinta Basin in northern Utah. The geologic map is shown on Figure 21 (Stokes and Hintze 1963). The geochemical traverses are shown as red lines and consist of four profiles. The Duchesne and Strawberry Rivers are clearly shown in the image as are the snow-capped Uintah mountains which are the only major east-west mountain range in the Rockies. The most prominent fault in the southern half of the Uinta Basin is the Duchesne fault zone which also runs east-west and lies just south of the Strawberry and Duchesne Rivers. As shown in the Landsat image the Duchesne fracture zone appears as a narrow lineament. Surface geological observations indicate that there is an abrupt change in the direction and amount of regional dip across this fracture. North of the zone the dip increases and becomes more E-W, parallel to the zone.

The extreme length and linearity of the Duchesne fracture zone, which can be traced for approximately 42 miles, suggest that it extends to great depth, perhaps even to basement. The surface geometry also indicates that the zone is essentially vertical, and this is certainly true where the zone can be viewed through some 800' of stratigraphic section in the east wall of Lake Canyon (Sec. 17, T4S, R6W). At its widest point the zone is about two miles across. The Duchesne fracture zone consists of a main line of normal faults which mark the south edge of the fracture zone proper. This zone of maximum deformation has the general aspect of a slightly asymmetric graben in that, although individual fault blocks within the zone may be up or down relative to juxtaposed blocks, the general sense of movement has been down relative to the terrain to the north and south. In places, minor normal faults are found north of the main zone for another quarter mile or so. Locally the individual surface faults may be offset slightly en-echelon. The faults are all high angle and of small displacement, maximum surface displacement of the bounding faults of the graben probably being not more than 50-75 feet. Faults which are secondary to the main faults appear to follow a regional trend of jointing. To the extent that there has been continued activity along the Duchesne zone through the Tertiary and probably also Cretaceous, it is possible that the graben at the surface reflects a single fault at depth. Thus the en-echelon spatial relations of the individual faults comprising the Duchesne fault zone may extend to depth forming a complex structural system.

Traverses across the Duchesne fault indicate that the largest soil-gas anomalies occur where a geochemical site is located on or immediately adjacent to one of the faults of the zone. The faults may have similar en-echelon spatial relations in section as well as in plain view so that, as conduits for gas leakage, they can be expected to have abrupt discontinuities both areally and at depth.

Duchesne Line 1 is a profile which extends from the town of Duchesne for about 16 miles along Indian Canyon (Figure 22). The only break in helium occurs where the line crossed the Duchesne fault zone at Sites 9 and 10. Hydrogen exhibits several anomalies in addition to the large anomaly of 110 ppm over the Duchesne zone.

Duchesne Line 3 (Figure 23) runs down Highway 53 and crosses the fault zone between stations 80 to 84. Hydrogen values are high all across the fault, while helium shows very little response.

Duchesne Line 2 extends along strike and follows the fault zone over its entire length (Figure 24). Two major helium anomalies of 681 and 470 ppm are noted. Hydrogen seems to be greater than 20 ppm over most of the zone, with numerous values over 150 ppm. The maximum hydrogen values range upwards of 3 to 5 thousand ppm.

The first three profiles were run during July, 1976. A second more detailed traverse was conducted in December, 1976.

The detailed traverse, Figure 25, consists of 10 closely spaced sites which cross the northernmost fault of the Duchesne zone that has a surface expression near the center of Line 2. The traverse continues south to cross a second fault, which is down to the north, as expressed in the topography, and forms a narrow graben with the first fault.

Sites 10 and 11 are north of the surface expression of the fault but they lie in small, shallow but topographically well-defined washes which are tributaries to Cottonwood Canyon. As shown by the outstanding helium anomaly of 372 ppm (Figure 26), they could be near or on zones of fracturing associated with the fault zone. Site 3 is definitely located on the surface expression of a fault. Sites 4, 5 and 6 are within the graben. Station 5 is on the crest of a little knoll and is located close to bedrock, while Site 6 is in the bottom of a shallow wash and is probably very close to or on a main fault, as indicated by the topography and subsidiary faults on the bluff to the south. Sites 7 and 8 were placed so as to lie above the secondary normal faults of slight displacement. These faults can be seen in the bluff east of these stations (Figure 27). Of particular interest is the large helium anomaly of 227 ppm at Site 7, which certainly indicates the possibility of a deep fault at this station. It should be noted that all the sites between 2 and 7 were observed to have considerable helium (50-100 ppm) associated with them. Thus the entire fault zone can be said to contain anomalous helium values. The presence of large hydrogen anomalies along this deep fault should be viewed with caution because the anomalies could be merely indicating the presence of excellent source rocks in, contact with the fault at depth.

The third, and final example is taken from an area of considerable structural complexity. Figure 28 shows the location of the survey area in Montana and the major structural features. The geochemical survey line is represented by the yellow line which crosses the Lewis Thrust near Glacier National Park. Figure 29 shows the location of the survey line on the Montana Geological Map (Ross 1958). The dot just NE of East Glacier marks the location of a "hydrocarbon spot". This site was initially flagged by a very high methane value of 108,500 ppm (10.8%). After this site was sampled, it was noted by the GS&T operator that the hole had a positive flow of about 30 cc/min. Detailed sampling on 5 foot centers indicated that this seep had a width of 40 feet.

Figure 30 shows this methane profile. Sampling was conducted at all marked sites on both sides of the road. A scale of 1" = 2000 ppm methane was chosen because of the very large magnitudes observed. The data clearly shows a NW-SE trend suggesting fault control. Because no outcrops exist at this site, it was not possible to map the suspected fault. This detail was finished on October 14, 1977 on the day in which the park was closed in East Glacier. This time limitation, coupled with the fact that this site is on the Indian Reservation, made additional sampling impossible.

Interpretation was complicated by the fact that a gas pipeline crossed the road about 150 feet to the northwest of this site. An analysis of the pipeline gas shown Figure 31 clearly indicates that the seep gas is not related to the pipeline. The seep gas appeared to be almost exclusively methane (75%) and ethane (25%). The observation that isobutane is greater than normal butane may suggest that this gas is an immature product of early maturation. The carbon delta value for the isotopic ratio of 13C/12C was found to be -55.5, again suggesting an early to middle stage of maturity, which the Kootenai non-marine shale could fit.

The final geochemical information was obtained when the evacuated cylinder was received at GS&T and found to contain 1243 ppm He, 27 ppm H2, and 2.4% CO2. This is the largest helium value as yet observed from a soil gas site.

The only fault and structural information available is contained in a paper by Robert W. Weimer. As shown on Figure 32, Weimer interprets this area as having thin-skinned tectonics with an upper plate which consists of a complex fault and drag-fold system involving Mesozoic and Paleozoic rocks ranging in the thickness from a few hundred feet at the eastern margin of the disturbed belt to 2 or 3 miles at the western margin. The lower plate is believed to be relatively unfolded and to continue its normal regional west dip for at least 15 miles under the edge of the folded rocks. The Lewis thrust overrides the younger sediments with pre-Cambrian aged rocks and is regarded as a third structural element. Thus, the structural pattern exhibited by the surface rocks in this area is principally one of west-dipping beds which have been cut by numerous west-dipping thrust faults. The thrust faults occur at intervals of one-half to one mile. The majority of the fault planes appear to have a dip greater than 40 degrees, but this dip is thought to decrease with depth. The throw is variable; on the majority of faults it probably ranges between 1000 and 4000 feet. Just to the east of this proposed thrust fault location is a zone of repeated Ellis, Kootenai and lower Colorado (Blackleaf member) formations, with a width varying from 2 to 9 miles. The strike of the formations and faults in this zone averages N 55º W., somewhat different from the strike of the formations further east. This strike of N 55º W would appear to fit the strike of the geochemical profile shown in the methane profile. Thus the assumption that the gas leakage is associated with one of these thrust faults. This last example illustrates the ability of these techniques to locate blind faults or fractures. The magnitude, 1243 ppm, of this helium anomaly would suggest either basement faulting or a very radioactive source in the near surface rocks. If organic source and decay minerals can be ruled out, then this data would have considerable impact on the interpretation of thin-skinned tectonics. Thus geochemical data can sometimes lead to a reevaluation of the geologic interpretations (Figure 33).


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