Hydrocarbon Flux Variations in Natural and Anthropogenic Seeps

Jones, V. T., and S. G. Burtell, 1996, Hydrocarbon flux variations in natural and anthropogenic seeps, in D. Schumacher and M. A. Abrams, eds., Hydrocarbon migration and its near-surface expression: AAPG Memoir 66, p. 203–221. Chapter 16 Hydrocarbon Flux Variations in Natural and Anthropogenic Seeps Victor T. Jones III Stephen G. Burtell Exploration Technologies, Inc. Houston, Texas, U.S.A. Exploration Technologies, Inc. Houston, Texas, U.S.A. Abstract Methodologies for conducting surface geochemical surveys and measuring the hydrocarbon flux rates of hydrocarbons migrating to the surface are addressed with examples from natural seeps and from anthropogenic seepage from underground gas storage reservoirs, leaky well casings, and underground coal gasification reactors. Natural gas flux was monitored for 1 year at Arrowhead Hot Springs, San Bernardino County, California, as part of an earthquake prediction program. The hot spring is on a splay of the San Andreas fault and releases 40 mL/min of free gases containing helium, hydrogen, light hydrocarbon gases, and radon. The volume of released gases varied by a factor of two within 7 months. Changes in gas flux could be a precursory signal of earthquake activity on the locked southern section of the fault and demonstrated that rapid changes were related to tectonic activity along this major basement fault. Gas flux associated with pressure changes in underground storage reservoirs confirms the rapid variations observed for natural seeps. Other changes in gas concentrations over a propane storage cavern are related to barometric and meteorologic variations. The rapidity with which natural gas can migrate through the earth was also demonstrated by measuring the gas flux in 122 boreholes over an underground coal gasification reactor. Baseline gas concentrations were established one month before the 180-m- (600-ft-) deep retort was pressured and fired. Leaked gases were detected at the surface in 2 to 15 days, depending on the location of the boreholes with respect to the retort at depth. The underground coal gasification (UCG) reactor provided an outstanding vehicle for migration flux measurements because of the unique gases generated in the reactor. Also, pressure and compositional changes in the reactor occur at known times in direct response to operational procedures. Individual gas pulses exhibited chromatographic effects as the gases migrated away from the source at depth. These chromatographic changes existed for only a few hours at the onset of a pressure pulse in the subsurface reactor and quickly returned to steadystate conditions in which the composition of the reactor gases matched those escaping at the surface. Oil Company discovery in the East Cameron area, offshore Louisiana. The accumulation is located downdip to the southwest, as shown by the bright spots marching down the fault (Weismann, 1980). The Gulf of Mexico sedimentation and water compaction rates were also evaluated and compared to calculated diffusion rates for migrating light hydrocarbon gases. The conclusion was that neither macro- nor microseeps could occur by diffusion since the downward flux of water and sediments exceeds the upward diffusion of gases (R. J. Mousseau and G. Glezen, personal communication, 1974). The presence of both macro- and microseepages in the Gulf of Mexico suggests that diffusion is not the dominant migration mechanism. The comparison of seep compositional information with known production proves that the microseeps are real since seeps INTRODUCTION During the early development of soil gas prospecting at Gulf Research, it became clear that the magnitude of variations in soil gas and dissolved gas sniffer data required an explanation that appeared to be strongly affected by fault and fracture systems (Jones et al., 1995). Migration of oil and gas from the subsurface follows a complex pathway that is rarely imaged, as illustrated by the “bright spots” in Figure 1. Migration of deep gases are expected to form shallow micro-accumulations along the pathway to the shallow subsurface. Thus, the sniffer anomaly in Figure 1 must be projected to depth to find the field associated with the surface anomaly. This example illustrates the expected relationship of sniffer anomalies to their subsurface sources and was an actual Gulf 203 204 Jones and Burtell Figure 1—(Top) Graph of dissolved methane and propane data and (bottom) seismic profile showing the migration of oil and gas from the subsurface following complex pathways that can only rarely be imaged by geophysical data. The bright spots in the profile show the direct relationship between sniffer anomalies and their subsurface sources. can be correlated to their respective source rocks at depth by comparing the compositional information of near-surface gases directly to their underlying sources (Williams et al., 1981; Jones and Drozd, 1983). Some typical reservoir gas analysis data that were instrumental in this comparison were published by Nikonov (1971), who compiled gas data from 3500 different reservoirs in the United States, Europe, and the former Soviet Union and grouped them into useful subpopulations. Gases from basins containing only dry gas were shown to have less than 5% heavy homologs, whereas gases dissolved in oil pools have an average of 12.5–15% heavy homologs. The heavy homologs plotted by Nikonov included ethane, propane, butane, and pentane. Compositional information from near-surface soil gas data sets collected in large gridded surveys of more than 600 sites each over the dry gas Sacramento basin, the gascondensate deposits in the Alberta foothills, and two oil fields (Abo Reef and Sprayberry) in the Permian basin showed that the concept put forth by Nikonov could be used to establish similar relationships between surface seepage data and their respective sources in these areas (Jones and Drozd, 1983; Drozd et al., 1981). Given this encouragement from compositional data, continued data gathering and generation of additional examples further demonstrated the strong structural control on microseep magnitudes. This is shown by examples from the Pineview and Ryckman Creek fields located in the Utah-Wyoming overthrust belt (Jones and Drozd, 1983). In both fields, many of the major faults mapped at the surface verified a direct relationship between the faults and the magnitudes of the anomalies. Additional examples from east Texas and the offshore Green Canyon area in the Gulf of Mexico (Figure 2) demonstrate the deflection and control of seepage pathways by growth faults (Pirkle, 1985). The Green Canyon example was collected in 1983 directly over the Jolliet field before the field was discovered. These examples helped in the reevaluation of basic concepts and proved that the conclusions reached about the association of macroseeps with production (Link, 1952) must also apply to microseeps. The Infantas field in Colombia had hissing gas seeps when first discovered. As the field was produced, the seeps disappeared. Later, during water flood operations, the hissing seeps reappeared upon repressuring, demonstrating a strong correlation with subsurface reservoir pressure. Chekalin and Timofeev (1983) published examples in which reservoir pressure could be directly correlated with seepage magnitudes and seismic activity, further demonstrating the relationship of seep magnitudes to tectonic activity and subsurface pressure. Hunt (1981) stated that over 70% of the reserves in the world are associated with visible macroseeps. According to Link (1952), the mechanisms by which macroseeps migrate to the surface are fractures, joints, fault planes, unconformities, bedding planes, and diffusion through porous beds. In the previously cited examples, diffusion is only one of six migration mechanisms and does not appear to be the main process for migration of hydrocarbons to the surface. MIGRATION MODEL Early surveys conducted in west Texas revealed that larger magnitude CO2 seeps occur with a higher frequency directly over deep-seated fault zones along the west flank of the Puckett field (M. D. Matthews and V. T. Jones, unpublished results, 1976). This observation was made in spite of the fact that the faults do not come to the surface and are covered with more than 4200 m (14,000 ft) of supposedly unfaulted sediments. A migration model pro- Chapter 16—Hydrocarbon Flux Variations in Natural and Anthropogenic Seeps (a) 205 Green Canyon Survey Figure 3—Contour map of gas concentrations (a 70/30% ethane/propane mixture) measured from about 500 permanent monitoring stations at 9 m (30 ft) depth located over a salt dome gas storage reservoir. The mixture has migrated more than 900 m (3000 ft) laterally from the leaking storage well. Legend: light gray = 1000–10,000 ppm, medium gray = 10,000–50,000 ppm, and dark gray = >50,000 ppm. EXAMPLES OF UNDERGROUND STORAGE RESERVOIR LEAKAGE Figure 2—(a) Gas geochemical and fluorescence results for C3/C1 × 1000, propane, and methane analyzed from gravity cores collected in 1983 over the Jolliet field in the Gulf of Mexico. (b) Seismic profile of the Jolliet field. Note the control of seepage pathways by near-surface faults. posed at that time was further demonstrated by a data set collected over the Patrick Draw field as part of the joint industry GEOSAT program (Matthews et al, 1984; Richers et al., 1986). Numerous studies have continued to suggest the validity of this model (Jones et al., in press). An outstanding visual example of this model was demonstrated by macroseeps in Oklahoma when a shallow gas well was overpressured (Preston, 1980). Many of the vents and bubble trains were clearly aligned parallel to fracture orientations, even in unconsolidated river deposits. Attempts to plug the surface vents would be expected to merely divert the migration path to the surface to an alternate fracture system. Many of the initial vents were apparently abandoned by natural plugging, and new vents formed during the later states of activity. Soil gas surveys have been used to detect leaks from a variety of underground storage complexes. One example outlined the leakage area affected by an ethane and propane mixture discharged into the cap rock through a hole in the casing at a depth of ~170 m (~570 ft) from an underground salt dome storage well. Deep relief wells were drilled to determine the vertical extent of the contamination and to relieve the pressure at depth. Finally, a nitrogen flood was conducted in the shallowest 9-m (30ft) aquifer to drive the contamination back toward the source area for final surface clean up. In addition, compositional data from gas chromatographic analysis indicated whether the soil gas was derived from the product well or from a previous spill or pipeline. Leakage from an Underground Salt Dome Topography (Figure 3) defines the surface expression of a Gulf Coast salt; the salt dome storage well that developed the casing leak is also shown, along with a contour map that defines the horizontal extent of the migration, as shown by the isoconcentration contours of the ethane and propane mix measured in 9-m- (30-ft-) deep permanent monitoring stations. The hook-shaped seep located in the northern part of this anomaly consisted mainly of propy- 206 Jones and Burtell (a) 100 SITE P 141 70 SITE 140 & 141 CONVERTED TO N2 INJECTION ON 12/7 80 %E+P % N2, O2 60 NITROGEN 50 40 30 20 10 OXYGEN 13 12 11 10 9 8 7 6 5 (b) % E+P 90 Red-Brown 4 3 2 1 0 20 21 22 23 24 NOVEMBER 25 26 27 28 29 30 1 5 6 DECEMBER Figure 4—A typical response curve illustrating the time required for lateral migration of a nitrogen flood past an ethane-propane charged monitor well. lene and was traced to a earlier reported spill, which was previously thought to have no known near-surface expression. The ethane and propane concentrations shown in Figure 3 were based on the analysis of gases collected from about 500 monitor wells, 9 m (30 ft) deep, installed on 15–30-m (50–100-ft) centers over and adjacent to the affected area. The relationship between topography and high gas concentrations suggests that the location of some of the surface drainages may be related to subsurface control that directs and focuses the migration of the lost gases. Based on the analysis of these near-surface gases, 32 relief wells, some as deep as 150 m (500 ft), were drilled and logged to determine the vertical extent of the contamination. Combining recent drilling data with all available well logs from the older production wells indicated that the gas had charged a sandstone 60 m (200 ft) below the surface. Two deeper and one shallower sandstone that had not been charged were also found above the cap rock. Several high-magnitude gas anomalies lay close to old, possibly uncased wells, suggesting that the main avenue from the cap rock to the sandstone at 200 ft depth was along the uncased wells. Clean-up was facilitated by turning the 30-ft-deep geochemical monitor wells into eductor sites by installing a venturi tube on the top of the well casing. Nitrogen was run through the venturi tube to produce a small vacuum on the hole and then injected into the ground through a monitor well located near the edge of the plume. The nitrogen migrated through the ground toward the eductor sites, where it escaped to the atmosphere. A typical response curve showing the advance of the nitrogen front and clean-up of the ethane and propane product mix are shown in Figure 4. The rate of clean-up response over the entire area illustrated lateral variations in lithology and permeability. Although visual inspection of the sands within the 30-ft aquifer did not exhibit any obvious stratigraphic differ- Figure 5—(a) Propane concentrations (from samples taken at 3 m, or 10 ft, depth) and (b) a corresponding lithologic cross section demonstrating soil color alteration caused by propane seepage over a mined underground storage cavern. Horizontal scale is 1 inch = 150 ft (1 cm = 18 m). ences, the variability in N2 flow clearly demonstrated a significant influence related to lateral variability. In some cases, the lateral permeability was so low that 15 lbs of nitrogen pressure at 9 m (30 ft) were not sufficient to push the gas 15 m (50 ft) laterally to the next soil gas station, even though water poured on the ground near the injection site would froth and bubble from the nitrogen escaping vertically through 30 ft of clay. Most of the recalcitrant hot spots also appeared to have a better association with deeper vertical migration pathways that charged the 30 ft sandstone rather than from lateral migration. Following clean-up operations, an advance warning system was installed to detect leaks from storage wells before they could become a problem, and two permanent monitoring stations were installed at the casing of every storage well. This type of monitoring system has been in operation since 1980 and, if properly sampled on a regular basis, allows early detection of well leakage before it appears at the adjacent boundary of the property. Application of such a system is beneficial for preventive maintenance and is highly recommended. Leakage from a Propane Storage Cavern Another excellent example of gas leakage from an underground storage reservoir is provided by a study conducted over a 60-m- (200-ft-) deep propane storage cavern. The immediate objective was to determine the leakage rates and to test whether ongoing remedial efforts to repair the leaks were successful. This particular case also provided an opportunity to determine the product leakage distribution and to conduct pressure pulse tests by injecting helium into the cavern as a tracer. Although this was a small facility, 455 geochemical measuring stations on 3-m (10-ft) centers were installed to depths of 6 m (20 ft) and lined with PVC pipe. Figure 5 shows the propane collected over the cavern plotted on a Chapter 16—Hydrocarbon Flux Variations in Natural and Anthropogenic Seeps 1.4" RAIN 2" RAIN 2.3" RAIN 5" RAIN 207 4.8" RAIN 1" RAIN 29.7 METR BARO PERCENT PROPANE 29.8 29.6 IC PR 29.5 ESSU 29.4 RE 29.3 29.2 29.1 MAY 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Figure 6—Graph of propane percentages by volume (two dashed lines) with respect to rainfall showing barometric pumping of propane flux under a ground sheet. Rainfall (vertical bars) expelled a significant amount of propane from under the ground sheets when first installed. Solid line shows barometric pressure. log scale. In addition to the propane anomalies detected, the soil changed color from red-brown to green-black over the top of the cavern, coincident with the largest gas anomalies. The color changes appear to be related to hydrocarbon seepage and confirm areas where the gas leakage has occurred over a long time period above this cavern. Relationship Between Barometric Pressure and Gas Flux An underground storage cavern is also a good place to observe gas flux related to atmospheric phenomena. Plastic ground sheets about 5 ft square were installed directly over known leaks to measure propane gas flux related to meteorologic and barometric changes. Rainfall (shown as vertical bars in Figure 6) produced a significant change when the ground sheets were first installed. The 2 in. of rain that occurred on the evening of May 9 caused the sheets to balloon up from soil gas being forced up under the sheets by rainwater infiltration adjacent to and around the edges of the sheets. The rain probably displaced the gas in the ground and caused it to come up underneath the ground sheet. Since this large gas flux did not occur again after the next rainfall events, it was assumed that this initial gas flux was caused by buildup of longer term gas leakage that was trapped directly under the sheet and forced out by the first influx of rainwater. Except for perhaps the 1-in. event on May 24, subsequent rain events did not again show such a dramatic change once equilibrium was established. Continued barometric monitoring (shown by the solid line on Figure 6) produced several small barometric changes on May 19 through 22. These barometric lows had clearly expressed positive gas fluxes (shown by the dashed lines). Every time the barometer took a dip, some gas flux popped up under the ground sheet. Gas flux was measured as the percentage of propane measured under the sheet. Data from two separate sheets are shown in Figure 6 for comparison. Migration Timing Related to Pressure Pulse Additional opportunities to observe pressure-related gas fluxes were encountered during this remedial sampling conducted over the mined propane cavern. The cavern pressure was decreased to ambient levels for repairs and then repressured to 80 psi, allowing the recharge leakage rate from the cavern to be measured. Upon recharging the cavern, we determined the time it took for the propane gas to reestablish its previously measured maximum leakage values at the surface. Following the recharge of the cavern with propane to its original pressure, a value of over 90% of the original soil gas propane concentration was observed in the observation test hole within 15 days. As shown by Figure 7a, most of this leakage appeared to come from around the central shaft. However, because the storage site had been known to leak for over 20 years, there was a large propane background in the soil that made it impossible to determine whether the product reappearing at the surface came directly out of the reservoir during the sampling period. Injection of Helium Tracer into Cavern As a second, more definitive test, helium was injected into the cavern to create a concentration in the cavern of about 600 ppm. Results showed that in 15 days, not only had the propane moved to the surface, but as shown in Figure 7b, the helium concentration detected at the surface was over 75% of that in the cavern. Helium injection not only showed the leakage around the central cavern, % of Maximum Propane 208 Jones and Burtell 90 70 (a) 0 PSI 15-80 PSI 80 PSI 50 30 10 0 -5 0 5 10 15 20 No. of Days After Pressuring NO OF DAYS UNTIL PROPANEEQUILIBRIUM % of Maximum Helium 1 2-4 90 0 PSI 5-8 10-18 >19 (b) 70 PSI 70 50 30 10 0 0 5 10 No. of Days After Pressuring 15 20 NO. OF DAYS UNTIL HELIUMEQUILIBRIUM 1 5-8 >15 Figure 7—(a) Graph and map showing oil gas propane recharge rates measured in permanent stations in direct response to a subsurface pressure pulse in a propane storage cavern and (b) in a propane storage cavern spiked with 600 ppm helium. The outline of the storage cavern is shown as a series of tunnels in the maps. but also found a leak at the end of one of the drifts that would have been missed looking only at propane. The amount of helium used for this test was not enough to damage the product for sale and yet still gave more than adequate sample for analysis. These two examples suggested that migration was rapid and controlled by faults, fractures, and other permeable migration pathways and not by diffusion. A similar case involving an underground coal gasification reactor is discussed next. Leakage from an Underground Coal Gasification Reactor Another educational example of gas leakage was taken from an underground coal gasification reactor near the Rock Springs uplift near Rawlins, Wyoming (Jones and Thune, 1982; Jones, 1983). The North Knobs underground coal gasification (UCG) facility is about 8 mi west of Rawlins, Wyoming. It is situated on the southwestern flank of the asymmetric Rawlins uplift adjacent to the Washakie Basin. Throughout the area, the exposed resis- tant sandstones have well-developed rectilinear joint sets striking N 14˚ W and N 49˚ W. The nearly vertical beds dip too steeply to do anything except gasify the coal inplace. The gasification reactor is about 180 m (600 ft) below the surface. A total of 122 permanent monitoring wells, 5.4 m (18 ft) deep, were installed over the general area of the retort to facilitate measurement of the soil gas concentrations. One objective of this soil gas survey was to determine if gases generated during the burn leaked into the near surface. We also wanted to know the rates of leakage, migration paths, and composition of the gases in order to evaluate the economic importance of such leakage and to assess their hazard potential to human safety and the environment. All geochemical monitoring wells (here referred to as sample sites) were drilled with a 3-in. diameter auger to a nominal depth of 18 ft and established as permanent observation sites. This was accomplished by installing a 20-ft length of 1 in. ID PVC pipe, perforated with about 30 1/4-in.-diameter holes in the lower 2 ft of the pipe. During installation, sufficient pea gravel was installed to provide a permeable zone for collection of soil gases leaking from the adjacent formations. Plots of methane and propane magnitudes with time are shown in Figures 8 and 9, respectively, for several sample sites selected to represent the typical changes noted in the response time of gas migration. Note that sites 22 and 27 exhibit an almost identical quick response to the retort pressuring even though they are about 15 m (50 ft) apart. Site pairs 1 and 2, 5 and 6, and 13 and 20 also show similar responses within close pairs, which are several hundred feet apart. The leakage patterns that emerge over time are clearly not random, but are systematically changing in relation to subsurface controls. It took about 3–5 days after the beginning of the system air pressure test and ignition of the coal before any significant increases in the magnitude of the hydrocarbon gases were recognized in the near surface. Propane leakage was related to migration from the retort of the initial products in ignition because propane was used to achieve ignition and was not a major retort gas generated during the burn. Figure 9 illustrates this because site 22 was directly updip from the retort and clearly showed a sharp rise in propane upon pressuring and a fairly rapid decrease during the initial phases of the burn. The maximum pressure in the retort at 180 m (600 ft) was 700 psi. This excess pressure, used to link the slant and vertical wells, caused a rapid change in the surface signature that occurred within 2 days. An examination of the change in methane flux from various sites suggested that the soil gas vapor data could initially be divided into at least four discrete time periods for mapping, as shown in Figures 10 through 14. These selected periods are defined by vertical lines in Figures 8 and 9 and are as follows: (1) Figure 10, preburn, prepressure, July 22 to August 16, 1981; (2) Figure 11, pressure, August 17 to 23, 1981; (3) Figure 12, burn, August 24 to November 10, 1981; and (4) Figure 13, postburn, Nov- Chapter 16—Hydrocarbon Flux Variations in Natural and Anthropogenic Seeps 1000000 Figure 8—Methane concentrations measured at four permanent stations installed over a subsurface coal gasification reactor plotted versus Julian days of the year. Shaded panels represent changes in operational activities: gray on left = preburn/prepressure; white = pressure; dotted = burn; gray on right = postburn. 10000.0 1000.0 SITE 22 SITE 27 100.0 SITE 5 SITE 6 10.0 10000 1.0 1000 202 232 262 292 322 352 10000 232 262 292 322 352 365 352 365 10000 1000 SITE 13 SITE 20 100 SITE 1 SITE 2 1000 1000 10 100 202 100 232 262 292 322 352 365 1 202 JULIAN DAYS ember 11 to December 12, 1981 (end of field survey). A final measurement was made 6 months later, from July 21 to 30, 1982, essentially 1 year after the initial measurements were made. Figure 14 shows this final data set. By selectively averaging the gas flux measurements from each of the five time windows for each site, we were able to construct a series of contour maps illustrating the most significant changes that occurred over the five time periods. As shown in detail in Figures 10–14, the leakage patterns changed with time in direct response to pressure variations in the subsurface retort (Jones and Thune, 1982; Jones, 1983). Evaluation of these contour maps allowed us to make estimates of how long gas remains in the surface sediments and on what magnitude of fluxes might occur under various pressure conditions. For example, Figures 13 and 14 were both measured after the retort had been depressured and filled with water. Within 6 months, the leakage directly updip at the outcrop had decreased an order of magnitude from about 10,000 to 1000 ppm. It was suggested to the Department of Energy that this site should be maintained for future research on soil gas analysis because we could conduct various depth probe measurements to check the influence of joints and soil types on both vertical and lateral gas migration and further investigate dissipation of the retort leakage gases with time. This site would also be particularly useful for a research study because of the uniqueness of the gases generated, including carbon dioxide, carbon monoxide, hydrogen, and methane. Because these gases are unique to the coal gasification process, they could have come only from the subsurface retort. The maximum concentration of gas in the bedding plane at the outcrop was about 50,000 to 100,000 ppm (5–10%) at the peak generation of the retort. As shown by Figures 13 and 14, this falls off as the reactor pressure was reduced and finally filled with water at the conclusion of operations. The raw data 232 262 292 322 JULIAN DAYS 100.00 PROPANE (PPM) METHANE (PPM) 0.1 202 365 10.00 SITE 22 SITE 27 1.00 0.10 0.01 202 1000 PROPANE (PPM) METHANE (PPM) 100000 209 232 262 292 322 352 365 352 365 SITE 1 SITE 2 100 10 202 232 262 292 322 JULIAN DAYS Figure 9—Propane concentrations measured at two permanent stations installed over a subsurface coal gasification reactor plotted versus Julian days of the year. Shaded panels represent changes in operational activities: gray on left = preburn, prepressure; white = pressure; dotted = burn; gray on right = postburn. produced adequate flux information for modeling, thus providing an excellent resource for further study (Department of Energy, 1982). Maximum gas values were observed in monitor wells 22 and 27, directly updip from the outcrop of the coals 210 Jones and Burtell Figure 10—Color contour map of methane concentrations measured at permanent stations installed over an underground coal bed retort. Data were averaged over preburn, prepressure time window, July 22 to August 16, 1981. Figure 11—Color contour map of methane concentrations measured at permanent stations installed over an underground coal bed retort. Data were averaged over pressure time window, August 17 to 23, 1981. Chapter 16—Hydrocarbon Flux Variations in Natural and Anthropogenic Seeps 211 Figure 12—Color contour map of methane concentrations measured at permanent stations installed over an underground coal bed retort. Data were averaged over burn (burn A) time window, August 24 to November 10, 1981. Figure 13—Color contour map of methane concentrations measured at permanent stations installed over an underground coal bed retort. Data were averaged over postburn (burn B) time window, November 11 to December 12, 1981. 212 Jones and Burtell Figure 14—Color contour map of methane concentrations measured at permanent stations installed over an underground coal bed retort. Data were collected on July 21, 1982, about 6 months (200 days) after shutdown of subsurface reactor. being gasified. This maximum value occurred in the sandstone directly over the coal with 100,000 ppm methane showing up at the surface. Figure 8 illustrates the location and change in shape of the methane magnitudes observed at the surface at four pairs of sites over the time windows selected for mapping. In contrast to sites 22 and 27, sites 85 and 86 had about 1200 ppm natural levels of hydrocarbon that had nothing to do with the active retort. They appeared to represent a natural seep that had existed previously in the area. The residual gases were pumped out of these two sites by the sampling process and apparently did not further recharge during the retort operation. The seep located to the northeast at sites 1 and 2 and along the baseline provided another anomaly that was not influenced by the gasification process. These latter two sites were charged by a previous UCG reactor (Department of Energy, 1982) that lies directly downdip from these sites. Both the vertical and slant hole product wells for both the 1979 and 1981 retorts are plotted on Figures 10–14 for reference. Injection of Helium Tracer into UCG Reactor Helium was occasionally injected during the stable part of the burn in order to estimate the transit time for gases passing through the reactor. This helium appeared in the surface seep gases at sites 22 and 27 within 2 days of its injection. It disappeared just as quickly. Hydrocarbon Spots Three major methane anomalies were observed along the strike of the bedding plane (Figures 13–17). These indicated that the leakage gases did not just migrate updip along the bedding plane and then laterally along the strike of the beds to fill the surface sediments with gas. Instead, the leakage gases came up almost simultaneously within three localized areas, or “hydrocarbon spots.” This concept suggests a migration pattern with gases moving along a complex mixture of bedding planes and fracture avenues at depth. The location of the hydrocarbon spots at the surface are then controlled by these complex pathways and are thus somewhat predictable from geologic and geochemical mapping. Once these hydrocarbon spots are determined by geochemical sampling at a site, they provide the pathways for all future pressure relief from depth and can be used to establish a permanent monitoring system. In this example, the vertical migration zones were apparent and formed the main hydrocarbon spots during both the charging and discharging periods, as the surface seepage gases depleted over time. Chapter 16—Hydrocarbon Flux Variations in Natural and Anthropogenic Seeps 213 Table 1—Gas Concentrations in Near-Surface Rocks Before (Above Line) and After (Below Line) Earthquake Activity at Mukhto Oil Field, Sakhalin Island a Date Strength of Shock Distance from Epicenter to Deposit (km) Well No. Time of Sample (days) CH4 (10-4 vol. %) HC (10-4 vol. %) H2 (vol. %) CH4 (% of HC fraction of gas) 91 X 1974 K=9 100 8 6 3 135.4 283.7 1.9 4.2 0 0 98.56 98.50 81 V 1975 M = 4b 12 8 2 4 73.6 213.5 0.81 2.34 0.53 1.22 98.35 98.96 24 V 1975 K = 6.2 25 8 2 1 188.5 525.0 3.18 2.52 3.10 7.10 98.50 99.50 4 VI 1975 K = 7.2 9 8 1 2 152.0 852.0 7.36 8.85 17.80 15.70 98.50 98.97 8 VI 1975 K = 7.5 25 11 5 2 935000.0 954000.0 11908.7 12465.0 0.09 0.26 98.70 98.70 5 X 1975 K = 9.5 100 8 6 1 58.8 369.0 3.8 5.8 26.8 33.6 98.90 98.60 100 61 5 1 256000.0 273000.0 1273.0 1399.0 3.6 4.2 99.54 99.54 aFrom Zorkin et al. (1977). bIntensity of 2 to 3. RELATIONSHIP OF DEEP MOBILE GASES TO EARTHQUAKES Large volumes of diverse gases continually escape from the earth’s crust into the atmosphere (Jones et al., in press). Areas of especially high activity appear to be related to zones of deep tectonic fracturing and the accompanying jointing in which mineralization is sometimes found. Typical gases derived from depth 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, carbon, oxygen and uranium have considerable potential for helping to define the sources of these gases. The magnitudes of deep gas anomalies are strongly governed by tectonic and magmatic activity, thus stronger patterns are encountered in seismically active areas of late orogenic activity. Accordingly, weaker patterns are observed in platform and shield areas (consolidated blocks of the crust) that are relatively quiescent. Numerous published examples of gas flux related to earthquakes have been reported, including Kartsev et al. (1959), Fursov et al. (1968), Elinson et al. (1970), Sokolov (1971), Eremeev et al. (1972), Ovchinnikov et al. (1972), Zorkin et al. (1977), Wakita (1978, 1980), Melvin et al. (1978, 1981), Barsukov (1979), Borodzich (1979), Mamyrin (1979), King (1980), Reimer (1980), Shapiro et al. (1981, 1982), Mooney (1982), and Pirkle and Jones (1983). That earthquakes are possibly preceded or accompanied by the escape of deep mobile gases was apparently first observed in the former Soviet Union in 1966. In a study of the Tashkent earthquake zone, Fursov et al. (1968) found air aspirating from boreholes over faults contained as much as 15 times more mercury than air not located over fault zones. This work points out that faults can be the channel ways through which mercury vapor migrates, but it also indicates that tectonic activity can release mercury not necessarily related to economic mineral deposits. Studies of this type were also undertaken in China at about the same time and in Japan in 1973. In other areas, the Soviets showed that soil gas values increase dramatically at faults shortly after earthquakes in which fault movement was involved (Zorkin et al., 1977). An extensive study involving 105 observation wells 3–5 m (~9–15 ft) deep was done over the Mukhto oil field in northeastern Sakhalin Island. A total of 3700 samples were collected and analyzed over a 4-month period, with the most active wells sampled daily. The range of hydrocarbon seepage gases varied from 0.2 to 271,000 ppm (27.1%) for methane and from 0.3 to 13,000 ppm (1.3%) for methane homologs. Hydrogen and carbon dioxide ranged as high as 90% and 30%, respectively. The largest anomalies occurred on thrust faults, and the concentrations increased in direct response to seismic shocks. As shown by the data in Table 1, the composition of the gases changed toward a gassier (higher methane relative to heavier hydrocarbons) signature immediately after an earthquake. Relative magnitude changes were greatest in anomalous wells, whereas background areas showed little or no change. Table 1 (from Zorkin et al., 1977) provides impressive evidence for the tectonic relationship of this leakage gas flux. His study also left no doubt that faults and fractures are the main control on the effusion of gases from the subsurface. Chemical monitoring of earthquake activity has not been widely practiced in the United States, where most efforts were geophysical until about 1975, when limited studies were initiated using radon. Gulf Research also 214 Jones and Burtell 100 (a) 0 1980.5 1981.0 1981.5 1982.0 1982.5 Pacoima Hydrogen Data in ppm 100 (b) along the San Andreas fault in the Cholame Valley, California (Jones and Drozd, 1983). These data confirmed helium as a deep basement or tectonic indicator that is commonly independent of oil and gas deposits. At Cholame, where these data were gathered, the fault moved in 1857, 1906, and 1922 and most recently in 1966 (Iacopi, 1976). At the time these measurements were made in 1975, it was reported by the foreman of the Hurst Ranch that nearby doors and gates changed their overnight fit on a daily basis, indicating that the San Andreas fault remained active in this area at the time of the survey. Earthquake Predictions Based on Gas Flux 0 1980.5 1981.0 1981.5 1982.0 1982.5 Figure 15— (a) Helium and (b) hydrogen concentrations (in ppm) sparged from monitor well at Pacioma Dam gas geochemical earthquake monitoring station. The concentrations are plotted versus time (three measurements per day) from June 1980 to June 1982. Note the hydrogen anomaly of about 75 ppm that occurred just before a 5.6 magnitude earthquake hit Westmoreland, California. conducted their first measurements of helium and hydrogen in 1975 (Jones and Drozd, 1983). Helium Spots A landmark publication in Science by Wakita (1978) has shown that helium was observed in anomalous quantities along faults. In 1978, helium concentration was observed to be as high as 350 ppm in a nitrogen vent on the Matsushiro fault swarm. Wakita proposed that these anomalous areas be called “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 × 50 m. Extensive experience in soil gas prospecting indicates that soil gas anomalies generally occur in irregularly shaped and spaced spots. This is because the migration of gases are dominated by faults and fractures, on either a macro- or microscale. A second paper by Wakita (1980) reported 70 measurements for hydrogen in the Yamasaki fault zone. These measurements, made in 0.5–1-m-deep holes, gave hydrogen anomalies ranging from 2 to 30,000 ppm in the fault zone, with ambient background values of 0.5 ppm measured outside the influence of the fault. Wakita postulated that hydrogen was formed by the reaction between groundwater and fresh rock surfaces created by fault movement. Limited programs using radon as an earthquake-sensitive gas began in 1975 at about the same time that Gulf Research and Development Company first made measurements of light hydrocarbons, helium, and hydrogen After studying both microseeps and anthropogenic macroseeps, it became apparent that the next step was to acquire gas flux data from an active fault, such as the San Andreas. The level of seismic activity associated with this geologic feature makes it an obvious choice. To accomplish our objective, Gulf Research approved a corporate level research project called “Gas Flux Related to Earth Motions.” Evaluation of the known earthquake prediction programs at the U.S. Geological Survey and various universities revealed that the Kellogg Radiation Laboratory at California Institute of Technology in Pasadena had the only computerized system that could provide automatic data collection of a series of geochemical variables. A network of automated radon-thoron monitors operated by a microcomputer collected data every 8 hr and stored it on-site in the computer memory and then transmitted the data back to a central laboratory over regular telephone lines on command from a remote computer (Shapiro et al., 1981). Since Gulf’s research objective was to map short-term flux changes, the computer link was essential. Operating stations were located at Fort Tejon, Lake Hughes, Pasadena, Santa Anita, Stone Canyon Reservoir, Big Dalton Canyon north of Glendora, Lyle Creek, Sky Forest in the San Bernardino Mountains, and Pacoima Dam. Pacoima Dam was chosen for a site because Caltech scientists thought microseismic activity might be generated by mass loading and unloading within this steep and fractured valley. Initial measurements at Pacoima Dam indicated that hydrocarbon concentrations were low, so a helium-hydrogen gas chromatograph (GC) was set up at this station. A computer and GC automatically sampled the dissolved gases sparged from the well three times a day. The data were archived on tape and transmitted daily by modem to the VAX computer at CalTech and ultimately plotted on a time axis, as shown in Figure 15. A strong hydrogen peak of 75 ppm was measured in April 1981, just before a 5.6-magnitude earthquake hit Westmoreland, California. This hydrogen anomaly lasted nearly 3 weeks and peaked sharply at about 75 ppm. Whether a coincidence or not, this classic response provided considerable encouragement to the joint Gulf Research–CalTech program. Chapter 16—Hydrocarbon Flux Variations in Natural and Anthropogenic Seeps Previous experience in using carbon dioxide to map soil gas anomalies encouraged Gulf to introduce instruments for measuring carbon dioxide at several established CalTech stations. Within a short time, results from the Lake Hughes station showed the presence of correlated radon and carbon dioxide anomalies. It appeared that carbon dioxide reached saturation levels in the water and then served as a carrier for the radon (Shapiro et al., 1982). Although not an earth-shaking result (no pun intended), each improvement in measuring and relating the natural gases emanating from these stations increased the possibility of producing interpretable data. ARROWHEAD HOT SPRINGS In 1975, Scripps Institute of Oceanography began earth gas monitoring studies for possible fluid phase precursors to earthquakes with sampling at Arrowhead Hot Springs. Grab samples of spring gases were collected at 1month intervals at the concreted hot spring and analyzed for dissolved radon, helium, and nitrogen, along with temperature and conductivity. Beginning in 1977, methane was also measured in each sample. Results of these compiled data reflected a variety of short-term variations in measured gas content for comparison with seismic events along the San Andreas fault in southern California. The most significant correlation identified was a large increase in measured gases (radon, helium, nitrogen, and methane) in 1979 before the Big Bear earthquake of magnitude 4.8 (Craig et al., 1980). This significant increase was interpreted as the result of an increase in the deep gas component dissolving into hot springs waters. The success of the Scripps’ grab sampling program suggested that this location would provide even more valuable data for earthquake prediction studies with on-site computer-controlled continuous monitoring of gases. Preliminary gas monitoring at Arrowhead Hot Springs began in early 1981 by collecting gas bubbles with a funnel and gas cylinder. Samples were analyzed by GC for methane, ethane, propane, i-butane, n-butane, ethylene, propylene, helium, and hydrogen. The initial results shown in Table 2 indicated that the hot springs gases contained 3918 ppm methane, 17.5 ppm ethane, and 1780 ppm helium. The overall high magnitude of measured gases observed with continued sampling suggested that the location was ideal for continuous gas emission monitoring and inclusion in the Gulf Research–CalTech earth gas research programs. A variety of sample collection and analysis methods were used at Arrowhead Hot Springs, including a cross-check analysis by J. Whelan of Scripps on a sample collected in May 1982. The Scripps data showed 6900 ppm (0.690%) methane, 96.68% nitrogen, 1.50% argon, 1.12% oxygen, and a trace of hydrogen. Methane homologs were not analyzed. On the basis of these early results, it was clear that Arrowhead was the type of active seepage site Gulf scientists wanted to sample and to include in the CalTech earthquake prediction program. 215 Geology of Arrowhead Hot Springs Arrowhead Hot Springs is located at the base of the San Bernardino Mountains in the Transverse Range Province of southern California (Hadley and Kanamori, 1977; Miller, 1979). The Transverse Ranges are a unique east-west structural and geomorphic belt that crosses the San Andreas fault. The province is bound by the Coast Ranges to the north, the Peninsular Ranges to the south, and the Mojave Desert to the east and northeast. Despite apparent strike-slip movement along the San Andreas fault, the Transverse Ranges seem to be continuous across the bend of the fault in southern California. The San Bernardino Mountains are located northeast of and are bound by the San Andreas fault zone. Hot springs are located in two canyons in the foothills of the San Bernardino Mountains on a splay of the San Andreas fault. The splay seems to be related to the bifurcation of the fault into northern and southern segments that continue southeast toward the Salton Trough. It is apparent that spreading and subsidence in the trough is moving northward along the San Andreas fault. Basement rocks of the San Bernardino Mountains, which were uplifted in late Quaternary time, can be divided into two structural blocks separated by the north branch of the San Andreas. Outcrops in the vicinity of the field area range from Precambrian granite to Paleozoic metasedimentary limestones and schists and Mesozoic intrusive rocks. This suite of formations reflects the complex history of the San Bernardino structural block. The Precambrian formations seem to be related to similar units in both the San Gabriel Mountains and the Mojave Desert region. The San Andreas fault cuts through the base of the San Bernardino Mountains as two distinct branches that continue to the southeast and a third branch that is truncated in the vicinity of Arrowhead Hot Springs. The San Andreas in this area shows extensive vertical displacement that has been active through recent times, as can be seen from terraces on alluvial fans in the two canyons of our study. Fault splays that cross the area do not show obvious recent movement. The faults crossing the field area were identified on the surface and from low-level areal photographs where not covered by recent alluvial sequences in the two canyons. The faults are most easily identified by linear drainage and contacts of metamorphosed Paleozoic carbonates with Precambrian gneiss formations. Small amounts of vertical displacement of ~0.3–1.2 m (1–4 ft) can be seen in the outcrop that continues until covered by alluvium. The three mapped faults are inferred for a short distance past the hot springs area and are not easily identified in the rough topography of this area. The hot springs in the field area are divided into two distinct groups and are located in two canyons about 0.5 mi apart on the Campus Crusade for Christ International property. The main group of springs to the east are located in and around Penyugal Canyon and can be divided into three groups by location as east of, 216 Jones and Burtell Table 2—Summary Results of Gas Concentrations ( in ppm) Measured at Arrowhead Hot Springs from December 1981 to December 1982a Obs. No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 Date C1 C2 C3 i-C4 n-C4 He 12/08/81 12/19/81 12/27/81 01/03/82 01/09/82 01/17/82 01/22/82 01/31/82 02/06/82 02/15/82 02/21/82 02/28/82 04/20/82 04/04/82 04/16/82 05/11/82 05/21/82 05/28/82 06/06/82 06/11/82 06/20/82 06/28/82 07/06/82 07/12/82 07/18/82 07/27/82 08/01/82 08/02/82 08/03/82 08/11/82 08/12/82 08/13/82 08/16/82 08/17/82 08/19/82 08/27/82 09/11/82 10/19/82 10/20/82 10/21/82 10/22/82 10/25/82 10/26/82 10/28/82 10/29/82 11/02/82 11/03/82 11/04/82 11/05/82 11/08/82 11/09/82 11/10/82 11/11/82 11/12/82 11/15/82 11/17/82 11/18/82 11/19/82 11/22/82 11/24/82 11/29/82 12/01/82 3918.08 3800.36 4374.00 4469.61 4313.75 4506.54 4721.68 4463.72 4495.50 4264.65 4721.63 4965.41 5016.73 5132.20 6030.57 5507.68 5240.56 2358.00 5431.39 5313.22 5299.27 5263.21 4959.05 5197.85 5210.53 5834.13 5767.66 5795.78 5827.74 5783.00 6193.13 6052.19 5958.46 5915.84 5976.75 5949.13 5765.50 5701.33 5587.96 5203.66 5527.42 5455.69 5642.88 5452.60 5478.15 5615.54 5417.59 5431.95 5396.20 5423.18 5628.28 5615.49 5544.38 5539.42 5431.11 5450.10 5760.84 5736.69 5789.43 5786.98 5798.82 5785.36 17.500 17.410 19.458 19.663 20.238 18.923 23.275 22.263 21.250 20.188 21.679 22.022 22.250 22.136 25.190 27.634 27.509 28.101 28.457 28.084 27.970 27.530 25.783 27.137 27.932 30.055 27.745 30.111 30.499 30.330 31.350 30.938 30.398 30.498 29.598 29.499 28.377 28.297 25.446 27.984 29.164 28.688 30.053 29.200 28.497 28.820 28.589 28.556 28.537 28.710 29.718 29.689 29.434 29.379 28.727 29.952 31.103 29.443 30.601 30.173 29.705 29.338 2.697 2.875 3.201 3.185 3.300 3.135 4.124 3.822 3.888 3.726 3.560 3.632 3.768 3.789 4.248 4.525 4.459 4.468 4.595 4.534 4.407 4.570 4.324 4.425 4.515 4.646 4.639 4.744 4.828 4.819 4.663 4.653 4.660 4.677 4.969 4.614 4.200 4.196 4.273 4.179 4.375 4.348 4.691 4.298 4.338 4.375 4.281 4.362 4.197 4.303 4.308 4.232 4.276 4.252 4.209 4.210 4.389 4.370 4.346 4.365 4.271 4.355 0.420 0.468 0.546 0.507 0.533 0.518 0.659 0.643 0.667 0.627 0.575 0.518 0.604 0.575 0.719 0.722 0.733 0.642 0.770 0.752 0.645 0.702 0.622 0.688 0.696 0.666 0.630 0.663 0.684 0.669 0.681 0.687 0.683 0.659 0.749 0.743 0.708 0.713 0.702 0.752 0.569 0.724 0.684 0.197 0.705 0.706 0.736 0.744 0.721 0.690 0.698 0.662 0.677 0.689 0.675 0.668 0.710 0.691 0.702 0.697 0.708 0.677 0.785 0.912 1.001 1.887 1.970 1.983 1.472 1.732 1.451 1.319 1.453 1.259 1.404 1.404 1.624 1.618 1.635 1.380 1.678 1.705 1.398 1.589 1.335 1.517 1.493 1.562 1.504 1.574 1.621 1.590 1.559 1.552 1.572 1.504 1.617 1.637 1.547 1.586 1.598 1.526 1.628 1.542 1.505 1.533 1.517 1.558 1.588 1.613 1.591 1.524 1.523 1.497 1.518 1.530 1.552 1.504 1.526 1.514 1.531 1.540 1.564 1.462 1784 1772 2022 2082 1978 1893 2161 2069 2049 1970 1811 1849 2015 2008 2212 2228 1745 1760 1783 1853 1912 1883 1835 1931 1924 1874 1857 1791 1794 1793 1789 1739 1739 1706 1827 1806 1858 1994 2067 2148 2110 2356 2066 2117 2274 2102 2102 2204 2239 2044 2077 2219 2150 2082 2124 2156 2188 2292 2180 2139 2180 2036 aFrom Burtell (1989). Observations were selected from a much larger data base in order to show the longer term variations observed over a year. long page H2 C3/C1 46 236 311 0 120 16 1253 60 92 0 4426 5701 0 61 0 127 66 74 177 154 58 60 54 61 93 60 82 92 96 97 100 117 65 66 70 63 76 89 89 96 89 82 84 83 89 66 113 85 83 92 78 88 84 93 99 84 89 80 108 101 90 76 0.69 0.76 0.73 0.71 0.76 0.70 0.87 0.86 0.86 0.87 0.75 0.73 0.75 0.74 0.70 0.82 0.85 1.89 0.85 0.85 0.83 0.87 0.87 0.85 0.87 0.80 0.80 0.82 0.83 0.83 0.75 0.77 0.78 0.79 0.79 0.79 0.78 0.77 0.74 0.77 0.80 0.79 0.80 0.83 0.79 0.79 0.78 0.79 0.80 0.78 0.79 0.76 0.75 0.77 0.77 0.77 0.76 0.76 0.75 0.75 0.73 0.75 Chapter 16—Hydrocarbon Flux Variations in Natural and Anthropogenic Seeps west of, and in the canyon. The area has been developed as a bathing spa and recreational area since the late 1800s, and most of the original hot springs have been altered from their original character and location by the construction of baths and collection pools. In the case of the springs to the west in Waterman Canyon, four caves were dug into the side of the alluvial fill covering the fractured bedrock, from which steam and hot water flow. The caves are presently bulldozed over to keep out trespassers and still show signs of warm ground and surface steam. The best descriptions of the area, before substantial development obliterated many surface geologic features associated with the springs, are provided by an assessment of the geothermal potential of hot spring areas adjacent to Southern Sierra Power Company transmission lines (Southern Sierra Power Company, 1925). The temperatures of the steam caves were described as being dependent on their locations with respect to the fault, which lies directly to the east. The temperatures were observed to decrease regularly as the distance from the fault increased. Geochemical Sampling and Analysis Field sample collection progressed in stages from simply filling an evacuated 1-L sample cylinder through an inverted funnel, to collecting and analyzing the gases continuously. To aid in this process, a Plexiglas sample collection bucket was installed above a stainless steel collection pan placed at the bottom of the spring. Samples collected from the collection bucket had a natural flow rate of 40–50 mL/min of free gas bubbles and provided an integrated sample over time. The water flows at a rate of ~53 L/min (14 gals/min) at a temperature of 87.8˚C. Gas chromatographic analysis of gas samples were completed using a dual GC with two columns and two detectors designed by Gulf Research. Light methane through butane hydrocarbons were analyzed using a 3ft alumina column and a Gulf-designed flame ionization detector. Helium and hydrogen were analyzed using an 11-ft mol sieve 5A column coupled to a thermal conductivity detector. Both columns were set up with a timer controlled backflush that prevented hydrocarbons heavier than butane from entering the alumina column and any components heavier than helium from entering the hydrogen-helium mol sieve column. This feature reduced contamination of the GC and increased the sensitivity and reliability of the C1-C4/He-H2 analysis. Samples could be analyzed either by flow-through or hand-injecting methods. Continuous Gas Monitoring As a preview to continuous monitoring, an on-site laboratory trailer was parked at the concreted hot spring and the monitoring instruments were run continuously during daytime hours. The continuous analyses, at 6-min intervals, provided a large body of data available for 217 interpreting short-term fluctuations in the gas emission magnitude and compositions with time. Real-time sample analysis was completed on March 16, April 22, and October 19 to December 2, 1982. These numerous measurements, along with multiple analyses from single days, have been compiled into graphs for evaluation (Burtell, 1989). To conduct these continuous monitoring tests, the Gulf Research–CalTech team had to design and build the first computer-controlled GC system. The system was controlled by an LSI 1123+ Digital computer with TU-58 tape drives for remote nonvolatile storage of data (Melvin et al., 1981). Methane As shown by the initial grab samples (Table 2) collected in 1-L cylinders from December 1981 to January 1982, there was a steady rise in methane from 3918 to 4507 ppm during the first month. More continuous measurements, which overlap these data, continued to show a steady but variable increase in methane content in the hot spring gases, ranging as high as 6000 ppm. As shown in Figure 16a, these data indicated gases migrating into the spring have a proportionately larger methane content throughout the sampling period. Propane Propane magnitude data from the concreted hot spring have also been plotted to examine propane content versus time and to compare these variations with the methane results (Figure 16b). Although propane exhibits a much narrower range of values and varies from 2.697 to 4.828 ppm, the propane data clearly mimicked the gross changes recorded by methane. However, the net propane increase with time was almost twice the initial magnitude, as observed for methane. Individual high-magnitude events identified for methane correlate well over the entire time period with propane magnitudes. However, on a sample-to-sample basis, propane does exhibit some independence from methane. Since ethane, i-butane and nbutane results show a close relationship with one another and parallel propane results, they were not plotted. Propane/Methane Compositional Ratio As a further means of evaluating light hydrocarbon seepage at the concreted hot spring, the composition of the migrating gases was closely examined. Compositional indicators included ratios of one gas to another and percentages of individual hydrocarbon gases as compared to the entire hydrocarbon content of the migrated gas. The propane/methane × 1000 light hydrocarbon ratio is commonly used in soil gas geochemical exploration activities to identify whether a seep originates from an oil, oil and gas, or natural gas type source in the subsurface (Jones and Drozd, 1983). This ratio (Figure 16c) ranges from 0.69 to 0.87, indicating a mature dry gas source. Although both methane and propane generally correlate with each other, their ratio clearly exhibits variations 218 Jones and Burtell 5.0 6 0 0 0 4.5 ppm ppm 5 5 0 0 5 0 0 0 4.0 4 5 0 0 3.5 4 0 0 0 3.0 3 5 0 0 2.5 1/5/82 3/6/82 5/5/82 7/4/82 9/2/82 11/1/82 1/5/82 3/6/82 5/5/82 7/4/82 9/2/82 11/1/82 DATE DATE C3C1 0.87 2 3 0 0 0.84 ppm ppm 2 2 0 0 0.81 0.78 2 1 0 0 2 0 0 0 0.75 1 9 0 0 0.72 1 8 0 0 0.65 1 7 0 0 1/5/82 3/6/82 5/5/82 7/4/82 9/2/82 11/1/82 DATE 1/5/82 3/6/82 5/5/82 7/4/82 9/2/82 11/1/82 DATE Figure 16—Gas concentrations in free gases evolving naturally from a concreted well at Arrowhead Hot Springs, near San Bernardino, California, measured during 1 year from December 1981 to December 1982. (a) Methane nearly doubled in that year. (b) Propane generally followed methane and also nearly doubled. (c) Propane/methane ratio (× 1000) varied significantly within variable time windows, suggesting a relationship to tectonic activity in the San Andreas fault zone. (d) Helium exhibited significant variations that do not follow those of the hydrocarbon gases. similar to those noted for the Mukhto oil field on the seismically active Sakhalin Island. These well-established changes are probably a result of changes in the earth’s stresses and its influence on gas emission. Ethane through butane also follow this trend and suggest that over this time period, a larger proportion of mature hydrocarbons was emitted from this spring. Helium Helium magnitudes fluctuated independently of the light hydrocarbon gases, suggesting some independence of the helium (Figure 16d). Overall helium magnitudes ranged between 1700 and 2350 ppm as free gases from the spring, indicative of a concentrated source of helium (more than 350 times atmospheric levels). Although the initial data from Scripps suggested a positive correlation between methane and helium for the 1979 Big Bear earthquake, this more detailed analysis showed that this correlation was much more complex and that helium may be affected by different geologic and tectonic events than methane. Stable Carbon Isotopes As noted earlier, the composition of the measured hydrocarbon gases reflects a dry gas signature. The presence of the methane homologs of heavier ethane through butane suggests a deeply buried sedimentary source for the hydrocarbons. Stable carbon isotope measurements made on two methane samples collected on January 9 and 17, 1982, had consistent results of –23.7‰, confirming the presence of a very mature source. The methane and higher hydrocarbon gases measured at the site suggest a migrated product derived by normal sedimentary processes, typical of very mature oil and gas accumulations. Small blocks of sediment could have been squeezed across the fault plane. A small sedimentary block could be thrust below the San Bernardino Mountains in the vicinity of Arrowhead Hot Springs, where the additional heat could produce the measured hydrocarbons. Helium Isotopes Isotopic measurements of He3/He4 in waters from Arrowhead Hot Springs were found to have an average of 0.431 ± 1.2‰ for five measurements (Craig et al., 1980). This value is not nearly as thermally mature as measurements made over hot spot thermal reservoirs in Iceland and in Yellowstone National Park, Wyoming, and over helium spots in the Matsushiro area of Japan. Although these measurements are in the range of 7 to 18 times atmospheric concentrations, their low ratios relative to other known thermal reservoirs suggest a dilution of normal granitic helium generated by radioactive decay within the crystalline rocks of the San Bernardino Mountains, mixed with limited mantle helium from depth. The presence of only a slight amount of mantle-derived helium suggests that the thermal waters are a result of either frictional heat and resultant fractures or possibly a small intrusive body. Chapter 16—Hydrocarbon Flux Variations in Natural and Anthropogenic Seeps 219 Figure 17—Contour map of adsorbed mercury extracted from surface soils at Arrowhead Hot Springs, California, illustrating its relationship to the concreted hot well. Mercury Associations To identify the size and shape of the hot springs system and its relationship to other geologic features, a lowtemperature soil mercury mapping program was completed. A total of 525 soil samples were collected at 15-m (50-ft) intervals on a grid of ten north-south and four eastwest survey lines 75 m (250 ft) apart and analyzed for adsorbed mercury content (Jones and Maciolek, 1984). Contoured mercury magnitude data (Figure 17) show two distinct and well-controlled zones of anomalous values that clearly define the area of thermal springs and wells. Anomalous zones are centered around the surface spring outlets in each area and on the northern edge of the study area where Precambrian rocks crop out. Anomalous zones have sharp boundaries and tend to be located within and north of mapped east-west trending fault zones that cross the property. The highest magnitude mercury anomalies are located in Penyugal Canyon where several samples had concentrations in excess of 250 ppb. The distribution of mapped mercury values conclusively located and confirmed the distribution of thermal springs and wells on the Arrow head Hot Springs property. Areas of anomalous mercury do not correlate with any particular mapped geologic unit, suggesting that the measured mercury migrates from depth and accumulates in the soils, rather than forming as a residuum of weathering of surficial rock units. The overall distribution of mercury anomalies correlates well with surface thermal spring outlets, indicating that the subsurface mercury source is controlled more by thermal systems and less by the east-west trending faults that splay from the San Andreas fault. High concentrations of mercury in the soil are coincident with surface thermal outlets, suggesting that mer- cury sampling can be used to identify potential thermal areas that are not otherwise evident. In the hot spring, mercury is more highly concentrated in the gas bubbles than in the spring water. Therefore, mercury may be enriched in soil as a result of vapor phase migration. The mapped mercury anomalies record the areal extent of the mercury rich vapor which probably migrated to the surface from a subsurface thermal reservoir. Origin of Arrowhead Hot Springs Gases Light hydrocarbons, helium, and mercury have been monitored over time at Arrowhead Hot Springs and found to exhibit significant changes in magnitude and composition. The origin of the measured helium appears to be primarily from crustal radioactive decay, with minor input from mantle sources. Methane and other light hydrocarbons have a very mature signature. This mature organic hydrocarbon gas strongly suggests that sedimentary rocks are present below crystalline and metamorphic rocks of the San Bernardino Mountains. The possibility is strongly supported by recent geologic research indicating that low-angle thrust faults of Laramide–Tertiary age occur in the Mojave Desert to the east. Tectonic activity may have thrust the San Bernardino block over sedimentary rocks that now lie buried below the San Bernardino Mountains. Subsequent burial and maturation of sedimentary source rocks may have produced significant quantities of hydrocarbons. If this possibility can be substantiated by additional data, exploration for an untested petroleum province with commercial potential below the San Bernardino Mountain and other parts of the Transverse Range Province may be warranted. 220 Jones and Burtell Discussion This project began as a spin-off of a joint Gulf Research–CalTech fault-migrated gas monitoring program to help CalTech develop vapor phase earthquake prediction techniques and to improve Gulf’s understanding of gas migration mechanisms. The resultant investigations have tested various assumptions and theories about deep fault and fracture system gas emanations and their expression at near-surface sampling sites. Each phase of the program was completed to help develop geochemical sampling techniques, to start a database for future programs, and to aid in the evaluation of potential earthquake prediction sites. This program included only the study of a single hot spring site in an attempt to relate gas emissions to earthquakes. Light hydrocarbons, helium, radon, hydrogen, carbon dioxide, and carbon monoxide were monitored for magnitude and compositional changes for correlation with earthquake occurrences. Although no clearly earthquake-related events were identified, significant gas magnitude and compositional changes were recorded. Regional stress variations are suggested as the most probable cause for the recorded changes in gas flux at this site. Plans for additional sites and longer term monitoring were made by Gulf and CalTech scientists and are highly recommended for future research. Industry downsizing and takeovers prevented the final implementation of these research plans. No comment needs to be made about the value these data might have had to California and to the nation if this program had been allowed to continue until today. CONCLUSIONS This paper presents a variety of near-surface geochemical measurements made over both natural and anthropogenic seeps, selected more by opportunity than design. The main advantage of these selected examples is that they generally represent opportunities to measure actual changes in flux rates of gases that are unique in chemical composition, and as such can be directly related to their respective sources. Understanding and mapping the actual migration pathway remains the difficult part of the process. However, in spite of these difficulties, these measurements do demonstrate that the migration model must be both pressure and permeability dependent. Magnitude changes related to pressure events at depth clearly occur in these examples on a time scale measurable in hours to days rather than in years. Suggestions that vertical migration from a field such as Hartzog Draw in the Powder River Basin might take a million years is no longer reasonable. In fact, measurements made in 1976 (before the field was discovered) and again in 1978 (after the wells were put on pump) indicated that the initial soil gas measurements made in 1976 had decreased by nearly an order of magnitude by 1978. These studies confirm that surface geochemical data reliably reflect subsurface hydrocarbon sources and their compositions. The magnitude of a microseep from a reservoir is related to the permeability of the migration pathway. Since we do not know whether the surface signal comes from an economic or an uneconomic reservoir, it is risky to speculate whether a particular seep necessarily represents an economic accumulation of hydrocarbons. A surface geochemical survey is not a stand-alone prospect tool. 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