Surface and Near-Subsurface Hydrocarbon Migration Patterns, Mechanisms and Seepage Rates Associated with a Macroseep
V.T. Jones, III, Exploration Technologies, Inc., D.F. Becker, Geoscientific Consulting Partners, D.D. Coleman, Isotech Laboratories, Inc., T.H. Anderson, University of Pittsburgh, P.A. Witherspoon, University of California, and G.A. Robbins, University of Connecticut
Previous assessments within a planned development area revealed methane macroseeps emanating from a natural wetland containing marsh deposits, Gas bubbles rising from an adjacent creek that crosses the property initially indicated the presence of methane gas emissions, Gas measurements made in five deep wells at multiple depths. extending down to the 50 feet adjacent to the creek confirmed the presence of gas in deeper sediments underlying the site. The methane was initially interpreted as biogenic, and non-hazardous although a natural gas storage reservoir lies within 3000 feet of the area where the methane seeps occur. An invcstigation was bcgun to address concern about possible leakage from the gas storage reservoir.
A soil gas survey, consisting of over 800 sites placed on a 100 foot grid spacing was completed over the site, defining an area of anomalous seepage. Within the area of seeps, methane concentration in some samples is as high as 100%. At each site gas was cxtractcd from a four-foot deep soil gas probe and analyzed for methane through butanes, carbon dioxide, and hydrogen sulfide. Selected soil gas samples were also analyzed for carbon and hydrogen isotopes. The distribution of the methane seeps was used to locate areas where deeper monitor wells might be installed in order to assess input from deeper sources. It was anticipated that mapping the presence of gas anomalies within an extensive gravel aquifer that completely underlies the site at a depth of approximately 50 feet could: 1) define gas sources located at intermediate depth below the marsh deposits and 2) provide a three dimensional picture of the natural gas seepage in the area.
Using the soil gas maps as a guide, 42 monitor wells were installed in the gravel aquifer. Both free and dissolved gases were collected from each of these wells using water displacement, with at least one sample collected from each of the five to seven well-volumes of water pumped from each of the wells. The well gases were analyzed for their methane through butane and carbon dioxide contents. The hydrocarbon data from the monitor wells collected and analyzed by this method provide reliable control points from which distribution of the light hydrocarbons within the aquifer may be estimated. Areas containing methane concentrations in the aquifer ranged from zero to 100%, with the quantity of free gases increasing significantly within the areas containing the higher methane levels. Carbon isotopes for methane, ethane, and carbon dioxide and hydrogen isotopes for methane were analyzed for all monitor wells, although only one sample from each well was analyzed. An excellent correlation between both composition and isotopic ratios was found between the soil gas seeps at four feet and the dissolved and free gases in the gravel aquifer at 50 feet. Gas collected from seeps and wells is derived from thermogenic sources.
Gases from nine deep wells within lhc storage field were collcctcd and analyzed for their chemical and isotopic compositions for comparison with the near-surface and deeper ground water aquifer gases. Comparison of the chemical and isotopic data indicates that the surface gas seeps and 50-foot gravel aquifer gases are not related to the gases injected into the adjacent gas storage field. The shallow gas is nearly identical to production gases from a field about 4.5 miles to the southeast. This field has produced over 23 BCF of dry gas from the Pico Forrnation_ A non-productive well drilled adjacent to the secpagc anomaly blcw out in 1930 while drilling at 1830 feet in the Pico Formation and flowed at the rate of 5 MMcfpd for a few hours. A 3-D seismic study indicates the presence of disrupted strata, possibly associated with a fractured slump0 block that appears to provide the vertical pathways for the observed macroseeps.
Using the soil gas contour maps as a guide, numerous additional macroseeps were located on land, where gas seeps are typically difficult to find. Measured seepage rates range upwards from about 1 ml/minute to as much as 9 liters/minute of gas into the atmosphere. This variation in seepage rates, coupled with the extensive data base from the aquifer below the oxidation zone, provides a unique opportunity to study, and document near-surface alteration effects on soil gases.
The seepage gases contain mainly methane and are very dry. However, ethane through butanes are sufficiently abundant for chemical trace analysis, and there is adequate ethane for carbon isotopes analysis on selected samples Although, the ethane is quite degraded within the 50 foot deep gravel aquifer, it exhibits only a small variation in the ethane carbon isotope within the aquifer, ranging from -18 to -2l parts per mil. Very near the surface. ethane carbon isotope values as low as -13.5 per mil have been documented at four feet, suggesting additional near-surface degradation related to the shallow depth. This type of degradation effect has also been documented for methane, where the methane carbon isotopes have been altered to values that appear to be more mature than they are in the underlying aquifer. These oxidation-related changes could easily be misinterpreted as being more mature than they really are without the additional supporting data.
In areas having very high advective seepage rates, both the methane and the ethane have chemical and isotopic values that are identical to those measured in the gravel aquifer. However, in areas having moderate seepage rates, where oxygen probably diffuses into the shallow subsurface, both methane and ethane isotopes have been documented to have carbon isotopes values that do not represent their true source, and/or maturity that is normally associated with isotope interpretations, Thus the carbon isotope data may be easily misinterpreted. In such samples, only the chemical composition; i.e., presence of ethane, propane, and butanes proves the presence of a thermogenic source.
Hydrogen sulfide does not occur in the gases vented directly from the gravel aquifer. It is, however, often present in the larger soil gas anomalies. The source for hydrogen sulfide appears to be natural, organic rich soils, and its areal distribution at the surface correlates almost entirely with rich seeps of methane gas, which apparently act as a carrier.
Figure 1. Soil vapor data collected on a 110 foot grid spacing from a depth of four feet below surface illustrates the size and shape of two very large magnitude methane seep. Methane magnitudes range from ambient air levels upwards of 89 percent. Actual gas bubbles have been visibly documented when the areas containing the larger magnitude concentrations are flooded.
Figure 2. Using the soil gas data as a guide, forty-one monitor wells were installed in the underlying gravel aquifer located approximately 50 feet subsurface. Methane concentrations in the free gases liberated by water displacement exhibit good correlation with the soil gas data shown in Figure 1. Free gas concentrations in the aquifer range upwards of 98 percent by volume.
Figure 4. A map view of the isotopic data shown in Figure 3 defines the two main methane anomalies, and suggests that the isotope values in the forty-one monitor wells are controlled by proximity to source. The two most thermally mature, and uniform isotope anomalies within the gravel aquifer at depth provide confirmation of the two major soil gas anomalies. Surface macroseeps, found by the soil gas survey have been collected and analyzed that match exactly to the compositions of the gases found in the aquifer.