Rates of Biogenic Gas Production
Case study: Illinois Basin shale gas field
In the Illinois Basin, 34 formation water and gas samples were analyzed for stable isotopes, Chloride, tritium, 14C and noble gases.
20Ne/36Ar ratios (0.08 to 0.33) are similar to the range found in air-equilibrated water (0.14-0.18). 84Kr/36Ar and 132Xe/36Ar ratios (0.035-0.056 and 13.8-35.0×10-4, respectively) are also similar to the range found in air saturated water.
In these samples we assume that there has been minimal fractionation of the inert gases, with near complete transfer of the noble gas in the water to the gas phase. When considering an external flux of 4He from the upper and lower crust, the average groundwater age range between 0.082 and 1.2 Ma (average 0.59 Ma) for representative crustal values.
Within the Illinois Basin, there is a clear spatial coherence between the different sample populations, the microbial gases which are closer to recharge zones have a younger 4He groundwater age than the deeper thermogenic methane which has an older 4He associated groundwater age. These results are consistent with chloride and δ18O data from the New Albany Shale, which show elevated values towards the basin center in the thermogenic methane area indicating limited flushing by fresh water, as compared to the microbial methane area nearer to the basin margins. All locations in the study area have been influenced by Pleistocene recharge; however, deep saline brines have not been completely flushed, suggesting that the basin fluids are fairly stagnant. Using 4He ages, the magnitude of in-situ microbial methane production is estimated to range from 10-1000 TCF/Ma, which is ~104 to 106 times slower than average laboratory methane production rates from coals, implying that laboratory experiments may be fundamentally different than in-situ conditions.Average groundwater age range from 0.082 to 1.2 Ma (average 0.59 Ma)
Thermogenic methane 4He ages are older than microbial methane 4He ages
Ancient Fluid Systems
The majority of the world’s surface and near-surface reservoirs of natural gas are generally accepted to have been produced by either microbial or thermal degradation of higher organic matter. These processes produce distinct chemical and isotopic signatures in gas reservoirs. There are however a number of gas reserves which do not display these typical signatures, including the large Urengoy gas field in western Siberia and the gas fields of the Songliao Basin, China. These fields produce isotopically light, methane-rich (‘dry’) gas (ILDG) that is not explained by the current understanding of biogenic gas production processes. ILDG has also been reported in high temperature fluids emanating from within sediment-poor mid-ocean ridges, in hydrocarbon seeps from areas upon ultramafic rocks, within fluid inclusions in mantle and igneous rocks and in fluids from the deep subsurface within Precambrian shield areas.
Previous stable isotope studies (Sherwood Lollar et al.) have shown that ILDG emanating from boreholes within the deep mines of the Abitibi Greenstone belt, Ontario, Canada display isotopic signatures consistent with abiogenic formation, and studies of fluids sampled from the deep gold mines of the Witwatersrand basin, South Africa, contain both abundant H2, a prerequisite for abiotic methanogenesis, and neon isotopic ratios indicating the fluid had been isolated in excess of 2 billion years (Lippmann-Pipke et al.).
Despite the biogenic sources of hydrocarbons dominating hydrocarbon reservoirs, the extent to which these deep subsurface abiogenic hydrocarbons interacts with the surface and near-surface biogenic hydrocarbons are unknown.
At the University of Manchester, we are applying our expertise in high-precision noble gas multicollector mass spectrometry to discerning the age, origin, extent and interactions of deep crustal carbon bearing fluids in the subsurface. By combining these results with stable isotope analyses performed by collaborators in the University of Toronto, we are building up a framework to quantify the fundamental processes that can distinguish abiogenic hydrocarbon input from biologically derived matter.
Working with geochemists, chemists, physicists, biologists and mathematicians as part of the international, multidisciplinary Deep Carbon Observatory, we are helping to increase the understanding of the cycling of carbon within the deep Earth.
Case study: Sanjuan Basin coalbed methane fieldIn the San Juan Basin study, 28 gas samples from producing wells in the artesian overpressured high production region of the basin were taken together with 8 gas samples from the underpressured low production zone as a control. Stable isotope and major species determination clearly characterize the high production region as dominantly biogenic in origin, and the underpressured low production region as having a significant admixture of thermogenic coal gas. 3He/4He ratios increase from 0.0836Ra at the basin margin to 0.318Ra (where Ra is the atmospheric value of 1.4×10-6), indicating a clear but small mantle He signature in all gases. Elemental ratios of water-derived 20Ne/36Ar and crustal 4He/40Ar* are different from the gas/water equilibrium solubility values and can be explained by a simple Rayleigh fractionation model where gas bubbles passing through water distill the noble gases into the gas phase. Low 20Ne concentrations compared to the model-predicted values can be accounted for by dilution of the groundwater-associated gas by desorbed coalbed methane. This Rayleigh fractionation and dilution model together with the gas production history allows us to quantify the amount of water involved in gas production at each well. The quantified water volumes in both underpressured and overpressured zones range from 1.7×103 m3 to 4.2×105 m3, with no clear distinction between over- and underpressured production zones. These results conclusively show that the volume of groundwater seen by coal does not play a role in determining the volume of methane produced by secondary biodegradation of these coalbeds. There is no requirement of continuous groundwater flow for renewing the microbes or nutrient components. Another potential implication may be related to well spacing. More production wells can be placed without affecting the presently producing wells. The observed strong mass related isotopic fractionation of 20Ne/22Ne and 38Ar/36Ar ratios can be explained by a noble gas concentration gradient in the groundwater during gas production, which caused partial diffusive re-equilibration of the noble gas isotopes. Excess 136Xe and 84Kr in our samples can only be accounted for by 136Xe and 84Kr associated with the desorbed coalbed methane. Xe and Kr isotopes are volumetrically trapped in the coal matrix and released as the coal is biodegraded to form methane.
Crustal radiogenic 4He accumulation in groundwater is an established dating tool. Based on physical models describing hydrocarbon/groundwater interaction developed by the noble gases in the hydrocarbon phase, initial crustal radiogenic 4He concentrations in the groundwater can be calculated. Considering both in situ 4He production and 4He from external crustal flux, this allows the 4He groundwater ages to be derived. In the San Juan Basin study, the solubility controlled Rayleigh fractionation model of noble gases is used to determine an initial 4He concentration in the groundwater associated with methane producing wells. The 4He groundwater ages in the San Juan Basin coalbed methane gas field range between 1.65×104 and 4.84×105 years. In the underpressured area of the San Juan Basin the groundwater ages increase as a function of the distance from basin margin recharge, but in the overpressured area, there is no clear trend. The dates derived for the groundwater in the underpressured area are consistent with 14C dates and hydrological modeling dates. This study illustrates the strengths and weaknesses of this technique. The principle uncertainties involved in the 4He dating of the groundwater associated with hydrocarbon reservoirs are the parameters used in the calculation of the 4He accumulation rate in the groundwater. They are aquifer thickness, porosity and radioelement concentrations and in particular an accurate quantification of the external 4He flux into the groundwater. However, there are many environments where no other dating tool exists, relative dates from systems interacting with the environment are valid. In these cases the hydrocarbon phase groundwater ages obtained give the only estimates available.
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