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USGS Workshop on a Global Assessment of Geologically Sourced Methane
Methane is the most abundant organic chemical in the Earth's atmosphere, where it is a powerful greenhouse gas and must therefore influence global climate change. The total annual source of methane to the atmosphere has been constrained to a range of 400-640 Tg (teragrams = 1012 g). Sources of methane to the atmosphere are varied. Much of the methane going into the atmosphere is being produced right now, from sources that include enteric fermentation, natural wetlands, rice paddies, biomass burning, termites, and landfills. Methane from these sources contains mainly modern carbon (with 14C). Some of the methane now going into the atmosphere was produced in the past. Sources of this methane, which we call geologically sourced methane, include hydrates; coal mining; and gas drilling, venting, and transmission. Methane from these sources contains mainly ancient carbon (with less or no 14C). The ancient-carbon, or geologically sourced, methane sources identified to date account for about 15% of the total methane in the atmosphere, which is half of the 30% needed to balance the observed atmospheric methane budget. The workshop participants addressed the hypothesis that naturally occurring methane seeps contribute a significant amount of methane, containing much ancient carbon, and may help account for the missing geologically sourced methane. Natural gas seepage rates have previously been ignored in inventories of atmospheric methane sources. However, natural gas seeps, commonly occurring with natural oil seeps, are found in both terrestrial and marine settings. These seeps range from microseepages (often considered in geochemical prospecting for petroleum) to macroseepages (so clearly evident in the Santa Barbara Channel, offshore from Southern California). Also ignored have been the natural exhalations of methane from exposed outcrops, particularly petroleum source rocks and coal beds. Workshop participants, informally named the Gaia's Breath Working Group, addressed issues of natural gas seep occurrence, measurements of rates of methane emission from seeps on local scales, estimates of rates of methane emission from seeps on a global scale, and methods for improved assessment of methane seepage rates.
The Working Group concluded that the contribution of geologically sourced methane to the atmospheric organic carbon cycle is significant and should be included in any global inventory of atmospheric methane. As a first approximation, the observed global rate of methane emission at the seabed was estimated to be about 50 Tg/yr, resulting in an atmospheric emission rate of about 30 Tg/yr. A theoretical estimate-based on the total reservoir of methane available for seepage over geologic time, steady-state conditions, and a half-life of methane in the system of 100 million years-resulted in similar rates of methane emission of 30 Tg/yr at the seabed and 10 Tg/yr to the atmosphere. These first-approximation rates are strongly influenced by methane oxidation, which plays a critical role in limiting the amount of methane available to the ocean-atmosphere system. Knowledge of methane emissions from terrestrial sources (volcanoes, rock outcrops, coal beds) is minimal, but the rate of methane emission from these sources is believed to be less than that from the oceans. In contrast, previous estimates of methane emissions from the exploitation of coal and petroleum are 35 Tg/yr and 45 Tg/yr, respectively. Results of this workshop will be presented at the AAPG Hedberg Conference "Near-Surface Hydrocarbon Migration: Mechanisms and Seepage Rates," to be held September 16th-19th in Vancouver, BC, Canada.
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in this issue:
Global Assessment of Geologically-Sourced Methane
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U. S. Department of the Interior | U.S. Geological Survey
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