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USGS Workshop on a Global Assessment of Geologically Sourced Methane

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photograph of the peak of Mt. Hood
Mount Hood: The largest naturally occurring gas vent near Portland, Oregon, and the site of a half-day field excursion.
A workshop dealing with global methane seepage was organized by Keith Kvenvolden, Tom Lorenson (both CMG), and Bill Reeburgh (University of California at Irvine) and hosted by Dennis Lynch and Jacky Varna of the Oregon District, WRD, in Portland, Oregon, May 2nd-4th. The USGS Energy Resources Program (Suzanne Weedman) and CMG (Mike Carr) co-sponsored the workshop. Diana Collins (CMG), Anne Gartner (CMG), and Arti Pringle (CVO) helped with logistics. The 18 participants represented government, academia and industry, and included two participants from Europe and two from Canada. Our main objective was to attempt to account for missing geologically sourced methane (methane that was produced in the past but is entering the ocean/atmosphere now) in the current global inventory of sources of atmospheric 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.

group photograph of the workshop participants
Workshop participants:
Top left to right: Ray Cranston, Geological Survey of Canada; Ira Liefer, UC Santa Barbara; Alan Judd, U of Sunderland, UK; Marta Torres, OSU; Keith Kvenvolden, USGS; Bob Garrison, UC Santa Cruz; Martin Hovland, Statoil, Norway; Jean Whelan, Woods Hole Oceanographic Institution.
Bottom left to right: Jordan Clark, UC Santa Barbara; Bill Reeburgh, UC Irvine; Michael Abrams, U of Utah; Ed Peltzer, MBARI; Michael Whiticar, U of Victoria, Canada; Ian MacDonald, Texas A&M University; Tom Lorenson, USGS.
Not pictured: Ed Brooks, Washington State University; George Claypool, USGS emeritus; Anne Trehu, OSU.

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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Channel Islands Cruise

Lake Mead Mapping

Research New Underwater Microscope System

Hurricane Display

Outreach Reston Open House

WHFC Outreach

Monterey Open House

School-to-Work Partnership

Acadiana Migratory Bird Day

Meetings SWICA-M³

Global Assessment of Geologically-Sourced Methane

Methane Hydrates

Metadata Workshop

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GIS 2001: Logan

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