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Shallow Gas Systems

GENERAL STATEMENT
Shallow natural gas systems can be usefully described in terms of general petroleum systems as outlined by Magoon and Dow (1994):
••The systems have a “pod” of source rock, a set of genetically related accumulations, and a network of migration paths.
••Main processes including generation, migration, accumulation and trap formation are put into time and space frameworks.
••“Critical moment” is the best time representation of gas generation, migration, and accumulation.

Shallow gas systems on basin margins fall into three distinct categories (Ideas: Figure 2) (Shurr, 2002; Shurr and Ridgley, in review):
1. Thermogenic gas--generated at depth within the basin and migrated up the basin margin as free gas or as gas associated with oil.
2. Early generation biogenic gas--generated shortly after deposition from source rock interbedded with reservoir rock. Unmigrated gas tends to be trapped in unconventional reservoirs in lineament blocks and migrated gas is in conventional reservoirs with stratigraphic and structural traps.
3. Late generation biogenic gas--generated in the relatively recent geologic past by microbes moving in ground water. Usually the flow is down a basin margin within rocks that act as both source beds and reservoirs.

COMMENTS AND EXAMPLES
Representative, archetype areas provide characteristic attributes for each of the three shallow gas systems:
••The Hugoton Embayment on the northwestern margin of the Anadarko Basin has nonassociated thermogenic gas in heterogeneous Permian rocks (Kansas Geological Survey, www.kgs.ukans.edu/Hugoton/).
••The southeastern margin of the Alberta Basin has early generation biogenic gas in Cretaceous, marine clastic reservoirs (USGS project, Ridgley and others, 1998-2001).
••The northern margin of the Michigan Basin has late generation biogenic gas in fractured black shales of the Devonian Antrim Shale (gastechnology.org, Search Icon, GRI Publication Data Base, Report GRI-97/0127).

Distinctive compositions characterize each of the three systems:
••Thermogenic gas has heavier hydrocarbons and may have larger amounts of helium and/or nitrogen.
••Biogenic gas is dominantly methane and has distinct ranges of isotopic composition for carbon and deuterium.
••Late generation biogenic gas can be distinguished by comparing hydrogen isotope values in the gas and co-produced water. Also, it tends to have higher CO2 content than early generation biogenic gas.

Ground water flow is important for all three systems:
••Thermogenic gas is transported by flow up the basin margin from the basin center.
••Late generation biogenic gas is produced by water flow down the margin from bounding outcrops or subcrops.
••Early generation biogenic gas may be transported in or isolated by ground water flow.
••Hydrodynamic traps appear to be important in many accumulations.

Development histories are similar for all three systems:
••Initially, small sweetspots are developed for local consumption.
••Subsequently, a supporting infrastructure is progressively built up.
••Eventually, large continuous-type accumulations are developed by step-out drilling from sweetspots.

FEEDBACK STARTER QUESTIONS
1. Is there a spectrum of time separations between deposition of source rocks and biogenic gas generation? For example, in Canada, deposition and generation are approximately contemporaneous during the late Cretaceous, while in Michigan source rocks were deposited during the Devonian and gas was generated during the Pleistocene.
2. Do the relatively young rock units in the Gulf Coast contain early generation or late generation biogenic gas?
3. Late generation biogenic composition signatures have been imposed on dominantly thermogenic gas in the San Juan Basin (Scott and others, 1994, AAPG Bulletin). Could a thermogenic signature be imposed on shallow biogenic gas that has been deeply buried? That is, do basin center accumulations originate as shallow biogenic gas (J. Ridgley, personal communication, 2000)?

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