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Basin Margin Accumulations

GENERAL STATEMENT
Large natural gas accumulations that are located on basin margins (Ideas: Figure 1) share the following broad attributes (Shurr, 2000):
1. Shallow depths, generally less than 3000 ft.
2. Historic local production and consumption, often dating back to the end of the nineteenth century.
3. Structural traps are common, including both local folds and regional fracture systems.
4. Ground water flow systems are important.

COMMENTS AND EXAMPLES
This summary of shallow gas accumulations has been extracted (Shurr, 2001, Expanded Abstract) from the atlas of gas reservoirs in the Rockies (Doelger and others, 1993):
••Nonassociated gas in Cretaceous reservoirs are the most common occurrences.
••The average depth for these Cretaceous reservoirs is about 2800 ft.
••Discovery dates peaked in the 1920’s and again in the 1950’s.

Basin margin fields in Cretaceous rocks of the central and northern Rockies are summarized from a more constrained data set (Shurr, 2001):
••Wells tend to be low volume with low flow rates, but have long production histories and are cheap to drill and complete.
••Migrated thermogenic gas is found in foreland structures as small fields with normal pressure gradients.
••Biogenic gas is found in larger, under-pressured fields at depths averaging less than 2000 ft.

There are two categories of biogenic gas fields in the constrained data set:
••Sweetspots in conventional reservoirs cover 10’s of square miles and have cumulative production of more than 2 BCF/sq mi.
••Laterally continuous, commodity-type fields have unconventional reservoirs, cover 100’s of square miles, and have cumulative production values of less than 1 BCF/sq mi.

Important examples of basin margin accumulations include:
••The Alberta, Michigan, and Anadarko Basins as described in the next idea section on shallow gas systems.
••The Williston Basin as described in detail in the section on leads.
••The Powder River, Denver, Illinois, and Appalachian Basin as described in less detail in the section on leads.

FEEDBACK STARTER QUESTIONS
1. Are there examples of shallow gas accumulations NOT located on basin margins? Remember that the “margin” for young formations may be located relatively close to the basin “center” defined for older units.
2. Are there other important attributes of basin margin gas accumulations?
3. What other basins have examples of basin margin shallow gas? You may not want to provide the details that would constitute a lead, but please provide some documentation of your example.
4. Does the northern Gulf of Mexico qualify as a basin margin with extensive shallow gas accumulations?

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