George W. Shurr1
(1) GeoShurr Resources, LLC, Ellsworth, MN
ABSTRACT: Attributes of Shallow Gas Accumulations in the Rocky Mountains
Published descriptions of gas fields in the Rocky Mountains provide a summary of the attributes of shallow gas accumulations. Fields described in the Atlas of Major Rocky Mountain Gas Reservoirs (1993) all have cumulative production of greater than 5 BCF. More than 200 fields in this compilation are at depths of less than 4000 ft.
The majority (130) of the shallow gas fields are in Cretaceous reservoirs and about two thirds of the Cretaceous reservoirs produce nonassociated gas. Nonassociated gas also dominates Tertiary reservoirs, but Paleozoic reservoirs mainly have associated gas. In general, the average area is larger for the nonassociated gas fields, while the average cumulative production per square mile is larger in fields of associated gas.
The frequency distribution of discovery dates for the 200 fields is bimodal. Nonassociated gas from Cretaceous rocks dominates a mode in the 1950's; associated gas from Paleozoic rocks defines a mode in the 1920's. The frequency distribution of depths is truncated with the maximum mode in the range of 3500-4000 ft. Cretaceous reservoirs have an average depth of 2300 ft, Tertiary reservoirs average 2700 ft, and Paleozoic reservoirs average 3100 ft. The majority of the shallow gas fields are located on the margins of structural basins.
A more tightly constrained data set is based on 70 nonassociated gas fields in Cretaceous reservoirs on basin margins in the central and northern Rockies. In general, they are less than 2000 ft deep, are underpressured, and consist mainly of biogenic gas. Conventional reservoirs confined to local structures characterize small (15 sq mi) "sweet spots". These "sweet spots" are embedded in large (1000 sq mi) accumulations in unconventional reservoirs located on regional arches.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado