Basement Block Influences on Shallow Gas Accumulations: a Reprise

George W. Shurr

More than a decade has passed since successful development of shallow biogenic natural gas in southeastern Alberta sparked interest in similar resources in the United States northern Great Plains. Regional stratigraphic syntheses, local sedimentologic studies, and development drilling documented natural gas accumulations in conventional reservoirs in Upper Cretaceous marine sand-ridge fields and in unconventional "tight" reservoirs in fine clastics that enclose sand bodies. Basement blocks defined by stratigraphic and structural patterns and by geophysical and remote-sensing data were shown to be paleotectonic hinges where vertical displacement influenced sedimentation. Recent refinements of the model suggest that block boundaries (1) act as fluid migration routes, and (2 display strike-slip displacement components.

In central South Dakota, ground water movement on block boundaries is demonstrated by geothermal and geochemical anomalies and by pump tests. Assemblages of geologic structures associated with basement deformation zones suggest local left slip on at least four east-northeast zones and on one north-northwest zone. Although there is no gas production in this area, potential reservoirs are present.

In eastern Montana, fluid migration on block boundaries is suggested by geothermal anomalies and the location of oil production at boundary intersections. Local left slip on northeast boundaries and right slip on northwest boundaries are interpreted from assemblages of geologic structures. Gas is produced on Bowdoin dome where initial pressures from the Phillips sandstone seem to have patterns related to block boundaries.

Recent refinements of the basement block model modify an earlier gas exploration strategy based mainly on paleotectonic influence of sedimentation. Ultimately, however, economic considerations may be more important than geologic considerations. Localized production and consumption of shallow gas may prove to be more economically viable than interstate marketing and consumption.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988