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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 58 (1974)

Issue: 5. (May)

First Page: 918

Last Page: 918

Title: Muddy Sandstone Environments, Powder River Basin, Wyoming and Montana: Outcrop and Core Study: ABSTRACT

Author(s): Roderick W. Tillman

Article Type: Meeting abstract

Abstract:

In the Powder River basin of Wyoming and Montana deltaic lobes have prograded from the east, northeast, and southwest at various times during the interval of time assigned to the Muddy Formation. Several recognizable sand-body types which yield hydrocarbons are associated with each of these deltas.

The major types of reservoir sands are delta-distributary channels, barrier islands and high sand tidal-flat deposits. Each of these environments of deposition can be recognized in outcrop and in slabbed cores. After the environment is identified in a core, log shape is useful to extend the environment laterally.

Examples from outcrops and cores from Wyoming and Montana illustrate the features which allow recognition of sands deposited in the various environments. In the barrier-island sands, such as those at Bell Creek field, low-angle crossbedding and a general coarsening upward from shale to silty sand to sandstone is present. Thalassinoides and a few Ophiomorpha burrows are locally present.

In the tidal-flat sandstones wave and current ripples predominate. Long vertical burrows designated as Skolithos are prominent and brackish deposits such as coal are locally interbedded. Corophium, a small U-shaped burrow, is present in some of the cleaner intertidal sand bodies.

Distributary-channel sandstones are typically medium to large scale trough crossbedded and locally have current ripples, climbing ripples, and interbedded clay drape in natural-levee deposits. Burrowing usually is limited to the upper part of the distributary-channel sands. Locally at the base of the channels is a conglomerate made up of clasts of marine shale. The uppermost part of the distributary-channel sands commonly are reworked and spread laterally as a thin transgressive beach or intertidal sand.

A subregional paleogeographic reconstruction can be made utilizing the probable areal distribution and trends of each of the genetic sand limits. Maps of this type aid significantly in improving the success ratio of both exploration and development drilling.

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Copyright 1997 American Association of Petroleum Geologists