--> ABSTRACT: Core, Well Log, and Seismic Integrated Stratigraphic Study of Humid and Arid Climate Lacustrine Oil Shales: Green River Fm., Washakie Basin, Wyoming, by K. S. Glaser, K. J. Miskell-Gerhardt; #91020 (1995).

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Core, Well Log, and Seismic Integrated Stratigraphic Study of Humid and Arid Climate Lacustrine Oil Shales: Green River Fm., Washakie Basin, Wyoming

K. S. Glaser, K. J. Miskell-Gerhardt

An integrated sequence-stratigraphic framework has been constructed for three lacustrine oil shale intervals in the Green River Formation of southwestern Wyoming. The framework was developed by integrating parasequences and parasequence stacking patterns identified in core and outcrop, with well logs, seismic geometries and seismic facies. Because base level in lacustrine environments is directly related to rainfall rather than eustasy, the development of facies assemblages within each systems tract of lacustrine depositional sequences, as well as the geometries of stratal units, differ from the traditional marine model. Recognition of these differences resulted in the development of two idealized depositional sequence models, one each for arid and humid climates.

Parasequences were identified in lacustrine sediments from vertical facies relationships and geochemical data. They appear to be driven by rainfall cycles and are of approximately 10,000 yr. duration. Parasequences formed during humid periods by shoreline progradation and flooding whereas during arid periods, prasequences formed by desiccation and flooding. Parasequence stacking patterns, facies relationships, and evidence of truncation were used to break out internal lacustrine depositional sequences within the gross lacustrine packages. Lacustrine sequence development also appears to be driven by climatic changes, at a frequency of approximately 100,000 yr.

Facies were calibrated to well log response, and the parasequences, parasequence sets and sequence boundaries were correlated in the subsurface throughout the Washakie Basin. The well log response to oil shale and interbedded clastics was unusual and differed among the three oil shale units studied.

Although most details of the internal lacustrine stratigraphic framework are below the resolution of seismic data, several reflections could be tied to significant surfaces within each lacustrine package and used to extend stratigraphic correlations away from well control. Subtle downlap could be observed in an offshore direction in the humid deposits, but only slight thickening between sub-parallel reflections was observed for the arid deposits. For both humid and arid lacustrine deposits, high amplitude, continuous reflections correspond with oil shales.

The alternating sets of lacustrine packages and clastic wedges are interpreted to be tectonically driven depositional sequence sets of approximately 1,000,000 yr. duration. The development of the sequence sets is related to changes in drainage patterns, due to activation of various Laramide uplifts surrounding the basin. This large-scale alternation between clastic and lacustrine packages is easily identifiable on seismic data from changes in reflection character.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995