--> Abstract: Predicting Subtle Traps with Sequence Stratigraphy and Synsedimentary Tectonic Analysis, Almond Formation, Southwest Wyoming: Old Concepts, New Plays!, by L. F. Krystinik and R. H. Mead; #90928 (1999).

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KRYSTINIK, LEE F.; RICHARD H. MEAD
Union Pacific Resources

Abstract: Predicting Subtle Traps with Sequence Stratigraphy and Synsedimentary Tectonic Analysis, Almond Formation, Southwest Wyoming: Old Concepts, New Plays!

Synsedimentary tectonics profoundly impact the distribution of depositional environments, stratal stacking patterns and trap types in the Upper Cretaceous Almond Formation within the Greater Green River Basin of Southwestern Wyoming. Early workers in the basin identified synsedimentary tectonics as an important component in understanding reservoir distribution, but data limitations constrained exploitation of these ideas. Integrated regional sequence stratigraphic analysis based on outcrop, core, well log, 2-D and 3-D seismic data in this study reveals a degree of complexity not previously recognized. This stratigraphic complexity provides numerous new drilling opportunities.

The Almond Formation is a net transgressive sequence set composed of lowstand shorelines and backstepping, storm dominated, microtidal barriers which locally onlapped a broad north-south oriented, paleohigh in the central part of the Green River Basin. During net transgression, highly erosive ravinement occurred across the relatively low sediment supply paleohigh. Up to 50% of the sedimentary record on the paleohigh has been removed by erosion along ravinement surfaces and sequences boundaries. In contrast, the tectonic lows flanking the paleohigh contain progradational to aggradational deltaic deposits which exhibit minimal truncation by subaerial exposure or transgressive erosion.

Transgressive erosion of shoreline deposits on the paleohigh supplied additional sediment via longshore drift to the south, accentuating the disparity between the prograding and retreating sections of the shoreline. This relationship is counter to conventional assumptions that subsiding areas should be transgressed and that relative uplift should cause shoreline progradation. These observations highlight the importance of the critical balance between sediment supply and accommodation, as well as their variability along strike.

Trap styles vary markedly in the two settings. The structural lows contain thick, extensive sand bodies with few stratigraphic trapping configurations and consequently have relatively low economic potential. The deltaic deposits are most prospective along the flanks of the paleohigh, where erosional truncation can exceed 20 m on several ravinement surfaces. On the paleohigh, stratigraphic traps occur against pinchouts of barrier sandstone below ravinement surfaces, against small synsedimentary structural uplifts and faults, and along sequence boundaries (Figure 1). These features are visible in 2-D and 3-D seismic data, are supported by well data and are presently the target of an ongoing exploration drilling program.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas