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Controls on Reservoir Architecture in Cross-Bedded Shallow Marine Siliciclastic Systems: A Subsurface Methodology and Case Study from the Cretaceous of Southwest Wyoming

Simon A. Lomas
Baker Hughes, Houston, TX

The President Beaches Formation (lower Cretaceous) is exposed on western Livingston Island as a c. 800 m thick interval of clastic deep-marine slope apron deposits characterized by soft-sediment deformation features related to episodic slope failure. This is a mudstone-dominated succession (c. 90% mudstone) accumulated at mean rates of >200 mm/1000 yr. Sandy deposits are chiefly confined to low-aspect-ratio channel fills, 7–30 m thick, composed of sandy turbidites with minor mud-rich debrites. Slope failure is recorded on a range of scales by widespread slump, soft-sediment collapse, injection and other deformation features.

This study examines the internal architecture of the mass-transport complexes (MTCs) within the President Beaches Formation and examines their relationships to sand dispersal pathways. Vertical and lateral facies trends indicate that the slope apron depositional system as a whole was not a simple progradational or well organized system, despite relatively high rates of sediment supply. However, deformation features within the MTCs (e.g. slump fold axes, disrupted bedding) show consistent, systematic patterns. Associated with the larger slope collapse deposits, distinct families of early fibrous and cone-in-cone calcite veins are conspicuous within the deformed mudrocks. From striated vein orientations and isotopic data we develop a model of episodic, localized bleed-off of pore fluids associated with overpressure and slope collapse. Paleocurrent indicators also show a strongly focused, unidirectional pattern suggesting that transport processes were strongly influenced by the structural morphology produced by the MTCs. Similarly, the channel-fill sand-bodies are elongate along the NW-facing paleoslope. Hence this depositional system produced a stratigraphy which appears disorganized to chaotic in terms of facies trends yet shows a very high degree of systematic organization in terms of deformation patterns and sediment dispersal paths, recording the overriding imprint of the high bathymetric gradients on slope processes.