--> ABSTRACT: Valley-Fill Deposits: Key to Stratigraphic Traps in Pennsylvanian and Cretaceous, Rocky Mountain and Mid-Continent Regions, by Robert J. Weimer; #91033 (2010)

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Valley-Fill Deposits: Key to Stratigraphic Traps in Pennsylvanian and Cretaceous, Rocky Mountain and Mid-Continent Regions

Robert J. Weimer

A valley fill is deposited during a rising sea level in an incised drainage that was cut during a relative sea level lowstand. Where thickest and most complete, the valley fill commonly has a vertical zonation from lowermost freshwater deposits to brackish and then marine deposits. The zonation results from deposition during rising sea level (transgression) by aggradational processes within the valley. This vertical facies order is contrary to the normal facies arrangement in typical regressive deposits.

Valley-fill deposits are bounded by surfaces of erosion. They overlie a lowstand surface of erosion associated with the valley incisement phase and a lowering of base level (this surface is a major sequence boundary in sequence stratigraphy analysis). They may be truncated at the top by a transgressive surface of erosion associated with shoreface erosion related to the transgressing marine shoreline. Marine shale, commonly an organic-rich condensed section (source rock), overlies the valley fill. Minor erosion surfaces that are found within the valley fill, associated with depositional processes, are called diastems.

Valley-fill deposits in the Pennsylvanian and Cretaceous are up to 120 ft thick, are less than a mile to as much as tens of miles in width, and may be hundreds of miles in length. The nature of the fill varies in percentage of sandstone, siltstone, and shale. The most important petroleum reservoir in the valley fill is the lenticular porous and permeable fluvial channel sandstones, normally in the lower portion. Of less importance are the estuary, tidal flat, and bayhead delta (estuarine) sandstones because they are thinner (< 10 ft) and "tighter." Typical stratigraphic and unconformity traps in valley fills are present in the Rocky Mountain and Mid-Continent regions. Some valley fills can be recognized on seismic, but a future exploration challenge is to integrate improved seismic and geologic models, to improve the predictability.

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