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The AAPG/Datapages Combined Publications Database

Wyoming Geological Association

Abstract


Stratigraphy Of Wyoming; 31st Annual Field Conference Guidebook, 1980
Pages 117-135

Depositional Environments and Sand Body Morphologies of the Muddy Sandstones at Kitty Field, Powder River Basin, Wyoming

G. M. Byrd Larberg

Abstract

Lower Cretaceous Muddy sandstones form a stratigraphic trap at Kitty Field, Campbell County, Wyoming. Porosity and permeability are generally low, but the best reservoirs develop maximum effective porosity of 17% and maximum permeability of 442 md. Reservoir sandstones average less than 15 ft. and rarely exceed 30 ft. in thickness. Ultimate recovery from the field is estimated at 23 million barrels of oil.

Based on electric log character, four easily recognizable zones within the Muddy interval at Kitty were numbered 1 through 4 in descending order. Sedimentary structures, observed in slabbed cores and petrographic analyses of selected core samples, suggest that sandstones in the second, third, and fourth Muddy zones were deposited as part of a sequence associated with the overall transgression of the Lower Cretaceous sea. Fourth zone sandstones are fluvial in origin, and were deposited in lows on the unconformable surface of the underlying Skull Creek Shale. Flood plain deposits, lateral to well-developed channel sandstones, are suggestive of a low-lying coastal swamp progressively inundated by a transgressive sea. The second and third zone sandstones represent strandline deposits, which were reshaped during continued transgression. Offshore bar, beach, estuarine, lagoonal, and fluvial sandstones are recognized.

Isopach mapping of high resistivity sandstones within each zone suggests reservoir morphologies consistent with the depositional environments determined from core data. Fluvial sandstones of the fourth zone display a north-south lineation in a narrow band through the central portion of the field. Reworked strandline deposits in the second and third Muddy zones demonstrate a wide, linear trend northward with a secondary northeasterly trend superimposed. Modern analog, with the Atlantic continental shelf off the northeastern United States, suggests that this type of trend development results from reworking and redistribution of sediments in response to interaction of longshore, wind, and wave-generated bottom currents.


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