--> ABSTRACT: Depositional Facies and Porosity Distribution in the Mississippian Sherwood Interval, Wabek Field, North Dakota, by Mark W. Longman, Mark Odegard, Susan Landon, and Mark Williams; #90906(2001)

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Mark W. Longman1, Mark Odegard2, Susan Landon3, Mark Williams2

(1) Consulting Geologist, Denver, CO
(2) Whiting Petroleum Company, Denver, CO
(3) Thomasson Partner Associates, Denver, CO

ABSTRACT: Depositional Facies and Porosity Distribution in the Mississippian Sherwood Interval, Wabek Field, North Dakota

Wabek field (Mountrail and Ward counties, ND) was discovered in mid-1985 and produced 5.71 MMBO, 3.9 BCF, and 2.9 MMBW through June, 2000, from 26 wells. Cores and wireline logs from the 15-m-thick Sherwood (Mississippian Mission Canyon) reservoir in the field reveal that the trap is a classic "shoreline trend" facies change. Blanket-like shallow-water coated- grain packstones and grainstones grade updip into lagoonal dolomite mudstones and then into massive anhydrites across a distance of about 1 km.

The best reservoir zone in Wabek field occurs about 6 m below the top of the Sherwood and has 7 to 15% porosity with 1 to 100 mD permeability. This zone becomes thinner downdip (westward), decreasing from 5 m to <2 m. In contrast, the tight limestones above and below this porous interval, which are coated-grain grainstones and packstones cemented with calcite and anhydrite, thicken westward from 1 m to >5 m. The reciprocal wedge-shaped geometries of the porous and tight zones in the reservoir are not due to changes in depositional facies, but rather to cementation processes. Production is mainly from preserved primary interparticle porosity that has been slightly enhanced during late-stage dissolution. Some vertical hairline fractures are also present.

Although the grainstones and packstones in the Sherwood "shoreline" fields have traditionally been interpreted as shoals or bar deposits, we see no evidence for positive relief on the sea floor. Instead, we interpret the grainstones as ramp deposits periodically affected by storms. The mixed grain types (peloids, ooids, pisoids, vadoids) in most beds, the poor sorting of the grains, the planar bedding with essentially no crossbedding, the common mud drapes, and the common geopetal micrite between the grains all argue against shoals being important in the Sherwood reservoir at Wabek field.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado