AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Cyclic Nature of Intracratonic Versus Passive Margin Mudstone Systems – A Reservoir Perspective
(1) Colorado State University, Fort Collins, CO.
(2) US Geological Survey, Denver, CO.
(3) U.S. Geological Survey, Oklahoma City, OK.
Mudstone sedimentary architecture varies greatly between intracratonic basins and passive margins as observed by comparing the upper shale member (USM) of the Upper Devonian-Lower Mississippian Bakken Formation (Williston Basin, ND) with the Woodford Shale (Arbuckle Mountains, OK) of similar age. In both cases, the mudstone environment is the downslope extension of a carbonate or carbonate-siliciclastic shallow-marine system. For the USM, laminated silt-rich mudstones with lag deposits—the most proximal facies—grade progressively basinward into finer-grained laminated mudstones and, most distally, into quartz-rich radiolarian cherts intercalated with laminated mudstones. In contrast, the Woodford proximal facies consists of siliciclastic mudstones with a successively increasing amount of Tasmanites algae downslope, and, most distally, radiolarian cherts.
Both formations show distinct internal cyclicity but their expression and architecture are very dissimilar. The Woodford passive margin succession developed distinct decimeter-scale parasequences representing Milankovitch-type cyclicity with both well defined lowstand mudstones and highstand cherts; the Milankovitch signal far outpaced the overlying million-year cycle, which is reflected in the gradual increase and decrease of mudstone and chert package thickness. In contrast, the USM intracratonic basin succession shows only vaguely defined parasequences at best, but a well developed million-year signal is expressed by the preferential accumulation of highstand millimeter-thick cherts in the central part of the unit. However, USM cherts are intercalated with mudstones, thinner, and less numerous than Woodford cherts.
The strongly differing expression of cyclicities likely results from relative distances to sediment sources. The intracratonic Williston Basin was largely surrounded by land during deposition of the USM so that even during short-term sea-level highstands, strong storms delivered sediment to the most distal reaches, resulting in a weak expression of the Milankovitch signal. In contrast, short-term highstands completely shut off mud delivery to distal parts of the passive margin during Woodford deposition, enabling good preservation of Milankovitch cycles in the formation. Thus, passive margin successions are more likely than intracratonic successions to contain numerous and distinct cherts throughout, making passive margins successions more promising for favorable “fracing” characteristics.