MORGAN, WILLIAM A.¹, MARK K. SHEEDLO¹, WAYNE M. AHR², DAVID L. BREWSTER³, and SUSAN W. YOUNG^3
1Conoco Inc., Houston, TX
²Texas A&M University, College Station, TX
³Conoco Inc., Midland, TX

Abstract: Sequence Stratigraphic Framework, Sedimentology, and Diagenesis of Waulsortian Mounds of the Lodgepole Formation (Mississippian) in the Dickinson Field Complex, Williston Basin, North Dakota

Waulsortian mounds and closely associated facies in the Dickinson field complex contained approximately 87 MMBOOIP when the field was discovered in 1993 and have produced 14 MMBO through March, 1998. The mounds are located within the oldest of two 3rd order (2+ My each) sequences that compose the Lodgepole Formation in the Williston Basin. The Lodgepole ranges upwards of 300 m in thickness and forms part of the trangressive phase of the 16 My Madison Group 2nd order sequence. The two 3rd order sequences comprise 9-10 ramp-dominated, 4th order (400 Ky) sequences. Mound complexes can be identified on 3D seismic data using 4 seismic criteria, but 4th order sequences are too thin to be seismically imaged. Consequently, detailed well-log correlations are necessary to identify the sequence hierarchy of the Lodgepole in the subsurface.

The Dickinson mounds appear to have nucleated on a subtle paleohigh that was situated some 80 km basinward of the toe-of-slope, unlike other mounds in the Williston basin which grew within a few kms of the toe-of-slope. An individual Dickinson area mound is typically 100 m thick. As inferred from seismic data, the smallest mounds, which are 800 m in diameter, coalesced to form circular and loaf-shaped complexes as large as 2300 m by 7500 m.

Mounds consist mainly of fenestrate bryozoan cementstones and microbial-peloidal micrite. Bioclastic mud/wackestones and skeletal grain/packstones are common as detrital interbeds. Neither shallowing-upward trends nor evidence of subaerial exposure were observed. Porosity within the mounds is dominated by fractures and diagenetically enhanced depositional porosity that includes stromatactis vugs and interparticle porosity in grainstones. Fractures appear to have formed in response to 1) slope failure during deposition, 2) differential compaction in and around mounds and, 3) tectonic stresses. Average porosity in the mounds is 5%, and reservoir permeability ranges from approximately 200 to 2000 md.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas