Print this pageSequence Stratigraphic and Tectonic Control on Growth and Reservoir Development in Lodgepole Waulsortian Mounds, Stark County North Dakota
STURM, STEPHEN D., ROBERT A. RIPPLE, DAVID F. LIST, and LAURA S. FOULK
Detailed analysis of cores, borehole image logs and well logs from productive Waulsortian mounds were integrated with a regional sequence stratigraphic framework, establishing that mound growth was controlled by a combination of tectonics and relative sea level changes during lower Mississippian time. Mound growth spanned at least two third-order sequences, a time span of 1.6 to more than 3.4 my.
Mound growth occurred in two distinct phases: 1) a lower mound platform during which sedimentation rates and depositional dips were very low and which spanned more than one third-order sequence, and 2) an upper mound core phase during which sedimentation rates and depositional dips were higher and spanned less than a single third-order sequence. Mound growth initiated on a local paleo-high, in low to moderate energy conditions surrounded by basinal anoxic conditions. The paleo-high coincided with the base of a carbonate ramp and mounds appear to have initiated growth along a paleo-bathymetric contour controlled locally by faulting, regional dip and relative sea level. Mound growth is minimal on the crest of the paleo-high and thickest on the flanks of the feature.
Comparison of core and borehole image data indicate the lower mound platform is comprised low-angle (<5 degrees) bedsets of generally poor reservoir quality limestones. The upper mound core is characterized by thick sequences of steeply-dipping (>40 degrees), vuggy, microbial boundstone and lime rudstone separated by thin lime grainstones. The compartmentalized reservoir architecture appears to have been influenced by fourth-order variation in relative water depths.