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Abstract: Mechanical Stratigraphy of the Madison Limestone: Bighorn Basin, Wyoming and Montana

Mark D. Sonnenfeld

"Fracture-controlled" contributions to reservoir heterogeneity tend to be viewed as non-fabric selective in nature. Given such an outlook, predictions of fracture overprints rely upon awareness of extrinsic controls such as past and present stress-fields, structural curvature, and fault proximity. The hierarchical sequence stratigraphy of the 300 m Madison provides the stratigraphic framework necessary to characterize the vertical distribution of early, fabric-selective dolomite; additionally, this framework assists in discriminating between fabric-selective and non-fabric-selective styles of fracturing. Whereas lateral reservoir heterogeneity within the Madison is dominated by non-fabric-selective fracture-styles, karst collapse dolines, and highly erratic late nhydrite plugging, vertical reservoir heterogeneity reflects the complex, yet relatively deterministic layering of early dolomites coupled with fabric-selective fracture and karst overprints.

Single, or perhaps even limited combinations of intrinsic (petrologic/stratigraphic) rock parameters cannot confidently predict variations in fracture styles and intensity within thick, vertically heterogeneous intervals such as the Madison Limestone. Consequently stratigraphic cycles provide an effective initial scale of analysis in the quest for true mechanical stratigraphic units which are defined by common fracture styles. This approach integrates numerous intrinsic factors governing rock fracturing such as 1) mineralogic composition (re. rock strength), 2) prevailing grain size, 3) porosity, 4) lithology contrasts, and 5) bed thickness because these attributes are likely to show a greater degree of organization and predictability when considered in a sequence st atigraphic context.

This perspective emphasizes the stratigraphic control of structural styLe and fracture distribution The mechanical stratigraphic approach should be viewed as a framework for fracture prediction distinct from, yet complementary to evaluations of extrinsic controls on fracturing. Mechanical stratigraphic units derived from sequence stratigraphic building blocks tends to emphasize the vertical distribution of flexural slip in concentric folding. The vertical distribution of fractures and small-scale faulting spatially associated with intraformational detachment zones and preferred horizons of drag-folding is more predictable when stratigraphically controlled variations in flexural slip are understood. Finally, close attention to the hierarchy of stratigraphic cycles should enhan e our appreciation for comparable mechanical stratigraphic hierarchies that may help explain distinct yet spatially coexisting fracture styles and lengths.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada