Abstract: Unraveling the Multiple Origins of Heterogeneity Within Lower Mississippian Madison Reservoirs: Bighorn Basin, Wyoming and Montana, USA

Mark D. Sonnenfeld

"Fracture-controlled" and "karst-controlled" contributions to reservoir heterogeneity tend to be viewed as non-fabric selective in nature. Given such an outlook, predictions of fracture and karst overprints depend on an awareness of extrinsic controls such as past and present stress-fields, structural curvature, fault proximity, and the positions and movements of paleo-water tables. Frequently this leads to a more stochastic view of reservoir heterogeneity than is necessary, especially given limited 1-D to 2-D control. The hierarchical sequence stratigraphy of the 300 m Madison provides the stratigraphic framework necessary to characterize the vertical distribution of early, fabric-selective platformal dolomite; additionally, this framework assists in discriminat ng between fabric-selective and non-fabric-selective styles of karst and fracturing. Whereas lateral reservoir heterogeneity within the Madison is dominated by non-fabric-selective fracture-styles, karst collapse dolines, and highly erratic late anhydrite plugging, vertical reservoir heterogeneity reflects the complex, yet relatively deterministic layering of early dolomites with fabric-selective fracture and karst overprints.

In the case of Madison karst, early meteoric lithification and subtle Mississippian tectonics resulted in a vertically oriented fracture-controlled karst on top of the Madison, yet this non fabric-selective system channeled waters into several fabric-selective, regionally widespread solution collapse zones and cave systems. The horizontally oriented regional dissolution was stratigraphically controlled by soluble evaporitic zones and/or argillaceous aquitards overlying intra-Madison sequence boundaries rather than occurring at various unconfined water-table stillstands. Evaporite solution collapse breccias presently form partial to complete barriers to vertical fluid flow depending on thickness and degree of associated argillaceous influx, while cave-roof "fracture breccias" ere preferential sites of late dolomitization within the giant Elk basin Madison reservoir.

In the case of Madison fracturing, stratigraphic cycles of several scales provide effective scales of analysis in the quest for true mechanical stratigraphic units defined by common fracture styles. This perspective emphasizes the stratigraphic control of structural style and fracture distribution and integrates numerous intrinsic factors governing rock fracturing such as 1) mineralogy, 2) prevailing grain size, 3) porosity, 4) bed thickness, and 5) lithology contrasts because these attributes are likely to show a greater degree of organization and lateral predictability when considered in a stratigraphic context. Mechanical stratigraphic units derived from sequence stratigraphic building blocks tend to emphasize the vertical distribution of flexural slip in concentr c folding, thereby enhancing prediction of permeability enhancement due to fractures that are spatially associated with intraformational detachment zones and preferred horizons of drag-folding.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France