Sedimentary and diagenetic controls on the multiscale fracturing pattern of a carbonate reservoir: The Madison Formation (Sheep Mountain, Wyoming, USA)

A multidisciplinary approach has been applied to characterize the Madison Formation in the Sheep Mountain anticline at different scales, in order to illustrate the relationship between sedimentology, diagenesis and fracturing pattern. Sedimentary facies are organized into three types of elementary facies sequences showing different vertical evolutions of their petrophysical properties. This is controlled by the double influence of the initial sedimentary facies and the diagenetic evolution. Three main sets of fractures (related to the Sevier and Laramide compressive phases and to the folding of Sheep Mountain) have been described at three orders of scale. At small scale, the Sevier-related set I and folding related set III are the most abundant. Set II, dated from the early stage of the Laramide compression, exhibits the highest fracture intensity. There is also a clear faciologic control on the fracture intensity. Small-scale mechanical units correspond to entire facies sequences when no or little petrophysical property contrasts between beds are observed. Conversely, small-scale mechanical units correspond to each bed of a facies sequence in the case of high contrasts of petrophysical properties between beds. At intermediate scale, set II is not present and sets I and III show the highest fracture intensities. The latter are partly controlled by the mechanical unit thickness. Intermediate and large-scale mechanical unit distribution is mainly controlled by the overall faciologic vertical stacking pattern and by the position across the Sheep Mountain anticline (forelimb or backlimb). Sheep Mountain outcrops provide a good spatial representation of the carbonate reservoir heterogeneities from the micro- to the field scale, illustrating the complexity of fractured-carbonate reservoirs.