Sedimentary Control from the Diagenetic Sequence Expression of Platform Carbonates and Its Impact from the Fracturation Pattern: The Example of the Madison Fm from Sheep Mountain (Wyoming, USA)
Carbonate reservoir properties are partly controlled by the initial sedimentary facies but are overall strongly modified during diagenesis and fracturing which are complex and interacting multi-stages processes, difficult to characterize and quantify. The aim of this study is to characterize relations between sedimentary facies, diagenesis and fracturation pattern.
In the Wyoming and the Montana, the Madison carbonate platform developed during the Mississippian into six third order sequences which outcrop in the core of the Laramide fold of Sheep Mountain (Wyoming). Petrographical and diagenetic analyses were carried out from field and laboratory data, along six meter-scale cycles specifically investigated within the two lower 3rd sequences.
At Sheep Mountain, these two sequences are organized in multiple high frequency cycles typical of (1) mid ramp environment; (2) storm reworking in subtidal conditions; (3) lagoonal environment (according to Sonnenfeld, 1996). Results from diagenetic and isotopic analyses show that (1) mudstones present an intense early dolomitization (proposed seepage-reflux processes); (2) oolithic grainstones display isopachous calcite cements with marine isotopic signatures; and (3) mud-supported facies (wacke- to packstones) are poorly impacted by early diagenesis. The shallow burial, and the deep burial diagenesis as well are characterized by recristallization processes (isotopic trend from −5 to −10‰ for δ18O and +3 to +1‰ for δ13C). During Laramide orogeny, the Madison Formation suffered from faulting, fracturing, and brecciation, associated with a late stage calcite precipitation, specifically found in the syn-orogenic set of fractures. Isotopic data (−20‰ for δ18O and −5 to −10‰ for δ13C) point to undersaturated meteoric ground waters, heated and re-injected in fractures during Laramide orogeny.
This work shows that: (1) the early diagenetic evolution was clearly controlled by the initial sedimentary facies; (2) a clear impact of the nature of carbonate on the fracture pattern is observed with a more diffuse, less persistent and regular pattern in the dolomite and a more localized and connected pattern in limestones; (3) density, shape and vertical persistence of fractures are influenced by the initial sedimentary facies and its diagenetic imprint; (4) macrofracturation is largely favored by the presence of the late-stage calcite.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009