Facies, stratigraphic architecture, and faults - The controls on the cement distribution in the Devonian Sappington Formation in southwestern Montana

Abstract

Diagenesis has a profound effect on the porosity and permeability of sediments, and it is necessary to holistically evaluate the diagenetic history in the context of depositional, stratigraphic, and structural frameworks. This outcrop study from the late Devonian to early Mississippian Sappington Formation of southwestern Montana, USA, investigates the complex interplay of these different geologic factors and their controls on the distribution of diagenetic alterations.Our results suggest that the principal control on the distribution of diagenetic minerals is the thickness of the Lower, Middle, and Upper Sappington Shale Members, respectively. The sedimentary architecture plays a secondary role as clinoform boundaries and stratigraphic surfaces compartmentalized fluid flow. Structural features are of local importance and depositional facies, other than the previously mentioned shales, display little effect on the distribution of diagenetic minerals.Six dominant cement types were recognized in the Sappington Formation. Quartz and clay cements are the earliest cements and formed on the surfaces of detrital grains during early diagenesis and are relatively minor phases (<5% vol). These are postdated by a series of carbonate cements that dominate the diagenetic mineral assemblage (>60% vol) and include two types of dolomite, and two types of calcite. Dolomite is the most abundant diagenetic mineral and is observed as zoned, euhedral to subhedral rhombs with a nearly stoichiometric core and a series of ferroan rims and as subhedral to anhedral ferroan rhombs without zonation. Calcite cements are most common as a rim on the outer extents of dolomite rhombs but also occur as a replacement of dolomite or detrital feldspar.The outcrop-based model presented here – the first comprehensive diagenetic model of the Sappington Formation in SW-Montana, USA – can serve as a guide to hydrocarbon development in tight hybrid plays, such as the Bakken Formation in the Williston Basin, USA. It provides a more holistic understanding of the relationship between the depositional architecture and the diagenetic alterations and helps to decrease uncertainty in the second and third dimensions.