A Petrographic Study of the Three Forks Formation (Upper Devonian), Williston Basin, North Dakota: Based on Thin Section Analysis, XRD and SEM

Deeply buried below 8,000？ft, the Three Forks Formation in North Dakota displays a variety of interesting diagenetic characteristics including dolomitization and hematite precipitation. Samples from three lithofacies are analyzed by thin section and SEM petrography and combined bulk and clay XRD analyses. Key aspects controlling the porosity and permeability of this formation are revealed by focusing on the detail mineralogy, rock type and diagenetic mineral distribution, and overall reservoir quality. Results prove that the Three Forks mineralogy is dominated by dolomite, along with substantial hematite, monocrystalline quartz, and mica flakes with trace feldspar, calcite, and pyrite. Clays mainly consist of illite together with minor chlorite and kaolinite and are associated with the scattered clasts. The reservoir quality is controlled by intercrystalline, rare microvuggy, and microporosity types that result from diagenetic and depositional events. Three stages of the dolomitization process are identified and discussed. Our hypothesis is that dolomitization commenced soon after deposition and was pervasive as no original carbonate texture is detectable. 1. Introduction The Three Forks Formation exists within the Williston Basin, occupying nearly two-thirds of North Dakota, underlying the Lower Mississippian Bakken Formation, and overlying the Upper Devonian Birdbear Formation (Figure 1). The Three Forks Formation is one of the most unpredictable petroleum systems within the Williston Basin with a combination of carbonates, clays, organics, and detrital material of poor to moderate reservoir quality, reflecting its depositional and postdepositional fabric. This study serves as an attempt to provide critical information on the prospectivity, in terms of key diagenetic processes that operate within the Three Forks, based on a derived dataset of results from thin section analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The aims of this study are to determine the following: (1) mineralogical composition, (2) differences in texture, (3) framework fabric, (4) sequence of diagenetic events, (5) pore system, and (6) reservoir quality of the Three Forks in three identified lithofacies [1]. Close-up photomicrographs are provided to stress (1) preserved primary and secondary pore types, (2) cements, (3) sedimentary structures, (4) fractures, and (5) pore plugging hematite. A brief summary of sample information, rock type, lithology, thin section porosity, analysis types, original texture, and reservoir quality are presented as Table 1.