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.