Deposition, stratigraphy, provenance, and reservoir characterization of carbonate mudstones: the Three Forks Formation, Williston Basin

Abstract

This thesis uses a drill core database from a 250 square mile area of North Dakota and Montana to evaluate the depositional environment, stratigraphic development, provenance, and reservoir quality of the Famennian Three Forks carbonate mudstones. Depositional environments and lithofacies include the following. Schizohaline, storm dominated intrashelf deposits defined by disturbed claystone, dolomudstone, sandstone, laminated mudstone, and distorted to brecciated dolomudstone lithofacies. These lithofacies often contain syneresis cracks, desiccation cracks, high depositional rate features such as convolute bedding and load features, and many scours. Ichnofacies vary from fairweather mobile and sessile feeding traces, early post-storm event escape traces, post-event opportunistic colonizers, and a final return to fairweather colonies. Arid shallow shelf deposits form during times of minimal fluvial and resultant minimal siliciclastic input into the basin, as well as times of evaporation conducive to broad salina, evaporative gypsum deposits. Mudflats rim these two shelf environments. During arid times they commonly contain abundant syndepositional nodular anhydrites and during less arid times they are full of intraclasts that range in abundance and size. Significant autogenic organization of these deposits is inferred to result in high frequency facies variability due to geomorphologic buildups. A major allogenically controlled shift progresses upwards in the stratigraphy as the deposits decrease in evidence of aridity and increase in evidence of fluvial inputs. Detailed correlations and calibrations to regional biostratigraphic studies delineate seven third-order transgressive-regressive cycles within the formation. Calibration to the biostratigraphic timescale facilitates comparison of [superscript 87]Sr/[superscript 86]Sr data to eustatic trends. These data indicate freshwater inputs that also provide more radiogenic Sr than the well-mixed global curve. Additionally [delta][superscript 34]S values across chronostratigraphically equivalent Famennian deposits in western North America corroborate this conclusion. Quantitative mineralogy trends, siliciclastic grain size comparisons, and [epsilon][subscript Nd] values support a primary fluvial system of sediments derived and progressively recycled from the Ellesmerian and Caledonian orogens north of the Canadian Shield as the primary siliciclastic sources. Some petrographic trends however do suggest some ephemeral drainage behavior from multiple directions surrounding the basin. Gypsum converted to anhydrite was precipitated directly from the fluvially-modified seawater indicated by radiogenically enriched [superscript 87]Sr/[superscript 86]Sr values. Dolomites predominantly formed as automicrites through freshwater mixing and meso- to hypersaline reflux based on elevated [superscript 87]Sr/[superscript 86]Sr values, fluvial input evidence including syneresis and sand pulses, ichnofacies and bedded evaporites. Automicrite nucleated through biological processes attributed to unique chemical conditions in the intrashelf basin. Details regarding the geologic formation of these rocks have significant implications for understanding controls on reservoir matrix characteristics. Heterogeneous samples with smaller grain sizes, variable sedimentary structures, or clay mottles consistently have superior reservoir quality than larger siliciclastic grain sizes, or homogenous textured samples. Subtle differences in pore sizes, shapes and resultant interconnectivity determine the potential for matrix oil saturation. Mercury porosimetry and nitrogen adsorption and desorption data indicate that for oil saturation to occur in this formation the reservoir matrix requires either high volumes of moderately sized nanopores with moderate connectivity or a moderate volume of large nanopores with some connectivity. Oil saturations in matrix with reservoir bitumen or in poor matrix quality that does not meet these criteria may be difficult to actually produce.