Browsing by Subject "Depositional systems"
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Item Depositional environment, diagenesis and reservoir quality of the middle Bakken member in the Williston Basin, North Dakota(2016-08) Ayhan, Oguzhan; Fisher, W. L. (Williams Lawrence), 1932-; Hammes, Ursula; Kerans, CharlesThe Upper Devonian-Lower Mississippian Bakken Formation in the Williston Basin is an important source rock for oil production in North America. The Bakken Formation is comprised of three units: Upper and Lower Bakken black shales and Middle Bakken Member. Upper and Lower Bakken shales are high quality source rocks which source reservoirs in the Middle Bakken, Upper Three Forks and lower Lodgepole Formations. The Middle Bakken Member, consisting of predominantly gray, silty and sandy dolostone, is under investigation in this study. The goals of this study are to determine the regional distribution of lithofacies and depositional environments of the Middle Bakken Member and explain diagenetic sequence and reservoir quality parameters in the Williston Basin. The reservoir quality of the Middle Bakken Member is mainly influenced by mineralogical composition and cementation resulting in low porosity and permeability and linked to lithofacies distribution in the basin. Dolomitization is pervasive throughout the unit, and also occurs as dolomite cement. Moreover, cementation occurred including quartz overgrowths, K-felspar, clay cement and pyrite as both cement and nodules. Not only dolomitization but also pyrite cementation plays an important role in reducing pore space in the reservoir. The pore types that were identified are intergranular, intragranular, fracture and moldic pores. Secondary intragranular porosity generally resulted from dissolution of biogenic fragments and dissolution of other unstable minerals including feldspar and dolomite. In this study, five lithofacies and one sandy interval within lithofacies C were described throughout the North Dakota portion of the Williston basin. The sandy interval in Lithofacies C was interpreted as bars or channel fills, which differentiates this study from previous studies in terms of core description. N-S, W-E, NE-SW, NW-SE oriented cross-sections drawn via cores suggest that the lithofacies of the Middle Bakken Member pinch out towards the edges. However, the anticlines in the basin affect their thickness distributions. Sandy interval in Lithofacies C reaches its thickest succession in the center of the basin. Lithofacies C and D consist of up to 80% of dolomite although the other lithofacies consist of relatively lower dolomite (up to 65%). While well logs indicate 4-8% of porosity, point-counting results show up to 5% of porosity. The sequence of diagenetic events in the North Dakota portion of the Williston Basin is from youngest to oldest: micritization, mechanical and chemical compaction, calcite cementation, dolomitization, pyrite cementation, microcrystalline quartz cementation, syntaxial calcite overgrowth, quartz overgrowth, K-Feldspar overgrowth, dolomite dissolution, feldspar dissolution, dedolomitization, fracturing, anhydrite cementation and hydrocarbon migration.Item Regional character of the lower Tuscaloosa formation depositional systems and trends in reservoir quality(2012-12) Woolf, Kurtus Steven; Wood, Lesli J.For decades the Upper Cretaceous Lower Tuscaloosa Formation of the U.S. Gulf Coast has been considered an onshore hydrocarbon play with no equivalent offshore deposits. A better understanding of the Lower Tuscaloosa sequence stratigraphic and paleogeographic framework, source-to-sink depositional environments, magnitude of fluvial systems, regional trends in reservoir quality, and structural influences on its deposition along with newly acquired data from offshore wells has changed this decades-long paradigm of the Lower Tuscaloosa as simply an onshore play. The mid-Cenomanian unconformity, underlying the Lower Tuscaloosa, formed an extensive regional network of incised valleys. This incision and accompanying low accommodation allowed for sediment bypass and deposition of over 330 m thick gravity-driven sand-rich deposits over 400 km from their equivalent shelf edge. Subsequently a transgressive systems tract comprised of four fluvial sequences in the Lower Tuscaloosa Massive sand and an overlying estuarine sequence (Stringer sand) filled the incised valleys. Both wave- and tide-dominated deltaic facies of the Lower Tuscaloosa are located at the mouths of incised valleys proximal to the shelf edge. Deltaic and estuarine depositional environments were interpreted from impoverished trace fossil suites of the Cruziana Ichnofacies and detailed sedimentological observations. The location and trend of valleys are controlled by basement structures. Lower Tuscaloosa rivers were 3.8m – 7.8m deep and 145m – 721m wide comparable to the Siwalik Group outcrop and the modern Missouri River. These systems were capable of transporting large amounts of sediment indicating the Lower Tuscaloosa was capable of transporting large amounts of sediments to the shelf edge for resedimentation into the deep offshore. Anomalously high porosity (>25%) and permeability (>1200md) in the Lower Tuscaloosa at stratigraphic depths below 20,000 ft. are influenced by chlorite coating the detrital grains. Chlorite coatings block quartz nucleation sites inhibiting quartz cementation. Chlorite coats in the Lower Tuscaloosa are controlled by the presence and abundance of volcanic rock fragments supplying the ions needed for the formation of chlorite. Chlorite decrease to the east in sediments derived from the Appalachian Mountains. An increase in chlorite in westward samples correlates with an increase of volcanic rock fragments derived from the Ouachita Mountains.