Sedimentology and Reservoir Characteristics of the Niobrara Formation (Upper Cretaceous), Kansas and Colorado
Published:January 01, 1985
Peter A. Scholle, Richard M. Pollastro, 1985. "Sedimentology and Reservoir Characteristics of the Niobrara Formation (Upper Cretaceous), Kansas and Colorado", Rocky Mountain Carbonate Reservoirs: A Core Workshop, Mark W. Longman, Keith W. Shanley, Robert F. Lindsay, David E. Eby
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The Niobrara Formation of the Western Interior consists of 100 to 250 m of interbedded chalks and calcareous shales. The formation is divided into two members: (1) a thin, basal, dominantly chalk-bearing sequence, the Fort Hays Limestone Member, and (2) an upper, thick, calcareous shale unit, the Smoky Hill Chalk Member. Both units represent widespread pelagic to hemipelagic sedimentation in an epicontinental seaway during a relative highstand of sea level. During this highstand, the shoreline migrated westward and large areas of the Western Interior were covered by relatively deep water and received little terrigenous influx. Productivity of nanno- and microfossils was sufficiently great to yield moderately thick sequences of regionally homogeneous chalk and marl. These strata are very fine grained, have high primary porosity, and contain few macrofosstls other than bivalves.
Sedimentary structures in the Niobrara consist mainly of laminations, fecal pellets, and burrows. Chalk-shale depositional cycles are found at scales ranging from millimeters to tens of meters, Cyclic sedimentation may have been related to variations in climatic patterns which, in turn, influenced terrigenous sediment influx and (or) biological productivity in the region. Climatic fluctuations probably influenced the salinity of the surface waters in the Western Interior seaway by altering circulation patterns and basinal water turnover. This resulted in periods of dysoxic and even anoxic bottom-water conditions within the seaway. Stagnation events are reflected by intervals with virtually no benthic fauna or burrows and with good preservation of millimeter-scale laminations and abundant organic matter.
The primary properties of the chalks and calcareous shales of the Niobrara have been greatly modified by burial diagenesis. Increased burial brought about rapid reduction of porosity by a combination of mechanical and chemical compaction and associated calcite overgrowth cementation. Authigenic pyrite is widespread in association with organic matter. Clay minerals of detritai origin and altered voicanogenic material originally underwent transformation from disordered, predominantly smectitic mixed-layer clays to ordered, predominantly illitic mixed-layer clays. During diagenetic alteration, brittle chalks and calcareous shales were deformed and significant fracturing occurred. Finally, thermal maturation associated with burial led to hydrocarbon generation from the organic-carbon-rich chalks and calcareous shales of the Niobrara. In areas of shallow burial, formation of biogenic methane was widespread and has proven to be economically important.
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Rocky Mountain Carbonate Reservoirs: A Core Workshop
This core workshop was organized to give geologists from across the country and around the world the opportunity to see a wide variety of carbonate reservoirs as well as some carbonate source rocks from the Rocky Mountain region. Cores displayed at the workshop range in age from Cambrian to Cretaceous and come from a number of the major oil-producing basins in the Rocky Mountains. Depositional facies represented in the cores range from sabkhas and tidal flats through algal and coral buildups to relatively deep water chalks. Dolomite and evaporite minerals are important in approximately half the cores described; the others are dominantly limestone. Porosity of many different types is discussed. Diagenesis, or lack of it, has played a major role in forming virtually all the reservoirs. Thus, the workshop offers the chance to observe and study a wide variety of depositional and diagenetic textures in a number of economically important rock units.