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Abstract

Detailed analyses of deeply-buried chalks from cores of both the Smoky Hill Chalk and Fort Hays Limestone Members of the Niobrara Formation were performed. The identified inorganic and organic constituents, and sedimentary structures helped to characterize the lithofacies, biofacies, burial diagenesis, bulk rock and clay mineralogies, and organic geochemistry of the Niobrara Formation in Weld County, Colorado.

Many depositional and diagenetic relationships are observed: 1) the chalks are comprised primarily of whole and fragmented coccoliths with subordinate planktonic foraminifera, inoceramid prisms, oysters, and other skeletal grains; 2) deposition was predominantly below wave-base in tens-to-hundreds of meters of water; 3) petrographic evidence supports the idea that subsequent burial, mechanical compaction, and pressure-solution (chemical solution-transfer) are the primary processes responsible for porosity reduction; 4) porosity of the chalk is inversely proportional to maximum depth of burial and to the amount of non-carbonate (acid-insoluble residue content) in the chalk; 5) total organic carbon content in the chalk is proportional to the amount of acid-insoluble residue and relative abundance of pyrite in the acid-insoluble fraction; and 6) mixed-layer illite/smectic clay as a geo-thermometer in bentonite beds from the Niobrara Formation indicate that these rocks are thermally mature with respect to oil generation.

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