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Abstract

Whole-rock and clay mineralogies of chalk, shale, and bentonite from the Smoky Hill Shale Member, Niobrara Formation near Pueblo and Florence, Colorado have been compared to mineralogies of this member in the Denver basin. Carbonate and noncarbonate mineralogies of chalk and marl from both areas are similar; however, there is less variation in the relative abundance of each phase throughout the Pueblo section. Acid-insoluble residues of chalks and marls primarily consist of clay minerals (particularly mixed-layer illite/smectite (I/S), discrete illite, and kaolinite) and quartz. Minor amounts of feldspar, pyrite, dolomite, and chlorite clay are also common. Bentonite beds typically consist of >90% clay minerals, about 5% quartz, and 5% feldspar. All bentonites were predominantly kaolinitic and crystal Iinity of kaolinite was var i abIe.

I/S clay from insoluble residues Is largely short-range ordered with an average composition of 65% illite layers and 35% smectite layers; some randomly interstratified I/S clay was also present in some samples. I/S clay from bentonite beds was randomly or short-range interstratified with compositions ranging from 30-60% illite layers. Some I/S clay In bentonites with 45-55% illite layers was defined as “transitionally” ordered; that is, the glycol-saturated clays showed characteristics of both random and short-range ordering. The percent illite layers and degree of ordering in I/S from bentonites along a 3 mile (5 km) continuous exposure of the Niobrara Formation at Pueblo, Colorado increased in a westward direction and downsection into the Florence basin, Indicating an increase in the extent of diagenetic reaction of I/S clay in this direction. The composition and degree of ordering of I/S clay in bentonites indicates that maximum pa Ieotemperatures were about 70°–100° C and, therefore, these rocks are marginally mature with respect to oil generation.

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