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Abstract

The Cambrian through Mississippian stratigraphic sequence of central Colorado comprises a thin seqnence (300-400 m thick) of interbedded quartz sandstones, carbonates, and minor shales separated by several unconformities. Extensive Paleozoic dolomitization of the carbonate units has occurred, increasing porosity and permeability in the Mississippian Leadville Formation. Karst solntion features, integrated cave systems with collection sinkholes and cutters, subhorizontal channels, and outlet chimneys, developed in the Leadville Formation and, to a lesser extent, in the Devonian Gilman and Dyer Formations beneath a regionally extensive karst erosion surface of Late Mississippian age. Although the sediments were deposited on a relatively stable cratonic shelf, several large-scale tectonic features, the Gore Range and ancestral Front Range-Wet Mountain uplift areas, were active along their bounding faults, as were the Homestake shear zone and other intrabasinal faults. These structures influenced sedimentary patterns and facies throughont the early and middle Paleozoic.

Pennsylvanian and Permian strata of central Colorado record a history of active basin subsidence along nnmerous faults and contemporaneous sedimentation of as much as 5,000 m of noumarine to marine strata. Central Colorado was broken into a mosaic of fault blocks, on which recurrent tectonic activity produced the sedimentary and structural basin of the northern Central Colorado trough, including many adjacent mountainous erosional areas, the ancestral Uncompahgre and Front Range uplifts, the basin-center ancestral Sawatch uplift, and many other uplifts within the basin. Abrupt thickness variations and facies changes, onlap and overlap relationships, and occasionally, angular discordances occur adjacent to faults and uplift blocks. Local volcanism apparently occurred in the Pennsylvanian using the intersection area of the Gore and Mosquito faults as a conduit.

Organic-rich source rocks occur abundantly in the Lower and Middle Pennsylvanian strata of the Belden, Gothic, Minturn, and Eagle Valley Formations. Thermal modeling of source-rock maturation data shows that the geothermal gradient locally within the rift basins in central Colorado during the Pennsylvanian and Early Permian was anomalously high (40°-48°C/km). Oil generation from Belden source rocks commenced in the Pennsylvanian and Early Permian and, in the local anhydrite-salt basin grabens of thickest late Paleozoic sediments, oil generation probably terminated in the late Paleozoic. Organic-rich mudstones, shales, and gypsum of the Minturn and Eagle Valley Formations were heated to the point of significant methane generation in the late Paleozoic. The maximum thermal maturity of Pennsylvanian strata was achieved during deepest burial near the end of the Cretaceous, prior to the uplift and erosion caused by Laramide and Tertiary tectonic activity and intrusion of the Pando Porphyry 72 Ma.

Major porosity reduction and water expulsion from the Pennsylvanian fine-grained sediments, source rocks, and gypsum and the associated development of impermeability occurred during the first 1,000 to 2,000 m of burial in the Pennsylvanian. With continued sedimentation and burial in the late Paleozoic, large volumes of organic acids, oil, and gas were generated from the solid kerogen of the organic-rich source rocks. It is likely that the significant volume expansion created overpressured hydraulic conditions. Fracturing of the impermeable source rocks probably provided hydrologic continuity with intrabasinal and basin margin faults and fluid migration along the faults to underlying and overlying permeable strata. The confining nature of the thick sequence of generally impermeable overlying strata and the excellent permeability of the underlying Leadville Formation with its karst solution features suggest that large volumes of basinal fluids, hydrothermal organic acid-rich waters and hydrocarbons, may have moved from the overpressured source rocks into and through the underlying Leadville strata.

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