--> --> A Hydrothermal Dolomite Model for the Cheyenne Wells / Smoky Creek Fields, Colorado

AAPG Rocky Mountain Section Meeting

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A Hydrothermal Dolomite Model for the Cheyenne Wells / Smoky Creek Fields, Colorado

Abstract

The adjacent Cheyenne Wells and Smoky Creek Fields, along the Las Animas Arch in southeastern Colorado, have together produced over 9.2 million bbls oil, largely from the Mississippian Spergen Fm. Hydrocarbons are produced from combination traps, where lime grainstones are dolomitized, resulting in enhanced porosity. Dolomitized zones are irregularly distributed across the field resulting in highly variable well recoveries. As a result of the Mississippian collision of the North and South American plates, the Mississippian section is densely block-faulted. Late Mississippian uplift of the blocks resulted in a paleotopography that provided the necessary relief for a well-developed karst topography on top of uppermost carbonate unit, the Mississippian St. Louis Fm and within the underlying carbonate units. This is evidenced by formation thickness changes shown in well logs. Previous workers documented that the paleotopography helped localize later overlying Morrow Formation channels. Sinkholes and caves in the Mississippian section may retain porosity or be sediment filled with porosity destroying Pennsylvanian sediments. Hydrothermal dolomitization is a porosity producing mechanism described for several fields in the United States and Canada. Dolomite porosity in the Mississippian carbonates at these fields is generated by this mechanism. With this model, basin sourced hydrothermal fluids rise along faults and through solution related features to create porosity in adjacent carbonates. Mineral indicators of this process include the presence of hydrothermal minerals such as saddle (baroque) dolomite, megaquartz and chalcedony. All are present in cores taken from the field complex. Fluid inclusion work on the cores confirms the alteration temperature range of hydrothermal dolomitization. Saddle dolomite fluid inclusions commonly range between 80 – 235 o C. Previous workers determined that saddle dolomite fluid inclusion data from cores in these fields give a temperature of 121.5–135 o C near the middle of the expected range. Two samples of megaquartz taken from cores provide fluid inclusion temperatures of 80–115 o C and 67.3–124.8 o C. The upper ends of these ranges correspond with published hydrothermal dolomite model homogenization temperatures for megaquartz.