The Mississippian Leadville Limestone Exploration Play, Utah and Colorado-Exploration Techniques and Studies for Independents

The Mississippian (late Kinderhookian to early Meramecian) Leadville Limestone is a shallow, open-marine, carbonate-shelf deposit. The Leadville has produced over 53 million barrels (8.4 million m{sup 3}) of oil/condensate from seven fields in the Paradox fold and fault belt of the Paradox Basin, Utah and Colorado. The environmentally sensitive, 7500-square-mile (19,400 km{sup 2}) area that makes up the fold and fault belt is relatively unexplored. Only independent producers operate and continue to hunt for Leadville oil targets in the region. The overall goal of this study is to assist these independents by (1) developing and demonstrating techniques and exploration methods never tried on the Leadville Limestone, (2) targeting areas for exploration, (3) increasing deliverability from new and old Leadville fields through detailed reservoir characterization, (4) reducing exploration costs and risk especially in environmentally sensitive areas, and (5) adding new oil discoveries and reserves. The final results will hopefully reduce exploration costs and risks, especially in environmentally sensitive areas, and add new oil discoveries and reserves. The study consists of three sections: (1) description of lithofacies and diagenetic history of the Leadville at Lisbon field, San Juan County, Utah, (2) methodology and results of a surface geochemical survey conducted over the Lisbon and Lightning Draw Southeast fields (and areas in between) and identification of oil-prone areas using epifluorescence in well cuttings from regional wells, and (3) determination of regional lithofacies, description of modern and outcrop depositional analogs, and estimation of potential oil migration directions (evaluating the middle Paleozoic hydrodynamic pressure regime and water chemistry). Leadville lithofacies at Libon field include open marine (crinoidal banks or shoals and Waulsortian-type buildups), oolitic and peloid shoals, and middle shelf. Rock units with open-marine and restricted-marine facies constitute a significant reservoir potential, having both effective porosity and permeability when dissolution of skeletal grains, followed by dolomitization, has occurred. Two major types of diagenetic dolomite are observed in the Leadville Limestone at Lisbon field: (1) tight 'early'dolomite consisting of very fine grained (&lt;5 {micro}m), interlocking crystals that faithfully preserve depositional fabrics; and (2) porous, coarser (&gt;100-250 {micro}m), rhombic and saddle crystals that discordantly replace limestone and earlier very fine grained dolomite. Predating or concomitant with late dolomite formation are pervasive leaching episodes that produced vugs and extensive microporosity. Most reservoir rocks within Lisbon field appear to be associated with the second, late type of dolomitization and associated leaching events. Other diagenetic products include pyrobitumen, syntaxial cement, sulfide minerals, anhydrite cement and replacement, and late macrocalcite. Fracturing (solution enlarged) and brecciation (autobrecciation) caused by hydrofracturing are widespread within Lisbon field. Sediment-filled cavities, related to karstification of the exposed Leadville, are present in the upper third of the formation. Pyrobitumen and sulfide minerals appear to coat most crystal faces of the rhombic and saddle dolomites. The fluid inclusion and mineral relationships suggest the following sequence of events: (1) dolomite precipitation, (2) anhydrite deposition, (3) anhydrite dissolution and quartz precipitation, (4) dolomite dissolution and late calcite precipitation, (5) trapping of a mobile oil phase, and (6) formation of bitumen. Fluid inclusions in calcite and dolomite display variable liquid to vapor ratios suggesting reequilibration at elevated temperatures (50 C). Fluid salinities exceed 10 weight percent NaCl equivalent. Low ice melting temperatures of quartz- and calcite-hosted inclusions suggest chemically complex Ca-Mg-bearing brines associated with evaporite deposits were responsible for mineral deposition. The overall conclusion from these analyses indicates late dolomitization, saddle dolomite, and dolomite cement precipitation, as well as sulfides and brecciation, may have developed from hydrothermal events that can greatly improve reservoir quality. The result can be the formation of large, diagenetic-type, hydrocarbon traps. The reservoir characteristics, particularly diagenetic overprinting and history, can be applied regionally to other fields and exploration trends in the Paradox Basin. Stable carbon and oxygen isotope data indicate that all Lisbon field Leadville dolomites were likely associated with brines whose composition was enriched in {sup 18}O compared with Late Mississippian seawater. The Leadville replacement dolomite's temperatures of precipitation ranged from about 140 to 194 F ({approx} 60 to 90 C). Saddle dolomite cements were precipitated at temperatures greater than 194 F (&gt;90 C).

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