Dolomitization of the Mississippian Leadville Reservoir at Lisbon Field, Paradox Basin, Utah

David E. Eby¹, Thomas C. Chidsey², Craig D. Morgan², Kevin McClure², John D. Humphrey³, Joseph N. Moore⁴, Louis H. Taylor⁵, and Virginia H. Weyland^6
1 Eby Petrography & Consulting, Inc, Littleton, CO
² Utah Geological Survey, Salt Lake City, UT
³ Colorado School of Mines, Golden, CO
⁴ Energy & Geoscience Institute, Salt Lake City, UT
⁵ Standard Geological Services, Littleton, CO
⁶ U.S. Department of Energy, Tulsa, OK

The Mississippian Leadville Limestone in Lisbon field of the northern Paradox Basin, Utah, has produced nearly 51.8 MMBO and 769 BCFG. The trap is an elongate, asymmetrical, northwest-trending anticline with nearly 600 m of structural closure. The field is bounded on its northeast flank by a major, basement-involved normal fault with nearly 760 m of displacement. In addition, multiple northeast-trending normal faults dissect the Leadville reservoir into segments. Several of the best producing wells are located close to these faults.

The Leadville Limestone was deposited as an open-marine, carbonate-shelf system highlighted with crinoid banks, peloid/oolitic shoals, and small Waulsortian mounds. Two major types of dolomite have been observed: (1) tight “stratigraphic” dolomite consisting of very fine grained (<5 µm), interlocking crystals that faithfully preserve depositional fabrics; and (2) porous, coarser (>100-250 µm), rhombic and saddle crystals that discordantly replace limestone and earlier “stratigraphic” dolomite. Predating or concomitant with late dolomite formation are pervasive leaching episodes that produced vugs and extensive microporosity. Solution-enlarged fractures and autobreccias are also common. Pyrobitumen and sulfide minerals appear to coat most crystal faces of the rhombic and saddle dolomites.

Stable carbon and oxygen isotope data indicate that all Lisbon Leadville dolomites were likely associated with brines whose composition was enriched in ¹⁸O compared with late Mississippian seawater. Oxygen isotope data constrain temperatures of the second dolomitizing event to >90°C. Fluid inclusions in calcite and dolomite display variable liquid to vapor ratios suggesting reequilibration at elevated temperatures. Fluid salinities exceed 10 weight percent NaCl equivalent.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005