DOLOMITIZATION OF THE MISSISSIPPIAN LEADVILLE LIMESTONE, PARADOX BASIN, SOUTHEASTERN UTAH

Various types of dolomitization and other diagenetic events resulted from a combination of diverse facies and a complex structural configuration in the Mississippian Leadville Limestone of southeastern Utah. The Leadville was deposited as an open-marine, carbonate-shelf system highlighted with crinoid banks, peloid/oolitic shoals, and small Waulsortian mounds. It is separated from overlying Pennsylvanian rocks by a major unconformity where a karst surface developed. The Leadville produces hydrocarbons at Lisbon and other fields in the Paradox Basin of southeastern Utah. Lisbon field is an elongate, asymmetrical, northwest-trending anticline with nearly 600 m of structural closure. The structure 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 into segments.

Two major types of diagenetic dolomite are observed in Leadville cores from Lisbon field: (1) tight “stratigraphic” dolomite consisting of very fine grained (<5 mm), interlocking crystals that faithfully preserve depositional fabrics; and (2) porous, coarser (>100-250 mm), 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 (50°C). Fluid salinities exceed 10 weight percent NaCl equivalent.