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Regional Diagenesis of the Mississippian Leadville Limestone: Southwestern Colorado


The regional trends in porosity and permeability in the Mississippian Leadville Limestone are of great importance to the energy industry currently producing from this unit in Southwestern CO, and Southeastern UT. Here we compare diagenetic trends from a core in a successful CO2 well in SW Colorado (Well YG-2 McElmo Dome Field) with those from a 65 mile transect of 13 measured stratigraphic sections stretching from Ouray, CO to Rockwood Quarry, 15 miles northeast of Durango, CO. In surface stratigraphic sections, diagenetic features include several phases of recrystallization, dolomitization, silicification, dissolution, cementation, stylolitization, compaction and multiple stages of fracturing. Low porosity (0-10%) and permeability are attributed late stage pore occluding blocky spar calcite, complete recrystallization into dolomite, and micritization. Fracture controlled porosity provided pathways for fluids which precipitated dolomite and later blocky calcite spar, and also recrystallized adjacent rock. Many fractures have been completely annealed with blocky spar calcite or filled with fine to medium crystalline dolomite, but rare, partially open iron-stained fractures occur. Hydrothermal dolomite occurs in both matrix and fractures and is most typical adjacent to the Silverton Caldera and other late Cenozoic intrusives. Minor intracrystalline micro-porosity in dolomite along stylolites has produced less than 1% porosity in the rock. In the YG-2 core, located fifty miles west in a more down-dip position, porosity reaches an astounding 20-50%. Both intercrystalline and moldic porosity occur in medium to coarsely crystalline dolomite in the lower portion of the Leadville Limestone in this core. Using image analysis software on thin sections, we can demonstrate that dolomitization fronts have recrystallized a previously very fine to fine crystalline dolomite (with 25% intercrystalline porosity) into a fine-medium crystalline dolomite (with ∼40-60% intercrystalline and moldic porosity). Up section in the middle and upper Leadville Limestone, pore-filling blocky spar calcite reduced porosity to only 1-10%. Other diagenetic features in the core include recrystallization, dolomitization, cementation, compaction, micritization and fracturing. The lack of porosity in the up-dip outcrop sections may be attributed to either more cementation during prolonged subaerial exposure, or to more recent alteration by modern groundwater in near-surface conditions.