The Mississippian Leadville Limestone Exploration Play, Utah and Colorado

Chidsey, Thomas C.¹, Craig D. Morgan¹, Kevin McClure¹, David E. Eby^2
1 Utah Geological Survey, Salt Lake City, UT
² Eby Petrography & Consulting, Inc, Littleton, CO

The Mississippian Leadville Limestone has produced over 826 BCF of gas and 53 million barrels of oil from six fields in the northern Paradox Basin of Utah and Colorado. This 7,500-mi2 area is relatively unexplored; only about 100 wells penetrate the Leadville (less than one well per township), thus the potential for new discoveries remains great. Gas and oil are produced from basement-involved, northwest-trending structural traps with closure on both anticlines and faults.

The Leadville Limestone is a shallow, open marine, carbonate-shelf deposit that thins from more than 700 ft in the northwest corner of the Paradox Basin to less than 200 ft in the southeast corner. Crinoid banks or mounds, dolomitized in the lower and part of the upper sections, accumulated in shallow-water environments on upthrown fault blocks or other paleotopographic highs. Initial intercrystalline and moldic reservoir porosity averages 6-8%. Reservoir quality is greatly improved by natural fracture systems.

To assist increased industry activity in the Leadville Limestone play by independent operators, the Utah Geological Survey has begun a study to: (1) develop and demonstrate techniques and exploration methods never tried on the Leadville (such as identifying potential surface geochemical anomalies); (2) provide the facies, hydrodynamic pressure regime, and oil show quality maps that will be used to target areas for exploration; (3) increase deliverability from new and old Leadville fields through detailed reservoir characterization; (4) reduce exploration costs and risk especially in environmentally sensitive areas; and (5) help add new discoveries and reserves.