Abstract

The McCracken Sandstone Member of the Upper Devonian Elbert Formation, an important oil and natural-gas producer at Lisbon field, Utah, ranges in thickness from 81 to 114 ft (25-35 m), and has a complicated depositional history and stratigraphic architecture. Sedimentologic evaluation of drill core, 841 ft (256 m) from 8 wells, indicates that the McCracken is mainly dolomitic sandstone (55 percent), sandy dolomite (38 percent), and dolomitic mudrock (5 percent). Cyclical fluctuations in relative sea level during McCracken time produced three coarsening- and thickening-upward intervals (parasequence sets) in the Lisbon area, which correspond to the main reservoir units. Within these three packages, 18 lithofacies units are recognized that were deposited in at least 10 subenvironments, ranging from intertidal-supratidal carbonate flat to siliciclastic prodelta and delta front. Reservoir flow units are strongly dominated by siliciclastic lithofacies, whereas carbonate lithofacies compose major flow barriers and baffles.

Petrographic and whole-rock x-ray diffraction data (208 and 282 samples, respectively) show that dolomite and quartz are the dominant minerals in the McCracken, followed by orthoclase, calcite, pyrite, anhydrite, and clay minerals (illite/muscovite, chlorite, and kaolinite). Productive intervals in the McCracken typically consist of fine- to medium-grained, cross-stratified, slightly dolomitic sandstone, and very fine to fine-grained, bioturbated, slightly shaley, slightly dolomitic sandstone. Petrophysical data (N = 521 analyses) indicate a porosity range from 0.3 to 16.9 percent, and a permeability range from less than 0.01 to 272 milliDarcys. Dolomite and quartz cement are the primary detractors of porosity and permeability. Fine-grained, moderately to well-sorted sandstone intervals usually have the highest porosity and permeability values, whereas coarser grained, well-sorted sandstones have lower porosity and permeability values because of pervasive pore-occluding dolomite and quartz cement.