PETROGRAPHIC AND PETROPHYSICAL CHARACTERISTICS OF THE MCCRACKEN SANDSTONE MEMBER OF ELBERT FORMATION, LISBON FIELD, PARADOX BASIN, UTAH

The McCracken Sandstone Member of the Late Devonian Elbert Formation occurs throughout the Paradox Basin of eastern Utah and western Colorado. At Lisbon field, the McCracken is a major hydrocarbon producer, consisting of an interstratified sequence (10-35 m thick) of shallow-marine sandstone, mudrock, limestone, and dolomite. Hydrocarbon production comes primarily from littoral to sublittoral sandstone beds in the lower McCracken. The present study documents a detailed petrographic evaluation of 25 representative thin sections made from core-plug samples collected from the Lisbon B-614 well, at depths ranging from 2,711 to 2,742 m. For the sample suite, core-plug porosity values range from zero to fifteen percent, and permeability values range from 0.001 to 63.0 millidarcies. The petrographic data show that the framework grains (6-91 modal percent) mainly consist of monocrystaline and polycrystaline quartz, with minor skeletal fragments. Dolomite (7-88 modal percent) and quartz (0-18 modal percent) are the dominant cements. Average grain size ranges from 125 to 1,410 micrometers, whereas sorting is variable (well to poor) and most framework grains are subrounded to subangular. Thin-section porosity is highly variable (0-25 modal %). Usually, the coarser grained and better sorted samples have the highest modal porosity values, and correspondingly, the highest measured porosity and permeability values (i.e., from core plugs). Some of this variability is also due to secondary porosity, which was produced by dissolution of carbonate cement and grains.