Gas Shale Reservoir Characteristics from the Pennsylvanian of Southeastern Utah, USA
Calcareous, organic-rich, dark brown-gray mudstones from the Paradox Basin of southeastern Utah are now recognized as potentially gas-productive. These comparatively thin deposits (less than 80 feet each), belonging to the Hovenweep, Gothic, and Chimney Rock members of the Paradox Formation cycles, possess modest TOC values (1-2%) and appear transitionally (oil to gas window) in terms of locally-pertinent thermal maturation. Nonetheless, gas production has been obtained through vertical stimulations in several wellbores.
Several cored intervals are beset by multiple types of subvertical, calcite-lined fractures that commonly propagate into stratigraphically-proximal brittle limestones or into silty dolostone interbeds. Because the mudstones themselves possess modest (2-3%) gas-filled porosity and nanodarcy permeability values in comparison to other well-known Middle to Upper Paleozoic black shale gas reservoirs, the hydrocarbon recoveries are perhaps greatly aided by natural fractures, and by the modest, but still substantial (3-5%), intercrystalline voids found in the associated dolostones. Thin section work and scanning electron microscopy have visually verified the quantitative results obtained through core-based, petrophysical methods.
As part of this study, rock mechanics data have also been assembled for a multitude of purposes, including calibration for logging-prediction, potential development of an optimal vertical stimulation program, as well as future design of horizontal and/or deviated drilling and completion methodology. Considerable work was accomplished in terms of measuring standard moduli (Young's Modulus, Poisson's Ratio) and ultrasonic wave velocities through triaxial compression testing.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009