Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Potential Paleozoic Shale Gas Resources in Utah
(1) Utah Geological Survey, Salt Lake City, UT.
Paleozoic shales in Utah have tremendous untapped gas potential. These include the Mississippian/Pennsylvanian Manning Canyon Shale and Pennsylvanian Paradox Formation of central and southeastern Utah, respectively. Shale beds within these formations are widespread, thick, buried deep enough to generate dry gas, and sufficiently rich in organic material and fractures to hold significant recoverable gas reserves.
The greatest Manning Canyon Shale potential is a 600-mi2 area at the north end of the San Rafael Swell. The unit is 300-1500 ft thick; the average depth to the top of the formation is 7470 ft. Four major lithotypes are: (1) carbonate, (2) fine-grained quartz sand and silt, (3) illite, smectite, and chlorite clays, and (4) organic matter composed dominantly of degraded fragments of terrestrial plants. This organic matter has good to excellent richness (TOC up to 15%) distributed throughout the shale, limestone, and even siltstone. Vitrinite reflectance indicates that the kerogen is in the dry gas thermal maturity window. The Manning Canyon lacks the cyclicity and lateral continuity found in many Carboniferous cyclothem units. It may have been deposited in a shallow restricted marine, brackish, and freshwater setting not unlike the modern Everglades and Florida Bay.
The organic-rich “black shales” of the Paradox Formation cycles are well known as the source for hydrocarbons in stratigraphically proximal carbonate reservoirs. Recent drilling has successfully produced gas emanating from the shales themselves. Individual shale units generally range between 25 and 50 ft thick at depths of 5800-6500 ft. Cores reveal several important parameters: (1) most shales are organic mudstones containing significant amounts of silt, pyrite, and fossil debris, (2) TOC values are modest (1-5%), (3) maturity values fall within the oil (or oil-gas) window, and (4) porosity (2-3%) and permeability values are low. The bounding and interbedded carbonate units are silty or muddy dolostones, in many cases possessing modest amounts of intercrystalline and microvugular pore space. These dolostones, as well as some shales, are also beset by numerous subvertical fractures. Therefore, the gas production very likely is derived not only from the shales, but also from the associated carbonates. Thus, this shale play is likely an intermixed series of reservoir types, all of which could produce upon successful stimulation.