Abstract

Six kerogen-rich shale units in Utah are recognized as having reasonable potential for commercial development as shale gas reservoirs. Four units are members of the Mancos Shale in northeast Utah - the Prairie Canyon, the Juana Lopez, the Lower Blue Gate, and the Tununk. The fifth is the Mowry Shale in the narrow strip of Utah immediately north of the Uinta Mountains, the southwest edge of the Greater Green River basin. The sixth is the black shale facies within the Hermosa Group in southeast Utah. The criteria for assessing the natural gas potential of these shale units include (1) probable gas content as estimated from kerogen/bitumen content, level of organic maturity and petrophysical properties; (2) thickness and lateral extent of the shale under conditions for gas generation/retention; and (3) suitability for successful fracture stimulation.

The Prairie Canyon and Juana Lopez Members are both detached mudstone-siltstone-sandstone successions embedded within the Mancos Shale in northeast Utah. The Prairie Canyon Member is up to 1,200 ft thick, but the stratigraphically deeper Juana Lopez Member is less than 100 ft thick. Both are similar in lithology and basinal setting to the gas-productive Lewis Shale in the San Juan basin. As in the Lewis Shale, the organic carbon-lean mudrock contains dominantly humic kerogen and is interlaminated with siltstone-sandstone. The high quartz content of the shale is likely to result in a higher degree of natural fracturing than the enclosing claystone-mudstone rocks. The quartz-rich units may respond well to induced hydraulic fracturing. Also the porosity of the sandstone interbeds, averaging 5.4%, can enhance gas storage. Both units extend beneath the southeast Uinta basin reaching depths sufficient for gas generation and retention from the gas-prone kerogen. At a few locations in the southeast Uinta basin, the Prairie Canyon has been completed and is currently producing natural gas. The potential for expansion of production from these units is enormous.

The Lower Blue Gate and Tropic-Tununk shales generally lack the abundant siltstone-sandstone interbeds that would promote natural and induced fracturing. However, they do have zones of observed organic richness in excess of 2.0% TOC (total organic carbon) that might prove to be sweet spots for shale gas where the rocks are sufficiently buried beneath the southern Uinta basin and perhaps parts of the Wasatch Plateau.

The Mowry Shale is kerogen-rich, locally gas-prone, and siliceous, all factors that should favor development as a gas reservoir where sufficiently deep for gas generation and retention. It is possible that this black shale may be exploited somewhere in the Green River basin or other parts of southwest Wyoming, but the small area for developing a play in Utah is possibly too limited for a stand-alone project that does not cross the state line. It is only north of the Uinta Mountains that the Mowry Shale is sufficiently thick to be considered an economically viable shale gas reservoir.

The black shale facies in the Hermosa Group of the Paradox basin is enigmatic. These shales contain humic type III and mixed type II-III kerogen that should favor gas generation, yet oil with associated gas dominates current production. They are relatively thin, just a few tens of feet thick on average, yet they are encased in excellent sealing rocks, salt and anhydrite. In the salt walls (anticlines) the shales are complexly deformed making them difficult to develop even with directional drilling methods, but where they are likely less deformed in the interdome areas (synclines) they are very deep. Yet in these deep areas one can expect peak gas generation. The shales are overpressured, which suggests generation currently or in the recent past. Prospects are good that shale gas reservoirs can be developed in the Paradox basin, but it may prove to be technically and economically challenging.