THE GEOLOGY OF SHALE GAS AND TIGHT OIL RESOURCES IN THE UNITED STATES

Over the past decade, unconventional resources have dominated U.S. oil and gas production, making the United States the world’s top producer of both natural gas and oil. Most U.S. production comes from ten plays: 1) The Barnett Shale is a Middle to Late Mississippian siliceous shale and limestone in the Fort Worth Basin that produces dry gas at depth and natural gas liquids (NGL) in shallower regions. 2) The Fayetteville Shale is a Late Mississippian black shale and limestone in the Arkoma Basin of Arkansas that is a dry gas producer. 3) The Late Jurassic Haynesville Shale consists of black shale with interbedded sandstones and redbeds in the Texas-Louisiana border region and produces dry gas. 4) The Woodford Shale is a Late Devonian to Early Mississippian shale that occurs in the Anadarko Basin of Oklahoma and produces NGL and dry gas. 5) The Middle Devonian Marcellus Shale extends across the Appalachian Basin from West Virginia to New York, where it primarily produces dry gas and some NGL. 6) The Bakken Formation in the Williston Basin of North Dakota, Montana and Saskatchewan consists of a Late Devonian basal black shale overlain by a limestone member, which is in turn overlain by an Early Mississippian black shale. Oil production is from the middle limestone member and an underlying limestone called the Three Forks. 7) The Late Cretaceous Niobrara Formation is a chalk and shale deposited in the Western Interior Seaway that reaches intermediate depths in the Denver and Powder River basins where it produces NGLs, and dry gas in the deepest part of the Denver Basin at the Wattenberg Field. 8) The Utica Shale is a Middle Ordovician shale above the Trenton Limestone that produces dry gas throughout the Appalachian Basin and abundant NGL in southeastern Ohio. 9) The Late Cretaceous Eagle Ford Shale consists of clay shales interbedded with limestones along the southern Texas Gulf Coast, and produces oil inland, NGL toward the coast, and dry gas at the greatest depths. 10) The Permian Basin of Texas contains six formations that form a large, stacked, unconventional play producing NGL and oil. These are the Early Permian Spraberry, Wolfcamp, Bone Spring, Glorieta, and Yeso formations, and the Middle Permian Delaware Mountain Group. Understanding the geologic factors that affect the behavior of shales can help to improve predictability of resource recovery.