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The Middle Devonian Marcellus shale play has emerged as a major world-class hydrocarbon accumulation. It has rapidly evolved into a major shale gas target in North America and represents one of the largest and most prolific shale plays in the world with a prospective area of approximately 114,000 km2 (44,000 mi2). Two major core areas have emerged, each with a unique combination of controlling geologic factors. Production from the Marcellus play reached 16 billion cubic feet of gas equivalent per day (BCFepd) in 2015, and it has been recognized as the largest producing gas field in the United States since 2012.

The organic-rich black shales comprising the Marcellus shale were deposited in a foreland basin that roughly parallels the present-day Allegheny structural front. The Marcellus shale accumulated within an environment favorable to the production, deposition, and preservation of organic-rich sediments. The key geologic and technical factors that regionally define the Marcellus play core areas include organic richness, thermal maturity, degree of overpressure, pay thickness, porosity, permeability, gas in place, degree of natural fracturing, mineralogy, depth, structural style, lateral target selection, completion design, and important rock mechanics issues such as the ability to be fractured, rock brittleness versus ductility, and the ability to generate complex fractures. Structural setting and deformation styles are critical to address natural fracture trends, potential geologic hazards such as faulting and fracturing in structurally complex areas, and fracture stimulation containment issues.

Since the Marcellus shale unconventional shale gas reservoir discovery in 2004 until May 2015, more than 8600 horizontal Marcellus shale wells had been drilled in Pennsylvania, West Virginia, and limited portions of eastern Ohio. Many decades of future drilling potential remain due to the enormous extent of the Marcellus shale play. Horizontal Marcellus wells report initial production rates ranging from less than 1 MMCFe/day to over 47.6 MMCFe/day. Despite the large number of wells drilled and completed to date and production of 16 BCFepd in 2015, the play is still in its infancy due to its vast geographic extent and production potential.

The Marcellus shale represents a continuous-type gas accumulation and when fully developed will comprise a large continuous field or series of fields. Over its productive trend, the Marcellus shale play has significant additional reserve potential in the overlying organic shales in the Devonian Age Rhinestreet, Geneseo, and Burket units as well as deeper potential in the Ordovician Age Utica/Point Pleasant units. Estimates of recoverable reserves from the world’s largest gas fields combine their reserve estimates for all key productive units in the field/play trend. Likewise, estimates of in-place gas resources for the Marcellus play range from 2322 tcf for the Marcellus (Hamilton Group) to over 3698 tcf for the combined Devonian Age Marcellus-Geneseo-Rhinestreet system. This represents the largest technically accessible in-place gas resources in the world.

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