Resource Assessment of the Marcellus Shale
Published:January 01, 2012
Estimates of gas in place (GIP) for the Marcellus Shale can be calculated from analysis of the organic-rich rock by programmed pyrolysis. The GIP is a function of several factors, including the original hydrocarbon-generation potential of immature shale (S2o) as measured by programmed pyrolysis; a conversion factor (C) to determine the gas-volume equivalent generated by cracking of kerogen, bitumen, and oil; the thickness (t) of organic-rich shale; organic richness (O) compared with a standard sample; the transformation ratio (TR) of kerogen to hydrocarbon; and percent retention (R) of gas after primary migration. In particular,
Technically recoverable reserves are computed to be 20% of the GIP. By way of example, our calculated GIP for Steuben County, New York, is 39 bcfg/mi2, and that for Broome County, New York, is 217 bcfg/mi2, a fivefold increase within 100 mi (161 km). Because geologic controls over GIP vary so greatly across the Appalachian Basin, gas resources and reserves should properly be calculated from local geochemical data.
Figures & Tables
Shale Reservoirs—Giant Resources for the 21st Century
In the early 1970s, most exploration geologists in the United States considered subeconomic or marginally economic petroleum resources such as coalbed methane, shale gas, and tight-gas sands as unconventional resources (Law and Curtis, 2002). Tax incentives and federally funded research beginning in the late 1970s helped make these resources economically viable in the last two decades of the 20th century. Economics aside, two important geologic attributes characterize most unconventional petroleum resources (Law and Curtis, 2002). Conventional petroleum systems are buoyancy-driven accumulations found in structural or stratigraphic traps, whereas most unconventional systems exist independent of a water column and are generally not found in structural or stratigraphic traps.
Shale reservoirs are not new. The first commercial hydrocarbon production in the United States was from a well drilled in 1821 in a shale gas reservoir. By 2000, more than 28,000 wells had been drilled in shale gas reservoirs. Rising gas prices and technological advancements in horizontal drilling and hydraulic fracturing associated with the development of the Barnett Shale led to a boom in shale gas development in the early years of the 21st century. Now the exploitation of shale reservoirs is turning to natural gas liquids, condensate, and oil. Far from being isotropic and homogeneous, as once naively envisioned, shale reservoirs are complexly layered accumulations of fine-grained sediment. Geologic variation on scales ranging from that of stratal architecture to that of lamination within individual beds must be understood in order to locate and exploid areas of higher production within shale reservoirs. Shale reservoirs remain largely geologic plays - notmerely lease plays or strictly engineering plays made possible by improvements in drilling and completion technology.