Title: The Perils of Pigeon Holing
Author: Ira Pasternack
Publication: The Outcrop, April 2011, p. 15-16
Article Type: President’s Column
Starting with Geology 101, earth scientists are trained in numerous classification systems of all things geological. For example, rocks are placed within three categories; igneous, metamorphic and sedimentary, unless, of course, the rock is a meteorite, for which there exist separate classification systems. Whether a mineral, ore body, aquifer, sandstone, depositional sequence, total petroleum system, plate margin, etc., geologists have devised classifications to describe key attributes that distinguish the different categories within an assemblage. Classification systems are a convenient way to organize observations of the earth, but they are not always without exceptions or controversies.
Petroleum accumulations have been classified as “conventional” and “unconventional” based on economic conditions, reservoir permeability, extraordinary extraction methods, or special regulatory status prevailing at a given time. All of these criteria change over time so what is considered an unconventional accumulation today is likely to become the conventional accumulation of tomorrow. It is also important to note that at any given time, global perceptions of what constitutes conventional or unconventional accumulations are likely to be different than what are considered conventional or unconventional in the United States (Law and Curtis, 2002).
The U.S. Geological Survey introduced the term “continuous accumulation” in its 1995 National Assessment of United States Oil and Gas Resources to develop a more rigorous and more geologically based differentiation between conventional and unconventional systems (Schmoker, 1995). A continuous gas accumulation typically is characterized by regionally pervasive gas saturation, abnormal pressure, low permeability, and is independent of structural or stratigraphic traps that are delineated with distinct downdip water contacts. Continuous gas accumulations include coalbed methane, basin-centered gas, shale (and chalk) gas, biogenic gas and gas hydrates. Although truly dry holes cannot be drilled within the boundaries of continuous gas accumulations, it is possible to drill wells that are not economic (Bartberger and Pasternack, 2010). Discrete commercial gas fields within some continuous accumulations are poorly understood and have been described as “sweet spots” that seem to be regions with locally improved reservoir properties (Schmoker, 2002). In contrast, Shanley et al. (2004) suggest the only truly continuous gas accumulations that exist are petroleum systems in which gas entrapment is dominated by adsorption, including coal-bed methane and some organic-rich shales.
The apparent pervasive gas saturation that presumably characterizes continuous gas accumulations has led some to consider these accumulations as providing “low geologic risk.” One need only look as far as recent lease sales in which record bids have been tendered for acreage considered prospective for shale gas (e.g., Barnett, Haynesville, Marcellus, and Fayetteville). Continuous accumulations are difficult to characterize because of subtle variations in attributes that are often at or below the resolution of traditional evaluation methods. All too frequently evaluations cannot discriminate between commercial and non-economic regions because appropriate or critical data was not gathered and/or proper analyses were not performed. Inappropriate characterization of continuous accumulations runs the risk of major economic failure— either missed opportunities or dry holes. It is much too premature to pigeon hole the diverse group of petroleum systems considered continuous accumulations as many key details remain poorly understood.
Bartberger, C.E. and I. Pasternack, 2010, Spontaneous potential: Key to understanding continuous and conventional gas in Upper Cretaceous sandstones, deep eastern Greater Green River basin, southwest Wyoming: AAPG Search and Discovery #90106 ©2010 AAPG Rocky Mountain Section, Durango, CO 13-16 June 2010.
Law, B.E. and J.B. Curtis, Introduction to unconventional petroleum systems: AAPG Bull., v. 86, p. 1851–1852.
Schmoker, J. W., 1995, Method for assessing continuous-type (unconventional) hydrocarbon accumulations, in D.L. Gautier, G.L. Dolton, K.I. Takahashi, and K.L. Varnes, eds., 1995
National assessment of United States oil and gas resources—results, methodology, and supporting data: U.S. Geological Survey Digital Data Series DDS-30, 1 CD-ROM.
Schmoker, J. W., 2002, Resource-assessment perspectives for unconventional gas systems: AAPG Bull., v. 86, p. 1993-1999.
Shanley, K.W., R.M. Cluff and J.W. Robinson, 2004, Factors controlling prolific gas production from low-permeability sandstone reservoirs: Implications for resource assessment, prospect development, and risk analysis: AAPG Bulletin, v. 88, p. 1083–1121.