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AAPG Bulletin


AAPG Bulletin, V. 86, No. 11 (November 2002), P. 1939-1969.

Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

Unconventional shallow biogenic gas systems

George W. Shurr,1 Jennie L. Ridgley2

1GeoShurr Resources, LLC, Rt. 1, Box 91A, Ellsworth, Minnesota, 56129; email: [email protected]
U.S. Geological Survey, Box 25046, MS 939, Denver, Colorado, 80225-0046; email: [email protected]


George W. Shurr is an independent geologist and partner in GeoShurr Resources, LLC. He recently retired from a thirty-year career of university teaching and consulting. His B.A. degree is from the University of South Dakota, his M.S. degree is from Northwestern University, and his Ph.D. is from the University of Montana. His research interests include shallow gas systems on basin margins, lineament block tectonics, and Cretaceous stratigraphy in the northern Great Plains.

Jennie Ridgley received her B.S. degree in mathematics from Pennsylvania State University and M.S. degree in geology from the University of Wyoming. She has been employed with the U.S. Geological Survey since 1974. Recently she headed a multidisciplinary team project to reassess the shallow biogenic gas potential of Montana. Her most recent research has focused on understanding the genesis and controls on shallow biogenic gas accumulation in Montana, Alberta, and Saskatchewan.


This article has benefited greatly from input by a diverse group of geologists. John Curtis and Ben Law were editors for this collection of articles on unconventional gas systems. Mark Longman and Jim Minelli acted as AAPG reviewers. Richard Pollastro and Charles Spencer also gave detailed reviews. Industry geologists who read an early version of the article and provided helpful suggestions included David Fischer, Dale Leckie, Timothy Maness, and James Morabito.


Unconventional shallow biogenic gas falls into two distinct systems that have different attributes. Early-generation systems have blanketlike geometries, and gas generation begins soon after deposition of reservoir and source rocks. Late-generation systems have ringlike geometries, and long time intervals separate deposition of reservoir and source rocks from gas generation. For both types of systems, the gas is dominantly methane and is associated with source rocks that are not thermally mature.

Early-generation biogenic gas systems are typified by production from low-permeability Cretaceous rocks in the northern Great Plains of Alberta, Saskatchewan, and Montana. The main area of production is on the southeastern margin of the Alberta basin and the northwestern margin of the Williston basin. The huge volume of Cretaceous rocks has a generalized regional pattern of thick, nonmarine, coarse clastics to the west and thinner, finer grained marine lithologies to the east. Reservoir rocks in the lower part tend to be finer grained and have lower porosity and permeability than those in the upper part. Similarly, source beds in the lower units have higher values of total organic carbon. Patterns of erosion, deposition, deformation, and production in both the upper and lower units are related to the geometry of lineament-bounded basement blocks. Geochemical studies show that gas and coproduced water are in equilibrium and that the fluids are relatively old, namely, as much as 66 Ma. Other examples of early-generation systems include Cretaceous clastic reservoirs on the southwestern margin of Williston basin and chalks on the eastern margin of the Denver basin.

Late-generation biogenic gas systems have as an archetype the Devonian Antrim Shale on the northern margin of the Michigan basin. Reservoir rocks are fractured, organic-rich black shales that also serve as source rocks. Although fractures are important for production, the relationships to specific geologic structures are not clear. Large quantities of water are coproduced with the gas, and geochemical data indicate that the water is fairly fresh and relatively young. Current thinking holds that biogenic gas was generated, and perhaps continues to be, when glacial meltwater descended into the plumbing system provided by fractures. Other examples of late-generation systems include the Devonian New Albany Shale on the eastern margin of the Illinois basin and the Tertiary coalbed methane production on the northwestern margin of the Powder River basin.

Both types of biogenic gas systems have a similar resource development history. Initially, little technology is used, and gas is consumed locally; eventually, sweet spots are exploited, widespread unconventional reservoirs are developed, and transport of gas is interstate or international. However, drilling and completion techniques are very different between the two types of systems. Early-generation systems have water-sensitive reservoir rocks, and consequently water is avoided or minimized in drilling and completion. In contrast, water is an important constituent of late-generation systems; gas production is closely tied to dewatering the system during production.

Existing production and resource estimates generally range from 10 to 100 tcf for both types of biogenic gas systems. Although both system types are examples of relatively continuous accumulations, the geologic frameworks constrain most-economic production to large geologic structures on the margins of basins. Shallow biogenic gas systems hold important resources to meet the increased domestic and international demands for natural gas.

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