Unconventional shallow AUTHORS George W. Shurr ϳ GeoShurr Resources, biogenic gas systems LLC, Rt. 1, Box 91A, Ellsworth, Minnesota, 56129; [email protected] George W. Shurr and Jennie L. Ridgley 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. ABSTRACT degree is from the University of South Dakota, Unconventional shallow biogenic gas falls into two distinct systems his M.S. degree is from Northwestern that have different attributes. Early-generation systems have blan- University, and his Ph.D. is from the University of Montana. His research interests include ketlike geometries, and gas generation begins soon after deposition shallow gas systems on basin margins, of reservoir and source rocks. Late-generation systems have ringlike lineament block tectonics, and Cretaceous geometries, and long time intervals separate deposition of reservoir stratigraphy in the northern Great Plains. and source rocks from gas generation. For both types of systems, the gas is dominantly methane and is associated with source rocks Jennie L. Ridgley ϳ U.S. Geological Survey, that are not thermally mature. Box 25046, MS 939, Denver, Colorado, Early-generation biogenic gas systems are typified by produc- 80225-0046; [email protected] tion from low-permeability Cretaceous rocks in the northern Great Jennie Ridgley received her B.S. degree in Plains of Alberta, Saskatchewan, and Montana. The main area of mathematics from Pennsylvania State production is on the southeastern margin of the Alberta basin and University and M.S. degree in geology from the northwestern margin of the Williston basin. The huge volume the University of Wyoming. She has been of Cretaceous rocks has a generalized regional pattern of thick, non- employed with the U.S. Geological Survey since 1974. Recently she headed a marine, coarse clastics to the west and thinner, finer grained marine multidisciplinary team project to reassess the lithologies to the east. Reservoir rocks in the lower part tend to be shallow biogenic gas potential of Montana. finer grained and have lower porosity and permeability than those Her most recent research has focused on in the upper part. Similarly, source beds in the lower units have understanding the genesis and controls on higher values of total organic carbon. Patterns of erosion, deposi- shallow biogenic gas accumulation in tion, deformation, and production in both the upper and lower Montana, Alberta, and Saskatchewan. 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 ACKNOWLEDGEMENTS much as 66 Ma. Other examples of early-generation systems in- This article has benefited greatly from input clude Cretaceous clastic reservoirs on the southwestern margin of by a diverse group of geologists. John Curtis Williston basin and chalks on the eastern margin of the Denver and Ben Law were editors for this collection basin. of articles on unconventional gas systems. Late-generation biogenic gas systems have as an archetype the Mark Longman and Jim Minelli acted as AAPG Devonian Antrim Shale on the northern margin of the Michigan reviewers. Richard Pollastro and Charles Spen- basin. Reservoir rocks are fractured, organic-rich black shales that cer also gave detailed reviews. Industry geolo- also serve as source rocks. Although fractures are important for gists who read an early version of the article and provided helpful suggestions included Da- production, the relationships to specific geologic structures are not vid Fischer, Dale Leckie, Timothy Maness, and clear. Large quantities of water are coproduced with the gas, and James Morabito. geochemical data indicate that the water is fairly fresh and rela- tively young. Current thinking holds that biogenic gas was gener- ated, and perhaps continues to be, when glacial meltwater Copyright ᭧2002. The American Association of Petroleum Geologists. All rights reserved. Manuscript received June 21, 2001; revised manuscript received June 6, 2002; final acceptance June 6, 2002. AAPG Bulletin, v. 86, no. 11 (November 2002), pp. 1939–1969 1939 descended into the plumbing system provided by frac- The purpose of this article is to review the litera- tures. Other examples of late-generation systems in- ture devoted to shallow biogenic gas systems, with a clude the Devonian New Albany Shale on the eastern particular focus on two production areas: Cretaceous margin of the Illinois basin and the Tertiary coalbed rocks in the northern Great Plains and Devonian shale methane production on the northwestern margin of in northern Michigan. These two areas represent two the Powder River basin. separate and distinct types of shallow biogenic gas sys- Both types of biogenic gas systems have a similar tems. This review is intended to provide the first steps resource development history. Initially, little technol- toward recognition of specific exploration strategies for ogy is used, and gas is consumed locally; eventually, shallow biogenic gas systems. sweet spots are exploited, widespread unconventional reservoirs are developed, and transport of gas is inter- state or international. However, drilling and comple- BACKGROUND tion techniques are very different between the two types of systems. Early-generation systems have water- Natural gas systems are vertically arranged into three sensitive reservoir rocks, and consequently water is distinct levels within an idealized basin (Figure 1). The avoided or minimized in drilling and completion. In deepest level is the kitchen, where thermogenic gas is contrast, water is an important constituent of late-gen- generated. The kitchen is bounded at the top by the eration systems; gas production is closely tied to de- thermogenic ceiling. At depths below the thermogenic watering the system during production. ceiling, conditions are right for generation of thermo- Existing production and resource estimates gen- genic gas. The shallowest level is where biogenic gas is erally range from 10 to 100 tcf for both types of bio- generated. This microbe nursery is bounded on the genic gas systems. Although both system types are ex- bottom by a biogenic floor. At depths above the bio- amples of relatively continuous accumulations, the genic floor, the environment is favorable for the mi- geologic frameworks constrain most-economic produc- crobes that generate biogenic gas. The intermediate tion to large geologic structures on the margins of level may have gas that has migrated upward from the basins. Shallow biogenic gas systems hold important deep, thermogenic kitchen or biogenic gas that has resources to meet the increased domestic and inter- been progressively buried below the shallow microbe national demands for natural gas. nursery. Shallow biogenic gas is natural gas generated by anaerobic bacteria from organic-rich, thermally im- INTRODUCTION mature source rocks. Environmental constraints on the microbes, especially temperature and water composi- Unconventional shallow biogenic gas systems represent tion, provide the biogenic floor that is analogous to the resources that commonly are unappreciated or even thermogenic ceiling over deep, basin-centered gas (Fig- neglected as possible solutions for increased natural gas ure 1). Biogenic gas accumulations are located at shal- demands. Wells completed in biogenic gas accumula- low depths above the floor, especially around the mar- tions commonly have low delivery rates that tend to gins of the basin. These shallow biogenic gas discourage many operators. However, the shallow accumulations generally are underpressured and host wells are inexpensive to drill and complete, and the large numbers of low-volume wells. In contrast, ther- accumulations commonly are in relatively undevel- oped frontier areas where leases are easily obtained. Consequently, unconventional shallow biogenic gas systems are ideal for small and independent domestic operators and for developing or emerging countries. There is, however, a significant problem in our un- derstanding of shallow biogenic gas systems. In contrast with deep and basin-centered gas systems, shallow bio- genic gas systems have had relatively little scientific in- vestigation. For example, the literature on deep gas sys- Figure 1. Sketch of a generic basin, comparing the location tems is much more extensive, and exploration models of shallow, biogenic gas accumulations above the floor with the have been clearly articulated. location of deep, thermogenic accumulations below the ceiling. 1940 Unconventional Shallow Biogenic Gas Systems mogenic gas accumulations at the basin center are (Rice, 1993a). A review of biogenic gas fields in the deep, exhibit anomalous pressure, and generally have western United States indicates that the average min- high-volume wells (for a review, see Law [2002]). imum depth is about 1600 ft (490 m) (Rice and Clay- Shallow biogenic gas accumulations occur in a va- pool, 1981). A compilation of attributes for shallow riety of unconventional reservoir types that also have gas accumulations on basin margins in the northern and deep thermogenic gas in the same basin (Figure 2). central Rocky Mountains and adjacent Great Plains Low-permeability clastic reservoirs in the Alberta ba- (Shurr, 2001) shows that most are biogenic and fall sin have biogenic gas on the southeastern margin into two broad categories: large accumulations