Unconventional Shallow Biogenic Gas Systems

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 relatively young. Current thinking holds that biogenic gas was generated, and perhaps continues to be, when glacial meltwater

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 interstate 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 understanding of shallow biogenic gas systems. In contrast with deep and basin-centered gas systems, shallow biogenic 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 that (Ridgley et al., 1999) and prolific thermogenic gas pro- cover more than 1000 mi2 (2600 km2) and have an duction in the basin’s center (Masters, 1984). Frac- average depth of about 1600 ft (490 m) and smaller tured shales on the northern margin of the Michigan sweet spots that average 16 mi2 (41 km2) and have an basin have economic accumulations of biogenic gas, al- average depth of about 2000 ft (600 m). However, the though the thermogenic gas at the basin’s center has perception of shallow depths also fluctuates with gas not yet been demonstrated to be economic (Walter et prices, pipeline access, and available technology. Eco- al., 1997). Low-permeability chalk reservoirs produce nomic basement is influenced by these nongeologic biogenic gas on the eastern margin of the Denver basin controls, as well as by the constraints of the geologic and thermogenic gas near the basin’s center (Rice, framework, including reservoir and source rock, geo- 1984a). Coalbed methane in the San Juan basin is logic structure, and geochemistry. dominantly thermogenic but includes a component of Biogenic gas is dominantly methane, but it may secondary biogenic gas along the northern margin contain up to 2% ethane, propane, butane, and pen- (Scott et al., 1994). tane (Rice and Claypool, 1981). Isotopic analyses are Most significant production of shallow biogenic gas used to verify a biogenic origin because methane-rich comes from depths of less than 2000 ft (600 m), al- gases are also produced by other processes. Isotopic though the depth of the biogenic floor may vary from compositions are expressed as ratios relative to analytic basin to basin and over time within a single basin. A standards for 13C and for deuterium in the methane. summary of worldwide biogenic gas accumulations Ranges in these values are used to distinguish fields of gives an average minimum depth of 1800 ft (550 m) composition that commonly characterize biogenic and thermogenic gases (Figure 3). Isotopic compositions of gases from low-permeability clastic reservoirs in the northern Great Plains, low-permeability chalks in the Denver basin, and coal beds in the Powder River basin plot within the field for biogenic gas (Rice, 1993a).

Figure 2. The Alberta, Michigan, and Denver basins have shallow, biogenic gas around their margins, as well as deep, thermogenic gas in the same stratigraphic unit at the basin’s center. The Williston, Illinois, and Powder River basins have biogenic Figure 3. Crossplot of carbon-isotope ratio (d13C) and gas on their margins. An early-generation system and a late- deuterium-isotope ratio (dD) for methane from several different generation system are also located (marked Figures 7 and 14, reservoir types (modiﬁed from Rice, 1993a). Values for shale respectively). are from Martini et al. (1998).

Shurr and Ridgley 1941 Compositions of gas from fractured shales in the north- and reservoir rocks are generally very close together ern Michigan basin plot within the thermogenic field within the same stratigraphic unit. Consequently, gen- (Figure 3). However, deuterium values in the methane eration and accumulation occur in close proximity, and do show a linear correlation with those in coproduced migration paths are extremely short. Finally, the over- water. This relationship, as well as other isotopic data, burden is not a particularly important consideration is interpreted to represent a biogenic origin for the because it is commonly very thin in shallow biogenic methane on the northern margin of the Michigan basin gas systems. (Martini et al., 1996). Some shallow biogenic gas accumulations are The generation of biogenic gas has been suggested found in conventional reservoir rocks, and many shal- to follow two broadly different scenarios (Rice, low gas accumulations are a mix of biogenic and ther- 1993a). Early generation is initiated shortly after de- mogenic gases. This discussion, however, focuses on position of the source and reservoir rocks. Subsequent unconventional shallow gas systems that are domi- migration and accumulation of the gas can occur over nantly biogenic gas. The early-generation and late-gen- an extended period of time. Clastics of the northern eration scenarios each constitute separate unconven- Great Plains and chalks of the Denver basin are both tional gas systems with distinctive attributes. In examples of these ancient biogenic gas systems. Late particular, these two systems have contrasting pod ge- generation occurs during the last few million years and ometries and critical moments. long after deposition of the source and reservoir rocks. Consequently, there is relatively little time for subse- Pod Geometries quent migration and accumulation. The fractured shales of the northern Michigan basin (Martini et al., Shallow gas accumulations in low-permeability clastic 1996) and the coal beds of the Powder River basin reservoirs in the Cretaceous strata of the northern (Rice, 1993b) are examples of late-generation biogenic Great Plains provide an archetype for early-generation gas systems. biogenic gas systems. The geometry of the active source rock pod corresponds to that of the reservoir, and the reservoir geometry has been characterized as a SYSTEM DESCRIPTION shallow blanket (Law and Spencer, 1993). Such accumulations have large-areal-extent, relatively low- Shallow biogenic gas systems can be described usefully permeability reservoir rocks in close association with in terms of the petroleum system concepts outlined by source beds and display other attributes of continuous- Magoon and Dow (1994). A petroleum system consists type gas accumulations (Schmoker, 1996). In the sim- of a pod of active source rock, all of the genetically plest sense, widely distributed stratigraphic units have related petroleum accumulations, and the distribution remained above the biogenic floor ever since the initial network of migration paths that connect the source deposition of source and reservoir rocks and the early rock pod with the accumulations. Essential elements generation of biogenic gas. The resulting continuous, of a petroleum system include source beds, reservoir or blanket, pod geometry is illustrated schematically in rocks, seals, and overburden. The main processes are Figure 4A. Note that the lower part of formation 2 is generation, migration, and accumulation of hydrocar- currently below the biogenic floor for the basin; how- bons; and trap formation. Elements and processes of ever, at the time of early generation, that is, shortly petroleum systems are described in terms of spatial as- after deposition, the unit was above the biogenic floor. pects, such as pod geometry, and temporal aspects, This is a consequence of the paleotectonic subsidence such as critical moment. The critical moment is the of the basin during generation and deposition. time that provides the best representation of hydro- The pod geometry for late-generation biogenic gas carbon generation, migration, and accumulation in a systems (Figure 4B) contrasts sharply with that of the petroleum system. early-generation systems. The archetype for late-gen- Shallow biogenic gas systems are, however, uncon- eration systems is the Devonian Antrim Shale in the ventional, continuous-type accumulations that do not northern Michigan basin. The Antrim acts as both fit neatly into all of the general system attributes. For source and reservoir. The Antrim pod and the associ- example, the formation of discrete traps and the pres- ated biogenic gas accumulations are found in a ring ence of seals may be relatively unimportant in uncon- shape near where the shale subcrops beneath glacial ventional biogenic gas systems. In addition, source beds sediments around the basin rim. Biogenic gas was gen-

1942 Unconventional Shallow Biogenic Gas Systems and erosion that are substantially postdepositional, 1 2 rather than by paleotectonism that affected the ge- Biogenic Floor ometry of the Antrim Shale. The diagrams in Figure 4 represent only an idealized version of pod geometries for early and late bio- A genic gas systems. In reality, the Cretaceous blanket geometries are not confined to a single basin, as shown in Figure 4A; the blankets extend beyond the Alberta basin and into the Williston and Powder River basins (see Figure 2 for locations of basins). Similarly, the An- Biogenic Floor trim Shale is not as deep in the Michigan basin as shown in Figure 4B. More significantly, from an explo- 1 B ration standpoint, although both the blanket and ring geometries are relatively continuous, they have small Figure 4. Cartoons contrasting pod geometries in basin cross sweet spots with higher productivity embedded within sections with biogenic gas accumulations above the biogenic the larger pod. The specifics of these geometric varia- floor that is shown as a dashed gray line. Numbers are generic formations discussed in the text. (A) Early-generation system tions are discussed in more detail in the descriptions of has a blanket shape. (B) Late-generation system has a ring the geologic framework for the two representative shape marked by black diagonal lines. systems.

Critical Moment erated during and after glaciation when relatively fresh water carried microbes down into the source rock pod The timing of events in a petroleum system can be where conditions were favorable for their survival described (Magoon and Dow, 1994) using a standard- (Martini et al., 1998). Salinities of formation water ized chart such as those shown in Figure 5. The chart provide the main constraint on microbe activity and, format is modified for unconventional biogenic gas sys- hence, define the biogenic floor for the pod. The re- tems by not including events such as deposition of seal sulting ring geometry is influenced more by subsidence and overburden rock and trap formation. Of particular

400 300 200 100 Geologic Time Paleozoic Mesozoic Cenozoic (Ma) Petroleum D MPIP Tr J K P N System Events

Source Rock

* * * Reservoir Rock ab c Generation, Migration, Accumulation Figure 5. Events charts for Critical Moment shallow biogenic gas systems. *A, 66; *B, 54-51; *C, 36 Format is modiﬁed from Magoon and Dow (1994). A (A) Early-generation system has all events in relatively close 400 300 200 100 Geologic time correspondence. Asterisks Time Paleozoic Mesozoic Cenozoic mark four new dates discussed (Ma) Petroleum in a following section in this ar- J System Events D MPIP Tr K P N ticle (see also Figure 11). Source Rock (B) Late-generation system has Reservoir Rock a wide separation between the Generation, Migration, Accumulation time of deposition of source Critical Moment and reservoir rock and the critical moment of generation, mi- B gration, and accumulation.

Shurr and Ridgley 1943 importance is the time that best represents gas gener- In the following sections, the general attributes of ation, migration, and accumulation. The time differ- early and late shallow biogenic gas systems are aug- ence between this critical moment and the deposition mented by descriptions of the geologic framework for of the source-reservoir rock provides another clear con- the two archetypes: the Cretaceous low-permeability trast between early- and late-generation systems, in ad- rocks in the northern Great Plains and the Antrim dition to the difference in pod geometries. Shale in the northern Michigan basin. Additional ex- Early generation occurs shortly after deposition of amples for each system type are described more the source-reservoir rock in the Cretaceous clastics of briefly. the northern Great Plains (Rice and Shurr, 1980; Rice and Claypool, 1981). Generation, migration, and accumulation occur continuously during deposition of GEOLOGIC FRAMEWORK: EARLY- the source-reservoir rock and may continue on into the GENERATION SYSTEM postdepositional history of the system (Fishman et al., 2001). As a result, the critical moment approximately Significant biogenic gas resources are found in shallow corresponds to the deposition of the source-reservoir Cretaceous reservoirs in southern Alberta and Sas- rock and subsequent burial by overburden deposition katchewan and in central and eastern Montana. The (Figure 5A). gas-prone rocks that cover this large area are part of a Late generation of gas in the Devonian Antrim complex sedimentary rock package that is thousands Shale of the Michigan basin is associated with relatively of feet thick. Reservoir rocks range in age from Ceno- recent glaciation (Martini et al., 1998). Consequently, manian through Campanian (Figure 6). Although the the critical moment is substantially later than the de- units are spread as a continuous blanket over the entire position of the source-reservoir rock in the Late De- northern Great Plains, the main hydrocarbon produc- vonian (Figure 5B). There is significantly less time for tion is limited to the margins of structural basins: the secondary generation, migration, and accumulation southeastern margin of the Alberta basin, the north- when compared with the early-generation system. western and southwestern margins of the Williston ba-

Figure 6. Correlation chart showing selected Cretaceous rock units from southeastern Alberta, southwestern Saskatchewan, and Montana. Asterisks mark gas-producing formations. The Shannon Sandstone Member (not shown) occurs in the Gammon Shale and is equivalent to sandstones found in the ﬁne-grained facies of the Milk River Formation in Alberta and Saskatchewan.

1944 Unconventional Shallow Biogenic Gas Systems sin, and the northern margin of the Powder River basin voir rocks tend to be ﬁner grained and source beds have (Figures 2, 7). higher amounts of organic carbon than the reservoir The huge rock volume can be subdivided into a rocks have in the upper cycles. lower and an upper part on the basis of geologic frame- Resource estimates vary widely. Original estimates work, reservoir and source rock attributes, and patterns of gas in place in the United States part of the system of development and production. An unconformity at were more than 100 tcf (Rice and Shurr, 1980); later the base of the Niobrara Formation (Figure 6) extends calculations suggested about 40 tcf of technically re- across the northern Great Plains (Dyman et al., 1995). coverable gas (Rice and Spencer, 1996). In the Cana- This stratigraphic break separates deposition during dian part of the system, an early estimate was 15 tcf contrasting cycles of transgression and regression and (Rice and Claypool, 1981). Differences between the during different paleotectonic regimes (Shurr et al., lower and upper cycles are reﬂected in summaries for 1989b). The single cycle of transgression and regres- individual areas. In and around Bowdoin dome, the sion that deposited the lower part was fairly symmet- lower cycle has produced about 220 bcf (Rice et al., ric; however, the several cycles that deposited the up- 1990), whereas the upper cycle is relatively unproduc- per part had rapid transgressions followed by slow tive. In southeastern Alberta, the reservoirs in the regression and progradation. In the lower cycle, reser- lower cycle have reserves estimated at less than 3 tcf,

Figure 7. Map showing distribution of early-generation bio- 114° 113° 112° 111° 110° 109° 108° 107° 106° 105° 104° genic gas accumulations cur- 52° ALBERTA SASKATCHEWAN rently producing in Canada and Pool E Montana. Gas production in the Southeast Southeast Alberta gas ﬁeld (dot Alberta gas field pattern) is from the Belle Red 0 51° Fourche, Medicine Hat, and -1000 River Deer tch Milk River formations; in the Bow Saska ewan 0 Brooks River Wymark pool it is from the River Belle Fourche Formation Wymark South Pool Williston Basin (black); in the Bowdoin ﬁeld 50° River 0 (diagonal lines), it is from the Medicine Oldman Hat Maple Creek Moose Jaw Carlile, Greenhorn, and Belle

Taber syncline

-2000 Fourche formations; in Tiger -1000 Lake

1000 Pakowki 0 Ridge and Cedar Creek Anti- Sweetgrass arch

0 0 cline ﬁelds (medium gray), it is 49°

2000 mostly from the Eagle Sand- Milk Battle Creek MONTANA stone, with minor production Havre Bowdoin Th ru Cut Bank dome from the Judith River Forma- s 1000 River t- fr o Malta 1000 tion; and in the Liscom Creek n Tiger Ridge 1000 t ° b and Pumpkin Creek ﬁelds, it is 48 o

n 2000 d from the Shannon Sandstone -2000 a 0

r 3000 y Great Falls Member of the Gammon Shale. Gas production from the Battle -1000 Cedar Creek Anticline Creek ﬁeld (medium gray) is 3000 47° 1000 mostly from the Eagle Sand- 2000 stone. In Canada, biogenic gas 0 Contour 0 is also produced in small pools Interval = 200 Ft Pumpkin Creek from the upper part of the 0 50 km

1000 Belle Fourche Formation ° 0 46 1000 -1000 0 50 miles Liscom Creek -1000 (crosses). Structure contours 2000 3000 0 are on the top of the Second White Specks and the Green- 2000 horn formations. See Figure 2 ° 45 for location.

Shurr and Ridgley 1945 but reservoirs in the upper cycles have estimates of The upper part of the Belle Fourche is also sub- almost 11 tcf (Canadian Gas Potential Committee, divided into three units (D1–D3 in Figure 8A), based 1997). on recognition of three upward-coarsening profiles observed in wire-line logs from the southern end of Lower Cycle Rocks Bowdoin dome (Rice, 1984b; Rice et al., 1990). Ap- plication of this subdivision is more difficult when cor- A single cycle of transgression and regression deposited relations are traced to the north and west, because of the formations that constitute the lower part of the facies and thickness changes and because of differential Cretaceous blanket reservoirs. From oldest to young- loss of section below an unconformity at the base of est, these formations are the Belle Fourche Formation, the Greenhorn Limestone (Figure 8A). In Montana, Greenhorn Limestone/Second White Specks Forma- reservoir units in the upper Belle Fourche are referred tion, and Carlile Shale (Figure 6). to as Mosby or Phillips (Rice and Shurr, 1980), and in Canada, the units are referred to as Second White Belle Fourche Formation Specks Sandstone by industry (Gilboy, 1988). Within The Belle Fourche Formation is one of the principal each of the major parasequence sets, the best reservoir- host formations for shallow biogenic gas in the north- quality sandstone (higher porosity and permeability ern Great Plains (Bloch et al., 1999; Ridgley et al., with less clay) is found at the top. These sandy intervals 2001a). Regionally, gas in the Belle Fourche is not pro- tend to be elongate on a northwest-southeast axis, ap- duced from the same part of the formation. Patterns proximately subparallel to the inferred paleoshorelines of gas production reflect the distribution and preser- that originally lay to the west within the thrust belt. vation of silty and sandy facies at the time that gas was Individual sandstone lenses are imbricate (Ridgley and generated, that is, the gas is early generational. The Gilboy, 2001) and pinch laterally into shale (Rice, Belle Fourche is divided into a lower and an upper part, 1984b). Trends in gas production in Canada have the each of which has different characteristics (Gilboy, same northwest-southeast orientation (Figure 7), re- 1988; Ridgley and Gilboy, 2001; Ridgley et al., 2001a). flecting the orientation of the sandy intervals. The upper part rests on an erosional surface (Figure The upper part of the Belle Fourche was mostly 8A) that appears to be regionally widespread and that deposited in shelf environments (Rice and Shurr, 1980; has little relief. Strata overlying this erosional surface Rice, 1984b). The clastics appear to have been derived differ from strata below with respect to sedimentary from western Montana and eastern Idaho (Ridgley et structures, vertical stacking of lithology, foraminiferal al., 2001a). As with the lower part of the Belle assemblages, and paleoenvironments. Fourche, the thickness and geometry of reservoir units The lower part of the Belle Fourche, which indus- are controlled by preservation beneath a regional un- try also refers to as the Second White Specks Sandstone conformity, namely, that at the base of the Greenhorn (Gilboy, 1988), is divided (Ridgley et al., 2001a) into and Second White Specks, and by movement on line- three units (A–C in Figure 8A). The basal, primarily ament-bounded basement blocks. For example, unit shale, unit separates the overlying sandy sequences D3 (Figure 8A) is best developed in the southern Bow- from the underlying Fish Scales and Mowry forma- doin dome area; it is truncated by the unconformity to tions. The two overlying units each consist of a very the north and west beneath the Greenhorn. As a con- fine grained to locally coarse-grained sandstone at the sequence, the reservoir is areally restricted. Production base that is overlain by shale and shale mixed with thin on Bowdoin dome comes from all three sandy units in sandstone lenses (Gilboy, 1988; Ridgley and Gilboy, the upper Belle Fourche, but in Canada most of the 2001; Ridgley et al., 2001a). Deposition was in shelf production is from unit D2, because unit D3 has been environments characterized by low to moderate en- removed by erosion. ergy. Regional cross sections show that the sandstone and shale sequences were deposited on and filled in Greenhorn Limestone/Second White Specks Formation regional erosional surfaces of variable relief. The relief Throughout their geographic extent in the shallow gas- on the unconformities and the geometry of the depo- producing area of Montana, Alberta, and Saskatche- sitional packages were affected by movement on fault- wan, the Greenhorn Limestone and Second White bounded basement blocks. Gas is produced from both Specks Formation consist of gray, calcareous, white- sandstone units in Saskatchewan in the Southeast Al- speckled, coccolith-bearing fissile shale. Some strata in berta gas field and Wymark pool (Figure 7). southern Alberta and Saskatchewan, formerly called

1946 Unconventional Shallow Biogenic Gas Systems Anderson Exploration Ltd. Mortenson 1-15 15-31N-28E SP R BRALORNE MERLAND 1500 MEDHAT 8-19-13-3w4

Bowdoin 1600 DENSITY GAMMA NEUTRON sandstone POROSITY RAY POROSITY Carlile COLORADO SHALE Shale 1700 1450

Greenhorn Fm MEDICINE D3 1800 HAT "A" b D2 1500

a 1900 D1 MEDICINE HAT C 2000 "C" B b Belle Fourche Formation Belle Fourche

A 2100 a

1600 1550 MEDICINE Fish HAT Scales "D" A B

ESSO et al. Cessford EX Tricentrol US 6-3-24-13W4 Roberts 28-4 28-31N-19E GR R C GR RT

PAKOWKI

300 FORMATION 800

xxxxxxxxxxxxxxxxxxxxxx Ardmore bentonites xxxxxxxxxxxxxxxxxxxxxx Eagle 350 VII MILK 900 RIVER FORMATION

III

IV 400 III-IV 1000 V

II Virgelle

I 450 1100 NIOBRARA FORMATION Telegraph Creek C D

Figure 8. Typical wire-line logs from Cretaceous formations that produce shallow biogenic gas. (A) Bowdoin sandstone member of the Carlile Shale and units A, B, C, and D1–D3 of the Belle Fourche Formation in the Bowdoin dome area. (B) Units A, C, and D of the Medicine Hat Formation in the southeast Alberta gas field. (C) Facies breakdown in the Milk River Formation in the Southeast Alberta gas field. (D) Shoreface sandstone facies in the Eagle Sandstone in the Tiger Ridge field. See Figure 6 for stratigraphic units and Figure 7 for field locations.

Shurr and Ridgley 1947 the Second White Speckled Shale or Second White through the Carlile show several upward-coarsening Speckled Sandstone (industry usage), are now assigned sequences (Figure 8A), although their exact nature is to the Belle Fourche Formation (Bloch et al., 1993). poorly understood because of lack of core. Thickness The base of the Second White Specks is taken to be at variations in these upward-coarsening sequences re- the base of the lowest widespread bioclastic limestone flect regional erosional surfaces. bed. The lithologic break between the Greenhorn/Sec- Two areas of shelf sandstone occur within the Car- ond White Specks and the underlying Belle Fourche lile in Montana (Rice and Shurr, 1980). The northern Formation is a regional unconformity (Figure 8A) (Rice area of shelf sandstone is represented by the Bowdoin et al., 1990; Ridgley et al., 2001a). sandstone and the southern area is equivalent to the Thin bioclastic limestone beds containing abun- Turner Sandy Member. Thus far, only the Bowdoin dant coccoliths and fish debris are commonly observed sandstone has produced biogenic gas. Other sandstone in cores. The limestones and the associated shale form sequences have not been exploration targets, and, con- parasequences that consist of a basal shale; a middle, sequently, the potential for gas in the Carlile is largely transitional interval of thinly interbedded shale and unknown. limestone; and a thicker bed of bioclastic limestone at The Bowdoin sandstone is areally restricted to the top. A prominent bentonite occurs in the basal part parts of north-central Montana and southwestern of the formation (Gilboy, 1988). Saskatchewan (Rice, 1981; Gilboy, 1989a, 1993). Pro- Production patterns in the Greenhorn Limestone duction is restricted to Bowdoin dome, although wire- are related to the geometry of lineament blocks. Thick line log responses indicate similar lithologic character- calcarenites provide better reservoir capacity to host istics for the Bowdoin sandstone west and north of the gas, and Greenhorn thickness is probably controlled, in dome. On Bowdoin dome, sandstone less than 1 in. (25 part, by block movement (Ridgley et al., 2001a). Lo- mm) thick is interbedded with siltstone and organic- cally thick accumulations of calcarenites and bioclastic rich black shale (Nydegger et al., 1980; Rice and Shurr, limestone result from gradual block subsidence. On 1980; Rice et al., 1990). The total sandy-silty interval Bowdoin dome, production is concentrated on the is more widespread than are individual sandstone lay- north end (Rice et al., 1990), where wells penetrate a ers that are laterally discontinuous (Rice, 1981; Gilboy, relatively thick Greenhorn section. On a large scale, gas 1989a). Productive zones appear to contain more clay traps appear to be related to block geometry that has and are not the cleanest intervals shown on wire-line controlled reservoir lithology and thickness. On a logs. Localized areas have porosity and permeability smaller scale, gas is trapped in the more porous calcar- values characteristic of conventional reservoirs. How- enites and bioclastic limestone by interbedded benton- ever, the producing intervals are generally tight reser- ite and shale beds that reduce permeability to gas mi- voirs that are improved by both natural and induced gration. In Canada, similar lithologic and thickness fracturing. variations in the Second White Specks Formation are attributed to paleotectonism on lineament blocks Upper-Cycle Rocks (Ridgley and Gilboy, 2001; Ridgley et al., 2001a). However, these intervals are rarely the sole target for The upper part of the extensive Cretaceous rocks cov- gas exploration. Tests are commonly done in conjunc- ering the northern Great Plains was deposited in three tion with tests of the upper part of the Belle Fourche cycles of transgression and regression/progradation. Formation. Any production from the Second White However, only the first two cycles have commercial gas Specks would be commingled with production from production. Reservoir units have different names in the upper Belle Fourche. Canada and the United States (Figure 6): the Medicine Hat Sandstone and Martin sandy zone of the Niobrara; Carlile Shale the Milk River Formation and Eagle Sandstone; and the The Carlile Shale thickens westward from Bowdoin Belly River and Judith River formations. dome into southeastern Alberta and thins to the north in southwestern Saskatchewan (Rice, 1981; Gilboy, Niobrara Formation 1989a, 1993). Its contact with the underlying Green- Regionally, the Niobrara Formation thickens toward horn Limestone or Second White Specks Formation is the northwest from Montana into Alberta and thins to conformable or unconformable, depending on the ef- the north in Saskatchewan (Rice, 1981; Gilboy, 1989a, fect of localized tectonics (Figure 8A). Wire-line logs 1996). The Niobrara is dominantly a gray, calcareous,

1948 Unconventional Shallow Biogenic Gas Systems coccolithic shale informally known in Canada as the 1981). In eastern Alberta, the formation is divided into First White Speckled shale. Shallow biogenic gas is pro- the Telegraph Creek, Virgelle, and Dead Horse Coulee duced from sandy intervals in the Medicine Hat and members. Some biogenic gas has been produced from Martin. Principal production has been from the Med- the Telegraph Creek Member, which is a succession of icine Hat in the Southeast Alberta gas field (Figure 7). offshore marine mudstone and thin-bedded, laminated The Medicine Hat Sandstone in Alberta and Sas- sandstone. The overlying Virgelle and Dead Horse katchewan is an interval of thin sandstone, siltstone, Coulee members document the successive seaward and interbedded mudstone in the upper 100 ft (30 m) progradation of shoreface sandstones and rocks de- of the Niobrara Formation (Hancock and Glass, 1968; posited in transitional tidal, deltaic, and estuarine en- Gilboy, 1989b; Hankel et al., 1989; Schro¨der-Adams vironments. This culminates with heterogeneous and et al., 1997, 1998; O’Connell, 1999). The Medicine carbonaceous lithologies formed in nonmarine envi- Hat is composed of a series of upward-coarsening para- ronments. These units have not produced commercial sequences; the principal gas-bearing intervals are units gas, although small amounts of gas are coproduced A, C, and D (Figure 8B). Units C and D are localized with water from the Virgelle sandstones. Environmen- in southwestern Saskatchewan and adjacent areas of tal interpretations (McCrory and Walker, 1986; Cheel northern Montana and southern Alberta. The Medi- and Leckie, 1990; Meyer et al., 1998) and stratigraphic cine Hat A unit contains the greatest volume of bio- correlations (Simpson and Singh, 1980; Meijer-Drees genic gas and is also the most regionally extensive. It and Myhr, 1981; Gilboy, 1987; Ridgley, 2000) for thins to the north and east in Saskatchewan and to the these three members provide the context for the lat- northwest in Alberta (Rice, 1981; Gilboy, 1989a, erally equivalent, fine-grained units of the Milk River 1996; Schro¨der-Adams et al., 1997). Overall, the Med- that produce gas in the Southeast Alberta gas field (Fig- icine Hat sands are composite units consisting of fine- ure 7). grained sandstone and siltstone interlaminated with The principal gas-bearing, fine-grained Milk River thin mudstone beds. Sandstone bodies are elongate on units are diachronous with the coarser Virgelle and a northwest-southeast trend (Gilboy, 1989b), subpar- Dead Horse Coulee members (O’Connell et al., 1999; allel to the shoreline located in western Alberta (Rob- Payenburg, 2000; Ridgley, 2000). In the gas-producing erts and Kirschbaum, 1995). The Medicine Hat was area, the Milk River Formation has been divided into deposited in shallow shelf environments that were in- seven lithofacies in the Southeast Alberta gas field fluenced by broad and regional syndepositional tecton- (Ridgley, 2000). Log responses on a typical wire-line ism on the Sweetgrass arch (Nielson and Schro¨ der- log are shown in Figure 8C. Lithofacies are interpreted Adams, 1999). to have been deposited in offshore to inner shelf en- The Martin sandy zone in the Bowdoin dome area vironments (I–V) and in shoreface and shoreline set- consists of thin, sandy lenses in the upper part of the tings (VI and VII). The main gas production occurs in Niobrara Formation (Nydegger et al., 1980). On re- the inner- to outer-shelf and shelf-sandstone facies (III gional cross sections, the Martin is the lateral equiva- and IV). Although the uppermost amalgamated shore- lent of the upper Medicine Hat A and undifferentiated line sandstone and mudstone facies (VII) also has po- Niobrara above the Medicine Hat A unit (Rice, 1981; tential, it has not been a historic exploration target to Simpson, 1981; Gilboy, 1989b). Limited available core date. data show that the unit consists of thinly interbedded At least two unconformities have been docu- sandstone, siltstone, and shale. Wire-line logs indicate mented in the Milk River Formation. The lower un- that thick and elongate sandstone bodies, such as those conformity (B in Figure 9A) occurs between facies II hosting Medicine Hat pools in Canada, are absent from and V (Figure 8C) and is characterized by the presence the Martin. On Bowdoin dome, old wells tested gas of chert pebbles (O’Connell et al., 1999; Ridgley, from the Martin, but commercial production has only 2000). The upper unconformity (A in Figure 9A), be- recently been established in a few wells on Bowdoin tween facies III and VII (Figure 8C), occurs within dome and at Battle Creek field (Figure 7). Campanian strata, and its presence documents significant movement on the Sweetgrass arch during the Milk River Formation/Eagle Sandstone Campanian. This unconformity merges with the un- The Milk River Formation in Canada consists of sand- conformity (as a ravinement surface) at the base of the stone, argillaceous siltstone, mudstone, and occasional Pakowki Formation in the area of the Sweetgrass arch lignite and bentonite beds (Meijer-Drees and Myhr, (Ridgley, 2000).

Shurr and Ridgley 1949 Figure 9. Cross section through the Milk River Formation and the Alderson Member of the Lea Park Formation, in the area of the southeast Alberta gas ﬁeld. (A) Schematic relationship between coastal facies of the Dead Horse Coulee Member and shoreface sandstone of the Virgelle Member of the Milk River Formation to the basinal ﬁne-grained facies of the Alderson Member of the Lea Park Formation. Vertical line separates the current gas-producing area from the nonproductive area. (B) Map showing location of cross section. Hatched line represents the seaward extent of shoreface sandstone (Meijer-Drees and Myhr, 1981).

The respective ages of the strata above and below above the lower unconformity (Figure 9A). To the the lower unconformity are not well documented. southwest, in the area of Writing-on-Stone Provincial O’Connell et al. (1999) have proposed that coarse- Park (Figure 9B), the Campanian–Santonian boundary grained Santonian units are cut out below the uncon- is within the coarse-grained Dead Horse Coulee Mem- formity and that fine-grained Campanian rocks were ber (Figure 9A). In this model, gas is trapped in the deposited directly above the unconformity (B1 in Fig- fine-grained facies as a result of high capillary pressure; ure 9A). In this model, gas is trapped in the fine- however, there would be hydraulic connectivity be- grained facies across the unconformity. This tends to tween the coarse-grained updip facies and the fine- inhibit updip gas migration (O’Connell et al., 1999), grained downdip facies, rather than a discontinuity as in contrast with interpretations of hydrodynamic gas proposed by O’Connell et al. (1999). More research is trapped below updip water (Berkenpas, 1991; Leis and needed to document the age of the strata directly Letourneau, 1995). above the lower unconformity, so that hydrologic Also possible, however, is that the lower uncon- models can be refined. formity occurs within Santonian rocks (B2 in Figure The Eagle Sandstone in Montana is a clastic wedge 9A) (J. L. Ridgley, 2002, unpublished data). To the of progradational sedimentary rock that separates the northeast, the part of the fine-grained Lea Park For- underlying transgressive Niobrara Formation from mation dated by ammonites as Campanian is well the overlying transgressive Claggett Formation (Rice,

1950 Unconventional Shallow Biogenic Gas Systems 1980; Rice and Shurr, 1983). The Eagle of north-cen- sandstones in fault-bounded gravity slide blocks in the tral Montana may be older than strata assigned to the Tiger Ridge area (Baker and Johnson, 2000). Eagle in southern and eastern Montana (Payenburg, The Belly River Group in Canada is divided into 2000). The two depositional systems were separated three formations, which are, in ascending order, the by a prominent paleogeographic embayment (Gautier Foremost Formation, Oldman Formation, and Dino- and Rice, 1982). Gas is produced from the Eagle at saur Park Formation (Hamblin, 1997; Bergman and Tiger Ridge field in north-central Montana and from Eberth, 1998). The lowest formation has nearshore Eagle-equivalent units on Cedar Creek anticline, at sandstone and shale that interfinger laterally with the Pumpkin Creek field, and at Liscom Creek field in underlying Lea Park/Pakowki Formation. The middle southeastern Montana (Figure 7). The last three ac- unit has stacked fluvial channels and associated coal cumulations are part of the southwestern margin of the deposits characteristic of alluvial to paralic depositional Williston basin and northern margin of the Powder environments. The uppermost formation has sedimen- River basin that are discussed in a following section as tary rocks from a variety of paralic coastal plain envi- other examples of early-generation biogenic gas in Cre- ronments; however, sandstones have evidence of tidal taceous reservoirs. processes. Sandstones in the Belly River Group pro- In the Bears Paw Mountain area, the Eagle Sand- duce gas that may be related to coals. stone produces biogenic gas from shoreface sand- The Judith River Formation in Montana consists of stones; a typical wire-line log is shown in Figure 8D. two generalized subdivisions (Rice and Shurr, 1980; In the Bears Paw Mountains, the shoreface sandstones Shurr et al., 1989a; Rogers, 1993). The lower part is are overlain by tidal-flat, coastal-plain, and nonmar- interbedded marine sandstone and shale that are tran- ine-fluvial sedimentary rocks (Rice, 1980). Syndepo- sitional with the underlying Claggett Shale. These clas- sitional tectonism on basement blocks influenced tic rocks are low-permeability, tight reservoirs that shoreline orientation and sandstone geometry in the contain gas locally. The upper part includes marine Eagle (Shurr and Rice, 1986). Eagle reservoirs both shoreface and nonmarine fluvial sandstone with minor north and south of the Bears Paw Mountains have coal. These lithologies were deposited in nonmarine to good porosity and permeability and, consequently, are shallow-marine environments during the main stages considered to be conventional (Rice and Shurr, 1980) of regression. Sandstones are conventional reservoirs reservoirs that contain gas with isotopic characteristics that produce gas at Tiger Ridge field. On the south- of biogenic gas (Rice, 1975; Rice and Claypool, 1981). western margin of the Williston basin, gas is produced The gas formed early and was initially concentrated in from Judith River sandstones on Cedar Creek anticline. stratigraphic traps that were subsequently broken in Gas shows, reservoir characteristics, and resource po- fault-bounded traps (Rice and Shurr, 1980). The tential also have been described in the area of Poplar faulted traps are on gravity-induced slide blocks dome (Monson, 1995), which is located between Ce- (Baker and Johnson, 2000) that compartmentalize dar Creek anticline and Bowdoin dome (Figure 7). production to a high degree. Lineament Block Controls Belly River Group/Judith River Formation The Belly River Group in Alberta and Saskatchewan Deposition and deformation of Cretaceous rocks in the and the Judith River Formation in Montana form a northern Great Plains were controlled by major tec- clastic wedge of progradational sedimentary rocks that tonic features that are mosaics of lineament-bounded were shed from uplifted source areas in the western basement blocks (Anna, 1986; Shurr et al., 1989b). thrust belt. The clastic wedge separates (Figure 6) the Biogenic gas accumulations are controlled by linea- underlying transgressive Lea Park/Pakowki Formation ment blocks in three major ways: (1) patterns of thick- and Claggett Shale from the overlying transgressive ness and lithology in reservoir rocks and source beds Bearpaw Shale (Shurr et al., 1989a; Eberth and Ham- reflect block geometry; (2) postdepositional deforma- blin, 1993). Most of the reservoirs in the Belly River tion along the lineament zones produces structural Group and Judith River Formation are conventional traps and compartmentalizes the reservoirs; and (3) sandstone reservoirs with good porosity and perme- lineaments are zones of increased fracturing and fault- ability. Consequently, trapping mechanisms com- ing that influence fluid migration. monly are conventional stratigraphic and structural Deformation along lineament zones has been re- traps. For example, gas is produced from Judith River lated to geologic structures in the Montana plains

Shurr and Ridgley 1951 (Shurr et al., 1989c). A size hierarchy of blocks exists geologic and hydrologic processes. The Judith River that ranges from major lithosphere blocks bounded Formation (Shurr et al., 1989a) and the Eagle Sand- by lineament zones down to small constituent blocks stone (Shurr and Rice, 1986) have been demonstrated within lineament zones (Shurr, 2000). On Cedar to be influenced by lineament blocks in Montana. More Creek anticline, gas reservoirs are compartmentalized recently, the effects of lineament blocks have been by the small constituent blocks within a lineament documented in the Greenhorn and Belle Fourche for- zone. On Poplar dome, tectonism on lineament- mations in Canada (Ridgley and Gilboy, 2001; Ridgley bounded blocks effectively compartmentalizes hydro- et al., 2001a). carbon reservoirs (Shurr and Monson, 1995), including Cretaceous shallow gas reservoirs (Monson, Summary of Reservoir and Source Bed Attributes 1995). Fluid flow may be enhanced or retarded by faults The attributes of reservoir rocks and source beds in and fractures within a lineament zone. Sealing faults Cretaceous rocks in the northern Great Plains are sum- commonly are developed in clastic intervals that con- marized in Tables 1 and 2. In these unconventional gas tain large amounts of shale. Groundwater movement accumulations, the reservoir rocks and source beds are may result in precipitation of fracture fillings that also interbedded, migration distances are short, and source- inhibit fluid movement. Alternatively, a lineament reservoir rock is an important element of the events zone may provide a conduit for cross-formational flow chart (see Figure 5). and/or long-distance lateral migration of water, oil, or The fundamental subdivision of Cretaceous rocks gas (Shurr and Watkins, 1989). Examples of the cor- into lower and upper cycles has expression in the res- respondence of lineaments, gas production, and pres- ervoir-rock attributes (Table 1). In general, the lower- sure variations have been documented in Cretaceous cycle reservoirs are characterized by lower porosity val- rocks on Bowdoin dome (Shurr et al., 1993). ues (Ͻ15%) and lower permeability values (10s md) Although deformation and fluid flow are impor- when compared with the upper-cycle porosity (Ͼ15%) tant aspects of gas accumulations controlled by linea- and permeability values (100s md). This conforms to ment blocks, Cretaceous paleotectonism is the most the generalization that the rocks in the lower cycle are important influencing factor for these early-generated finer grained and more clay rich than rocks in the upper biogenic gas accumulations. Early movements on the cycles. Sandstone reservoirs vary in quality because of lineament blocks influenced patterns of deposition and changes in rock fabric, such as those produced by bio- erosion so that thickness and lithology variations in res- turbation. Although some porosity and permeability ervoirs and source beds follow the block outlines. In values in both the lower and upper cycles may be char- addition, paleotectonism on the blocks may set up ini- acteristic of conventional reservoirs, these represent tial traps that collect the early-generated gas. Subse- only localized areas within otherwise tight, continu- quently, the gas distribution may be modified by later ous-type reservoirs.

Table 1. Summary of Reservoir Rock Attributes

Stratigraphic Unit Porosity (%) Permeability (md) Reference(s)

Upper Cycle Judith River Formation 15–30 10s–100s Shurr et al., 1989a Eagle Sandstone up to 26 up to 150 Rice and Shurr, 1980 Milk River Formation 14–26 Ͻ1–259 Simpson and Singh, 1980 Myhr and Meijer-Drees, 1976 Niobara/Medicine Hat Sandstone 5–37 0.1–500 Martin and Yeung, 1991 Hankel et al., 1989

Lower Cycle Carlile/Bowdoin Sandstone 8–14 0.1–0.7 Nydegger et al., 1980 Greenhorn 6–13 0.1–6 Nydegger et al., 1980 Upper Belle Fourche 5–21 0.1–43.5 Gilboy, 1988 Nydegger et al., 1980 Lower Belle Fourche 13–23 0.4–9 Gilboy, 1988

1952 Unconventional Shallow Biogenic Gas Systems Table 2. Summary of Source Bed Attributes

Total Organic Carbon Hydrogen Index Stratigraphic Unit (%) (from Rock-Val) Reference(s)

Upper Cycle Milk River Formation 0.56–1.18 23–29 Ridgely et al., 1999 Niobara/Medicine Hat Sandstone 0.5–2.1 89–152 Ridgely et al., 1999 Schroder-Adams et al., 1998

Lower Cycle Greenhorn and Second Specks 1.11–3.69 58–420 Ridgely et al., 1999 Ridgely, 1998 Upper and Lower Belle Fourche 1.0–4.0 40–300 Bloch et al., 1999 Ridgely, 1998

The upper- and lower-cycle subdivisions also are etate fermentation. Most biogenic methane is probably reﬂected in the attributes of source beds (Table 2). The produced during carbon dioxide reduction by hydro- lower-cycle rocks generally have higher values of total gen, except in very recent freshwater environments, organic carbon than do the rocks of the upper cycles. where acetate fermentation appears to be the preferred Furthermore, hydrogen index values are higher in the pathway for methane generation (Schoell, 1980; Whi- lower-cycle rocks than in the rocks of the upper cycle. ticar et al., 1986). The differences in hydrogen index indicate differences Carbon isotopes in the methane commonly are in the type of organic matter. Lower-cycle source beds used as a criterion for identifying biogenic gas. How- are capable of producing both gas and liquid hydro- ever, this method is not entirely reliable, because bio- carbons, whereas rocks of the upper cycle have a genic gas can have a range of carbon-isotope values, greater tendency to generate gas. including those normally assigned to mixed gas or thermal gas. Because most biogenic gas is formed by carbon Geochemistry of Early-Generation Systems dioxide reduction, the isotopic composition of the initial and subsequent carbon in the carbon dioxide is the The critical moment of gas generation, migration, and controlling factor for the isotopic composition of the accumulation is an important distinguishing character- resulting carbon isotope of the methane. This may vary istic for unconventional biogenic gas systems. Geo- over geologic time. In marine settings, there is gener- between (75‰מ to 50מ) chemical data from both gas and water in Cretaceous ally a greater fractionation reservoirs in the northern Great Plains clearly demon- the carbon-isotope composition of the methane and strate that the gas is early generation. the carbon-isotope composition of the coexisting carbon dioxide than that observed in freshwater settings Whiticar et al., 1986). Figure 10) (50‰מ to 40מ) Gas Geochemistry Gas produced from shallow reservoirs in the Belle shows the carbon-isotope data of coexisting methane Fourche through the Judith River formations is of bio- and carbon dioxide pairs of recently sampled gas and genic origin and was produced during the breakdown coproduced water in the Bowdoin dome and Tiger of organic matter by anaerobic bacteria. The d13C iso- Ridge (near the Bears Paw Mountains) areas (Figure topic values of the methane in this gas in the shallow 11). Dissolved carbon in bicarbonate in water is used reservoirs of Montana, Alberta, and Saskatchewan because measurable carbon dioxide gas was too small Rice, 1975; Rice and for isotopic analysis. The data fall within the marine) 72‰מ to 64.6מ range from Claypool, 1981; P. G. Lillis, 2001, personal commu- depositional setting. Note that, in general, as the car- nication). In addition, the hydrocarbons in the gas are bon isotope of the methane becomes heavier, the cor- Ͼ dominated by methane [C1/(C1–C5) 0.98]. Both responding carbon isotope of the dissolved carbon also these characteristics are typical of biogenic gas. Bio- becomes heavier. genic gas can form in marine and freshwater environ- The carbon-isotope data also support the premise ments. Generation of methane by microbial action that the gas is old (Rice, 1975; Rice and Claypool, takes two pathways: carbon dioxide reduction and ac- 1981) and began to form and accumulate soon after

Shurr and Ridgley 1953 5 Field of marine ration of gas generation (Fishman et al., 2001). Thin sediments sections of several samples from the Milk River For- 0 mation were examined, and the paragenetic sequence

-5 of pore-ﬁlling cements was described. Principal pore- ﬁlling cements are siderite and calcite (ferroan to non- -10 ferroan). Carbon isotopes of the carbon in the siderite Field of freshwater and the calcite indicate that they could have been pro- -15 sediments duced as a by-product of methanogenesis. The siderite C Dissolved Inorganic Carbon C Dissolved 13

δ and carbonate cements also were found to contain

-90 -80 -70 -60 -50 methane-bearing fluid inclusions, which suggests that 13 the process of methane generation occurred over a sig- δ C Methane nificant period of time (about 15 m.y., as reported in 13 13 Figure 10. Crossplot of d C in methane gas and d Cof Fishman et al. [2001]). Principal production of meth- dissolved carbon (as bicarbonate) in the coproduced water. The ane appears to have ceased in the early Tertiary when data plot within the field for biogenic gas generated in marine maximum burial occurred, based on evidence of environments. Note that as the methane carbon isotope be- compaction. comes heavier (more positive), the corresponding carbon isotope of the dissolved carbon also becomes heavier. This indicates that during the process of methanogenesis, the isotope of Water Geochemistry carbon dioxide controls the fractionation of the carbon isotopes Water in shallow Cretaceous reservoirs is of two types: between methane and the dissolved carbon. See Figure 11 for (1) in-situ water that is coproduced with gas, and (2) locations of samples. water that occurs updip from gas accumulations. The latter has been proposed to form an updip barrier to gas migration in the Medicine Hat Sandstone (Hankel deposition. A recent petrologic study of the Milk River et al., 1989) and the Milk River Formation (Berkenpas, Formation in the gas-producing area of Alberta and 1991; Lies and Letourneau, 1995). Recent studies by Saskatchewan addresses the question of time and du- the U.S. Geological Survey (P. G. Lillis, 2001, personal

M O N T A N A

3E 45 6789101112 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 37N 3 ... 36 * BLAINE . . 35 1 . Sweetgrass . .. 34 Hills * . Bowdoin 33 2 . Dome . 32 *.. 31 . HILL . 30 4 * 29 LIBERTY Bears Paw PHILLIPS 28 Mountains 27 26 Figure 11. Map showing lo- 25 24 24 N cation of gas samples with Little Rocky 23 coproduced water and water CHOUTEAU Mountains 23 22 22 samples dated by 129I/I ratios. 21 21 Sample 1 is 65.6 Ma; sample 2 20 20 1 Water samples with 129 I/I dates is 53.6 Ma; sample 3 is 51.0 19 Ma; and sample 4 is 35.6 Ma. .* Water samples with oxygen and deuterium isotopes

1954 Unconventional Shallow Biogenic Gas Systems communication) examined the equilibrium relations Fourche Formation in the Canadian Monchy pool, and between the biogenic gas and its coproduced water. it yielded an uncorrected age of 51.0 Ma. Sample 4 is During methane generation by the carbon dioxide re- from the upper part of the Belle Fourche at the south duction pathway, hydrogen from the water is incor- end of Bowdoin dome; its uncorrected age is 35.6 Ma. porated in the methane. The fractionation of hydrogen The younger sample is from shallow depths in an area isotopes between water and the coexisting methane where units above the lower Claggett have been re- has a linear relationship (Schoell, 1980; Whiticar et al., moved by erosion. Additional sampling in this area is 1986), and this linear relationship can be used to es- needed to understand the disparity in age of water from tablish whether equilibrium exists between the gas and the Belle Fourche Formation on and near Bowdoin coproduced water. Using this linear relationship, all dome. but three of the sample pairs were found to be in On a crossplot of the deuterium isotope vs. the equilibrium. d18O isotope, the coproduced water data set (P. G. The 129I isotope of iodine was obtained for four of Lillis, 2001, personal communication) plots below the the water samples to date the water. The 129I isotope modern meteoric and the Montana meteoric water has a half-life of 15.7 m.y., and the age of the water lines (Figure 12). Also shown on Figure 12 are copairs can be extrapolated at five to six times this rate (G. of deuterium and oxygen isotopes from the Milk River Snyder, 2001, personal communication). The gas and aquifer (Drimmie et al., 1991) that cluster in three coproduced water appear to be in equilibrium for three fields: of the samples. Deuterium was not obtained in the methane for the fourth sample, and, thus, it is not 1. Water younger than 50,000 yr is found closest to known whether the gas and coproduced water are in the outcrop and represents the most recent recharge, equilibrium. based on 36Cl isotope data (Phillips et al., 1986) Samples 1 and 2 in Figure 11 are from the western 2. Water between 50,000 and 600,000 yr is found far- part of the area. Sample 1 from the Eagle Sandstone ther downdip but updip from the main gas field, yielded an uncorrected age of 65.6 Ma, and sample 2 based on 36Cl isotope data (Phillips et al., 1986) from the upper part of the Belle Fourche Formation 3. Older water from the Bow Island Sandstone, which gave 53.6 Ma. The location of the samples to the north- was beyond the 36Cl isotope dating technique (36Cl east of the thrust belt (Figure 7) and the age range of isotope has a half-life of 300,000 yr), was consid- about 66 to 54 Ma indicate that fluid movement in the ered to approximate the water in the Milk River formations may relate to the events in the thrust belt. gas-producing field Sears (2000) has suggested that growth of the imbri- cated thrust system forced fluids out from beneath, and The crossplot of the deuterium isotope vs. the to the front of, the tectonic slab, from about 79 to 59 d18O isotope in the coproduced water from the Eagle Ma. These perturbations in the regional flow system Sandstone and Belle Fourche Formation are similar to may have extended out to the area of shallow gas pro- those obtained from the Bow Island Sandstone, al- duction. Early-generation gas may have migrated and though they are consistently more enriched in deute- been trapped as a part of the tectonic expulsion of flu- rium (Figure 12). Dating the water using the iodine- ids from the southwest. In fact, the huge Southeast isotope technique may be useful in understanding the Alberta gas field is strategically located at the northeast variability between the deuterium and the d18O iso- end of the Sweetgrass arch (Figure 7). Fluids squeezed tope copair values and how these relate to times of gas from under the thrust loading may have migrated generation, duration of gas generation, and time of gas northeastward along the trend of the arch and its as- migration and exsolution. sociated lineaments, mixing with or displacing connate The relatively old ages determined for fluids in water. Gas generated soon after deposition of Creta- Cretaceous reservoirs have implications for the pro- ceous rocks was remobilized and ultimately concen- posed hydrodynamic trapping mechanisms. If the pre- trated and trapped in the Southeast Alberta gas field. liminary ages determined for coproduced water rep- This may have been a progressive, multistage process, resent the shallow biogenic system, except in areas of the dates and the accumulations being just the last ma- active recharge, then updip water may not be a control jor migration and trapping event. factor for holding gas in place. Instead, incursion of Samples 3 and 4 in Figure 11 are from farther east updip water could be a destructive process that pro- near Bowdoin dome. Sample 3 is from the Belle gressively destroys the gas accumulation, along its

Shurr and Ridgley 1955 Figure 12. Crossplot of d18O -35 and d2D in water coproduced with biogenic gas in the Tiger -55 Ridge and Bowdoin dome areas, Montana (see Figure 11 for locations). Similar data from -75 water in the Milk River aquifer and Bow Island Formation in -95 Canada (Drimmie et al., 1991) Northern Montana Bow Island waters are also shown. The global meteoric water Global Meteoric Water Line (GMWL) -115 D water meteoric water line (Craig, 2 1961) and the northern Mon- ␦ Bowdoin area Tiger Ridge area tana modern meteoric line -135 (Sheppard et al., 1969) provide Past recharge GMWL 50,000–600,000 years support for the old age of the Meteoric water -155 Montana and Bow Island water. Milk River waters The cluster is displaced off the Recent recharge < 50,000 years line and is on trend with pro- -175 gressively older dates. -22.0 -20.0 -18.0 -16.0 -14.0 -12.0 -10.0 -8.0 -6.0 ␦18O water

margins, over geologic time. This theory assumes that from upper-cycle Cretaceous rocks (Figure 6) on Ce- the gas can overcome capillary pressure, which is the dar Creek anticline (Figure 7) and adjacent areas of dominant force for holding the gas in place, and that it North and South Dakota. Conventional shelf-sand- can migrate (Berkenpas, 1991). Such an interpretation stone reservoirs equivalent to the Eagle Sandstone pro- is supported by the fact that gas is produced where no duce shallow gas at West Short Pine Hills field in north- updip water has been defined. Furthermore, there ap- western South Dakota (Shurr, 1998). Unconventional pears to be a significant time difference between the reservoirs in interbedded siltstone and shale equivalent age of updip water and in-situ water. The iodine-iso- to the Eagle produce at Little Missouri field in south- tope technique may permit examination of this ex- western North Dakota (Gautier, 1981; Shurr, 2001). change at water-gas boundaries. The major production has been on Cedar Creek anticline, where initial production was from conventional Other Examples reservoirs in shelf sandstones equivalent to the Judith River Formation (Shurr et al., 1989a). More recently, Cretaceous clastic reservoirs on the southeastern mar- unconventional reservoirs similar to those at Little Mis- gin of the Alberta basin and the northwestern margin souri field have had expanded production using mod- of the Williston basin have been described in detail ern stimulation technologies (Green et al., 1997). because they are the archetype accumulation for early- On the eastern margin of the Denver basin (Figure generation biogenic gas. However, numerous examples 2), more than 30 shallow gas fields produce from chalk of shallow, nonassociated gas accumulations exist on reservoirs in the Niobrara Formation (Hemborg, basin margins throughout the western United States. 1993). The chalk is a high-porosity, low-permeability Although not all of these accumulations have detailed reservoir that loses porosity with increasing depth and geochemical data to verify a biogenic origin, the ma- diagenesis (Pollastro and Scholle, 1986). These accu- jority has attributes very similar to those of the Cana- mulations are similar to those in the northern Great dian and Montana accumulations. Specifically, these Plains, because early-generation biogenic gas is pro- basin margin accumulations are dominantly methane, duced from localized sweet spots within a reservoir occur at shallow depths, cover large areas, commonly that is a relatively continuous blanket. However, the are underpressured, and have relatively low cumulative chalk reservoirs differ from the clastic reservoirs in one production values per square mile (Shurr, 2001). major aspect: depth of burial is important in the chalk On the southwestern margin of the Williston ba- and should be included in the event chart (see Figure sin, gas of probable biogenic origin has been produced 5A).

1956 Unconventional Shallow Biogenic Gas Systems GEOLOGIC FRAMEWORK: LATE- GENERATION SYSTEM

The Devonian Antrim Shale yields significant biogenic N gas production on the northern margin of the Michigan basin. Hydrogeochemical studies have demonstrated that the gas was generated in the relatively recent geologic past (Martini et al., 1996). Consequently, this accumulation is taken as the archetype for late-genera- S tion biogenic gas systems. In contrast with the early-generation system in the Cretaceous of the northern Great Plains, the Antrim is thin, has little lithologic variability, and possesses a well-developed groundwater flow system that controls gas generation. The biogenic floor in the Antrim is defined (1) by contemporary groundwater conditions that provide favorable environments for methanogenic microbes and (2) possibly also by fracture permeability. The biogenic floor in the early-generation system depends on environmental conditions that existed at or immediately after the time of deposition. These differences in timing are associated with pods of gas-generating rocks that have distinctly different shapes (see Figures 4, 5): the gas-prone Antrim has a ring shape around the basin margin, whereas the Cretaceous gas- prone units are continuous blankets. In both systems, A viable commercial production is limited to specific parts of basin margins where geologic structures exert influence. SN Although the Antrim Shale is an example of a fractured shale reservoir (Curtis, 2002), it is different from other fractured shale plays. The Antrim has a low thermal maturity and a high total organic carbon, is thin and shallow, and has large amounts of coproduced water when compared with other fractured shales (Hill 10 km and Nelson, 2000). A small thermogenic component 100 m 10 mi is likely in the methane produced on the basin margin; 1000 ft fractured shale production elsewhere tends to be dominated by thermogenic gas. Although resource estimates for the Antrim have generally included the B deeper, thermogenic gas (for example, 19 tcf recoverable resources [Dalton, 1996]), successful produc- Figure 13. (A) Area of late-generation biogenic gas production on the northern margin of the Michigan basin (modified tion is concentrated on the northern margin of the ba- from Walter et al., 1997). (B) Cross section through the northern sin. In 1999, there were 6500 Antrim gas wells in the margin of the Michigan basin, showing the gas-prone Devonian Michigan basin, and production was 190 bcf (Hill and Antrim Shale subcropping beneath glacial deposits (modified Nelson, 2000). from Martini et al., 1998). Biogenic gas production on the northern margin of the Michigan basin (Figure 13) is from the Upper De- vonian Antrim Shale. Below the Antrim, Lower and units are mainly clastic. However, the overlying mantle Middle Devonian rocks are dominantly carbonates; of glacial deposits is critical to the hydrogeochemical above the Antrim, Upper Devonian and Mississippian conditions that result in biogenic gas generation. The

Shurr and Ridgley 1957 following description of the geologic framework for the time, deltaic sediments prograded eastward from low- late-generation Antrim gas system is largely drawn lying cratonic source areas along the western seaway from a series of publications (Martini et al., 1996, margin. 1998; Walter et al., 1996, 1997) that are based on Antrim black shales serve both as source beds for work funded by the Gas Research Institute. biogenic gas and as reservoir rocks. The Norwood and Lachine members have total organic carbon values of Rocks of the Antrim Shale 0.5–24%, and the organic material is dominantly marine. The gray shales of the intervening Paxton Mem- The Antrim Shale is divided into four units, starting ber have lower total organic carbon (0.3–8%), and the with the Norwood Member at the base and progressing organic material includes a greater abundance of ter- upward through the Paxton Member, Lachine Mem- restrial matter (Martini et al., 1998). The shales have ber, and the informal upper Antrim member (Gut- a low thermal maturity, as indicated by vitrinite reflec- schick and Sandberg, 1991a). The main targets for gas tance values. The Antrim Shale generally has low po- production are the Norwood and Lachine members, rosity, but fractures greatly enhance the effective po- which act as both source beds and reservoir rock. They rosity and permeability. Fracture sets constitute the are laminated, silty, organic-rich black shales with plumbing system for groundwater movement and are abundant carbonate concretions. Between the Nor- an important element of the geologic structure in this wood and Lachine, the Paxton consists of interbedded late-generation biogenic gas system. gray calcareous shale and argillaceous limestone. The upper Antrim is also a black shale, but it has few con- Geologic Structure cretions, and it intertongues toward the western part of the basin with a gray shale interbedded with lime- In the simplest terms, the geologic structure in the stone. It has recently received attention as a secondary area of production is dominated by a gentle dip south- exploration target in areas where gas is produced from ward off the northern basin margin (Figure 13). De- the Norwood and Lachine members (T. Maness, 2001, tailed subsurface mapping shows several small anti- personal communication). Throughout the area of pro- clines superimposed on the regional trend (Walter et duction on the northern basin margin, the organic-rich al., 1996). The anticlines trend northwest and north- Norwood and Lachine have fairly uniform thicknesses east and plunge southward, down the regional dip. of about 30 ft (9.1 m) and 80 ft (24 m), respectively Although little work has been done relating these spe- (Walter et al., 1996). However, thicknesses of all An- cific structures to large-scale structural blocks or to trim members are influenced by erosional cutout at fracture patterns, fractures have been studied locally the base of the overlying glacial sediments (Figure because they are critical to the groundwater flow sys- 13B). tem. For example, influence of fractures on ground- The paleogeographic setting of the Devonian An- water flow could be one of several controls on the trim (Gutschick and Sandberg, 1991b) is similar to the depth of the biogenic floor (T. Maness, 2001, personal Cretaceous Western Interior seaway, in which the communication). rocks of the early-generation gas system were depos- Outcrops, deviated core, and borehole-imaging ited. During the Late Devonian, a transgression logs have been used to characterize fracture patterns flooded the Eastern Interior seaway, and several dein very limited areas (Walter et al., 1996). The domi- pocenters received laterally equivalent, organic-rich nant directions are northeast and northwest for vertical black shales: the Antrim Shale in the Michigan basin, fractures; there is also a northeast set that has moderate the New Albany Shale in the Illinois basin, and the dips. Good agreement exists among the surface and widespread Chattanooga Shale along the western mar- subsurface data sets, although, not unexpectedly, the gin of the Appalachian basin. In the central part of the imaging logs from vertical boreholes provided poor seaway, times of stagnation and anaerobic conditions data on the vertical fractures. Organic-rich black shales produced the Norwood and Lachine members. The of the Lachine and Norwood members have major other members represent times when prodelta clastics throughgoing fractures; gray shales in the other two and lime mud prograded into the center of the seaway. members have many minor fractures that terminate on The Catskill delta complex prograded westward from bedding contacts. Apotria et al. (1994) suggested that the erosion of mountainous uplands in the convergent the fracture patterns were regional in nature and not margin east of the Appalachian basin. At the same related to specific geologic structures. However, dis-

1958 Unconventional Shallow Biogenic Gas Systems tinct flow paths are delineated within the groundwater should show a linear relationship. Gas-water pairs from flow system, and the groundwater flow is thought to the Antrim do cluster along the expected straight line be controlled primarily by fracture sets. This implies (Walter et al., 1997). Points off the linear trend result that definite local patterns do exist within the fracture from mixing with thermogenic gases that have mi- sets. One study (Decker et al., 1992) has directly re- grated updip from deeper parts of the Michigan basin. lated fractures to small-amplitude folds and regional This demonstration of biogenic origin is important be- structural trends. cause other carbon isotopic criteria for biogenic meth- Despite the important controls that fractures exert ane (d13C, 60‰) are not clearly met by Antrim gas on formation fluids, there does not appear to be a clear (Figure 3). relationship between specific geologic structures in the Establishing the age of water is a two-step process Antrim Shale, and gas production (Walter et al., 1996). (Martini et al., 1998). First, tritium, which has a very However, production data are aggregated for large, short half-life, was used to identify young water within multiwell project areas, and differences in individual the glacial drift. Older water in the drift and the An- well performances are masked. Within specific project trim was then dated using 14C measurements. Several areas, however, the differences in well productivity different geochemical models were used to interpret have clear relationships to fracture sets (T. Maness and ages from the 14C data, but the maximum age inter- J. Morabito, 2000, personal communication). preted for any sample is 21,600 yr (Walter et al., Successive loading and unloading by the advance 1996). and retreat of Pleistocene glaciers may have influenced Patterns of variation in salinity and of gas compo- the fracture system and the fluids within it. Regional sition are very similar along the northern margin of the pressure anomalies in Ordovician rocks on the eastern basin (Martini et al., 1998). Highly saline brines, from side of the lower peninsula of Michigan have been re- deeper parts of the basin to the south, mix with rela- lated to glacial loading (Bahr et al., 1994). Overpres- tively fresh water from the subcrops to the north. The sure conditions in this area are believed to be dissipat- result is a fairly sharp salinity gradient (Figure 14A) ing by vertical leakage through zones of increased that generally corresponds to a significant change in gas permeability associated with anticlines and basement composition (Figure 14B). Gas in the southern part of faults (Bahr et al., 1994). In the area of gas production, the area has greater proportions of ethane and propane, glacial meltwater probably provided an influx of rela- whereas methane dominates in the north. Migration of tively fresh water into the plumbing system provided thermogenic gas from the deep basin and mixing with by fractures. This infusion of ground water was im- biogenic gas generated along the northern margin ac- portant in establishing the hydrogeologic conditions re- count for this pattern. Salinity conditions constrain the quired to support methanogenesis. microbial environments to the shallower parts of the basin and thus help to provide the floor for the late- Geochemistry generation system (Figures 1, 4).

The fundamental conclusion from geochemical studies Other Examples in the area of production from the Antrim is that the gas is generated by relatively recent microbial activity. The Antrim Shale on the western and southern mar- Hydrogen-isotope values for gas-water pairs are con- gins of the Michigan basin has geochemical attributes sistent with microbial fractionation of the hydrogen in of gas and water that suggest a close afﬁnity with the methane from the coproduced water. In addition, iso- area of major production on the northern margin (Wal- topic studies indicate that the formation water gener- ter et al., 1997). Speciﬁcally, isotopic values and pat- ally is less than 22,000 yr old. This interpretation of terns in gas and water composition, including salinity, late generation explains why low salinity of water is are similar to the late-generation gas system on the optimal for supporting microbes in the same general northern margin of the basin. Production has been in- area that methane gas proportions are high: the mi- creasing on the western margin, but development on crobe communities established in subsurface environ- the southern margin has been slower. ments are currently generating gas or have done so in The Devonian New Albany Shale in the Illinois the relatively recent geologic past. basin (see Figure 2 for basin location) is also a target If microbes take hydrogen from water to make for shallow gas exploration. The New Albany is a thin, methane, then deuterium values in the gas and water thermally immature black shale similar to the Antrim

Shurr and Ridgley 1959 85° 84° Powder River basin is different from other coalbed

0.05 methane accumulations (e.g., see Ayers, 2002). The 0.1 0.1 0.05 0.5 0.1 Powder River accumulation has a lower coal rank and 45° 1 gas content and a larger thickness and permeability 2 (Nelson, 2000). Isotopic data show that the methane 3 4 0.5 1 4 5 2 in the Powder River basin is biogenic and is the result 5 3 4 10 km of late-stage generation (Rice, 1993b). Other coalbed 10 mi basins have dominantly thermogenic gas, but biogenic A components have been demonstrated to be important in some basins (Scott et al., 1994; Smith and Pallasser, 85° 84° 1996). A recent summary of the Powder River basin coal- 3.5 bed-methane play (Montgomery, 1999) provides the 3.5 45° 3.0 geologic framework for this late-generation accumu- 2.5 lation. Thick, low-rank coals in the Tertiary Fort Union 2.0 3.0 1.5 1.5 2.5 Formation host the gas. Successful development is 2.0 10 km mainly adjacent to the outcrop, on the gently sloping 10 mi eastern margin of the basin. There is a westward com- B ponent of groundwater ﬂow from the adjacent Black Figure 14. Maps showing geochemical variations in the area Hills uplift. As shallow artesian aquifers, the coals have of late-generation biogenic gas production (simpliﬁed from Wal- substantial amounts of water. Dealing with the water ter et al., 1997). See Figures 2 and 13 for location. (A) Salinity coproduced with the gas is an important part of com- in coproduced water, shown by chloride ion concentrations in pletion procedures. moles per liter. Note that salinity decreases toward shallow ar- ם eas to the north. (B) Gas composition shown as log C1/(C2 C3). Note that the driest gas is produced in the shallow areas ASPECTS OF EXPLORATION AND to the north. DEVELOPMENT

Early-generation and late-generation biogenic gas sys- and dissimilar to other fractured shale reservoirs (Hill tems share similarities but also have striking contrasts and Nelson, 2000). Most production is on the eastern with respect to exploration and development. Devel- margin of the basin in western Indiana and Kentucky opment histories are broadly similar for both types of (Walter et al., 1997). Gas compositions and salinity system. Drilling and completion techniques are ex- patterns indicate that Kentucky has high-salinity basin tremely different. brines and thermogenic gas. In contrast, the production area in western Indiana has lower salinity water, and Development Histories the gas is dominantly biogenic (Walter et al., 2000). Freshwater incursion into the subcrop belt has proba- Development histories in biogenic systems follow a ge- bly resulted in subsurface environmental conditions fa- neric three-phase cycle. Although timing of the cycle vorable for methanogenic microbes. However, large may vary slightly from area to area, in general the initial quantities of water are produced with the gas, and the phase of historic production occurred in the late nine- production of gas is irregularly distributed. Although teenth and early twentieth centuries; the middle phase these factors have limited full-scale development of the of expanded production occurred in the middle twen- New Albany biogenic gas reservoirs, the play continues tieth century; and the later phase of much expanded to receive attention. A consortium of gas exploration production occurred late in the twentieth century and and service companies has recently made available the continues on into the twenty-ﬁrst century. results of a multiyear study of the play (Hill, 2001). Initial historic production of biogenic gas involved Coalbed methane produced on the northeastern a minimum of science and technology. Because the gas margin of the Powder River basin in Wyoming (see is shallow, the drilling of water wells commonly dem- Figure 2 for basin location) is another example of late- onstrated a potential for gas production. Although in- generation biogenic gas. Gas from Tertiary rocks in the tentional exploration was dominated by serendipity

1960 Unconventional Shallow Biogenic Gas Systems and luck, the first gas wells generally were for individ- ultimate production of a well may be adversely af- ual domestic use. Eventually, local consumption ex- fected by rock-fluid interactions with reactive clays, tended to small municipalities, and production focused fines migration, or water retention (Bennion et al., near population centers. 1996). Drilling and stimulation practices have evolved Subsequent evolution beyond local historic pro- to minimize problems such as these. During the early duction and consumption depended on development 1970s, standard techniques included water-based of a gathering and transportation infrastructure. In drilling and frac fluids or air drilling. Formation dam- general, exploration emphasized buoyancy traps in age as a result of swelling clays was minimized by ad- conventional reservoirs on specific geologic structures. dition of potassium chloride, hydroxyaluminum so- The result was a series of small fields developed in lutions, and carbon dioxide (Anderson, 1974). In the limited sweet spots. Distribution to regional centers early 1980s, wire-line logs were used to pick sandy of consumption generally required intrastate pipe- layers, and perforations were expanded above and be- lines. low these layers. The larger intervals commonly were The latest stage of biogenic gas production in any stimulated by a single fracture treatment (Evans, given accumulation relies on modern technology to 1984). By the middle 1980s, multistaged fracture develop unconventional reservoirs. Often, technolog- treatments were done by dividing the reservoir into ical improvements can be used because tax credit pro- lower and upper parts and doing independent frac grams provide additional economic incentives. Uncon- jobs on each. Later in the decade, water-free frac flu- ventional reservoirs cover large areas, and expanded ids based on the gelling and crosslinking of methanol production commonly comes with step-out drilling in were developed. These water-free fluids reduced clay regimented, factory-type drilling and completion pro- swelling, fines migration, and capillary pressure (Jabs jects. Hundreds of shallow wells are tied into large et al., 1991). In the early 1990s, oil-based drilling mud gathering systems in a relatively short period of time. was used to better determine reservoir properties and Transportation by interstate or international pipelines minimize formation damage (Georgi et al., 1991). supplies the gas to centers of consumption far from Multiple directional wells were drilled from single commonly remote gas fields. pads in the Medicine Hat pool during the middle of the decade (Becker, 1995). Recent technologic ad- Drilling, Completion, and Stimulation Techniques vances in drill bits and coiled tubing have improved penetration rates (Pinney and Rodrigues, 1999). Early- and late-generation systems have sharply dif- Fine-grained Cretaceous reservoirs in the United ferent drilling and completion techniques. In early- States are also water sensitive, and a variety of drilling generation systems, epitomized by low-permeability, and completion techniques have been attempted. On high-capillary-pressure Cretaceous reservoirs in the Bowdoin dome, Montana, drilling has employed low- northern Great Plains, water is an anathema. Late- fluid-loss, polymer-based muds and stimulation fluids generation systems, such as the Antrim Shale in containing a potassium-chloride, water, and carbon- northern Michigan, routinely deal with copious water dioxide mixture; log interpretation commonly has during drilling and completion. been based on crossovers of porosity logs (Rice et al., During initial phases of historic exploration and 1990). On Cedar Creek anticline, Montana, tradi- development, drilling in both early and late systems tional practices included perforation over a 500 ft commonly used cable tool rigs, and stimulation of the (150 m) interval and a single-stage frac job; later, reservoir rocks was minimal. During the middle more-limited intervals were perforated, and three- phases, rotary rigs were standard, wire-line logs com- stage frac jobs were used to greatly improve produc- monly were used, and stimulation techniques were tion and enhance economics (Green et al., 1997). On unexceptional. However, the most recent phase of ex- the eastern margin of the Denver basin, drilling and ploration and development generally has involved completion techniques in Cretaceous chalks are simi- new technology in drilling and completion to effi- lar to those used in the fine-grained clastics of the ciently exploit shallow, unconventional biogenic gas northern Great Plains. The chalks are drilled with reservoirs. low-water-loss mud, are not acidizing because of wa- Early-generation systems in fine-grained, high- ter sensitivity, and are fracture-stimulated using a capillary-pressure Cretaceous reservoirs in Canada are foam-frac or a carbon dioxide/gelled water fluid (Ab- particularly prone to formation damage by water. The bott, 1993).

Shurr and Ridgley 1961 Late-generation biogenic gas requires water in the Production and Resource Estimates system, and, consequently, drilling and completion techniques must deal with the water. A review of his- Estimates of shallow biogenic gas resources have varied toric practices in the Antrim Shale emphasizes simi- widely because of different assessment methods and larities between Antrim gas wells and coalbed meth- because the biogenic component is not always clearly ane wells and provides a summary of the evolution in recognized. For example, Antrim resource estimates completion technologies in the Antrim (Frantz et al., range from more than 100 tcf gas in place (Frantz, 1996). Antrim wells, like coalbed methane wells, are 1995/1996) to less than 20 tcf of recoverable resource dewatered. Initial high rates of water production (up (Dalton, 1996). Both these estimates include possible to 500 bbl/day) decline progressively, with an atten- thermogenic gas in other parts of the Michigan basin. dant increase in production rates of gas. Before the Recoverable resources of late-generation biogenic gas 1990s, Antrim wells routinely had open-hole comple- on the northern basin margin are estimated to be about tions and employed a single-stage stimulation on one 5 tcf (Dalton, 1996). producing interval. High-pressure gas was injected in Resource estimates are commonly very different places to help remove water, but the resulting bot- from cumulative production. Again, some of these dif- tom-hole pressures limited gas production. During the ferences may be artifacts of the estimation techniques, 1990s, Antrim completions were cased hole, and a but a control also appears to exist that is intrinsic two-stage stimulation was applied to two separate within the geologic framework. This is particularly true production intervals. Nitrogen foam-frac fluid was in the early-generation systems of the northern Great used in conjunction with a sand proppant. Water was Plains. removed by pumping from a several-hundred-foot rat hole that acts as a sump below the producing interval. Montana Thus, there are two flow lines from the wellhead: one At seven different localities in Montana (Figure 7), ap- for water and another for gas. Evolution of technolo- proximately 880 bcf of biogenic gas has been produced gies employed in Antrim gas wells has resulted in im- from shallow Cretaceous reservoirs (Figure 6) since proved production and enhanced economics for the record keeping began (IHS Energy Group, 2001, un- play. Similar economic benefits were realized when published data). On the west side of the Sweetgrass technologic changes improved production in the Cre- arch, about 13 bcf has been produced from sandstones taceous reservoirs in the northern Great Plains. that correlate with the lower part of the Belle Fourche Formation. On the east side of the Sweetgrass arch, sandstones in the upper Belle Fourche have produced ECONOMIC ASPECTS approximately 7 bcf. In the area of Bowdoin dome, about 234 bcf has been produced, mainly from the up- The economic viability of unconventional biogenic gas per Belle Fourche, the Greenhorn Limestone, and the systems has benefited greatly from improved technol- Bowdoin sandstone. At Battle Creek field, nearly 50 ogy. Ironically, however, low prices in the past have bcf has been produced from the Eagle Sandstone. The limited early applications of some expensive new tech- majority of shallow biogenic gas production in Mon- nologic tools. For example, the costs of three-dimen- tana has been from the Tiger Ridge area, where almost sional seismic programs are difficult to justify with the 525 bcf has been produced. Most of the production is shallow, low-volume wells that characterize biogenic from conventional reservoirs in the Eagle Sandstone, accumulations. Furthermore, exploration programs with some contribution from the Judith River, Nio- based on new geologic concepts or improved technol- brara, and upper Belle Fourche formations. In and ogy rarely are sustained directly by development of around Cedar Creek anticline, approximately 44 bcf biogenic gas fields. Most regional geology and experi- has been produced from the Judith River Formation mental engineering is performed by academic or insti- and Eagle Sandstone. The southernmost production in tutional research teams. Most increased production is eastern Montana is from the Eagle at Liscomb Creek the result of step-out drilling or in-field exploitation of and Pumpkin Creek fields, where approximately 7 bcf deeper targets. Increased natural gas prices may gen- has been produced. erate more investment in exploration, but the results Cumulative production of shallow biogenic gas has must be clearly effective in generating expanded pro- not yet exceeded 1 tcf from all the reservoirs in central duction and realized resource potential. and eastern Montana. However, various estimates of

1962 Unconventional Shallow Biogenic Gas Systems gas reserves or resources in place indicate that the area cover large areas, have source rocks in close association contains significantly more potential. This difference with unconventional reservoir rocks, and are probably may result in part from market and economic forces gas charged throughout their extent. These accumu- and lack of infrastructure that tend to drive exploration lations commonly require assessment techniques that and production. Past estimates suggest that as much as are different from those used for conventional accu- 41 tcf could be added to reserves in continuous-type mulations. For example, a recent U.S. Geological Sur- Cretaceous reservoirs in northern and central Montana vey assessment of biogenic gas in Montana produced a (Rice and Spencer, 1996). A more recent estimate mean estimate of more than 40 tcf of potential addi- places the resources at less than 10 tcf (Ridgley et al., tions to reserves (Rice and Spencer, 1996). It used a 2001b). However, the more recent estimate is for re- cell-based assessment technique especially designed for sources that have the probability of addition to reserves continuous-type accumulations (Schmoker, 1996). A in the next 30 yr. It does not, therefore, represent total much larger estimate, of more than 100 tcf (Rice and gas in place or total technically recoverable gas. Shurr, 1980), resulted from simply applying established production numbers over the total large area of Southern Canada reservoir extent. Both estimates are much larger than In southern Canada, the total gas in place is estimated the area’s total cumulative production of less than 1 to be about 4 tcf in the Belle Fourche Formation (Rein- tcf. son, 1995) and about 16 tcf in the combined Milk At least some of the differences between estimated River–Medicine Hat units (Hamblin and Lee, 1997). resource and established production may result from The majority of the production has been in the South- exploration strategies that are inappropriate for un- east Alberta gas field, which extends eastward into conventional accumulations. Alternatively, particular southwestern Saskatchewan (Figure 7). aspects of the geologic framework appear to impose Approximately 6 tcf has been produced in Alberta limits on the extent of the continuous-type accumu- through October 2000 (IHS Energy Group, 2001, un- lations. The resulting sweet spots are larger than con- published data). This includes 1.5 tcf each from the ventional accumulations (Shurr, 2001) but are still Milk River Formation and Medicine Hat Sandstone substantially smaller than the total prairie area visual- and an additional 2.3 tcf from the combined units. Iso- ized as having potential in the model of a true contin- lated pools in the upper Belle Fourche provide addi- uous-type accumulation. tional production. Sweet spots are an intrinsic aspect of the geologic Nearly 2 tcf has been produced in Saskatchewan framework. For example, the best reservoirs may be through October 2000 (Saskatchewan Energy and concentrated on discrete paleotectonic blocks because Mines, 2001, unpublished data). Within the Southeast of facies changes or erosion on an unconformity sur- Alberta gas field, about 0.6 tcf is from the Milk River, face. Additionally, groundwater flow patterns may 0.3 tcf is from the Medicine Hat, and 0.5 tcf is from have modified the original distribution of early-gener- the combined units. An additional 0.3 tcf comes from ation gas. However, geologic structure has the most the lower Belle Fourche within the field. The Belle pervasive influence in subdividing the continuous-type Fourche is also productive in smaller fields scattered accumulations. Deposition, erosion, and fluid flow are outside of the Southeast Alberta field (Ridgley and Gil- all influenced by geologic structures that range in size boy, 2001). from major lineament-bounded lithosphere blocks The difference between the total gas in place, 20 down to small blocks that are components of a fault tcf, and the total cumulative production, 8 tcf, pro- zone. Current production in the northern Great Plains vides an estimate of the remaining reserves plus pos- is clearly related to individual structural features, some sible future resource potential, not all of which may be of which are regional in nature (see Figure 7). Shallow technically recoverable. gas accumulations, the majority of which are biogenic, tend to be associated with specific geologic structures Resource Estimates and Geologic Framework on the margins of the Williston basin and other basins in the Rockies (Shurr, 2001). The Cretaceous reservoirs that host early-generation The early-generation gas in the Cretaceous of biogenic gas in the northern Great Plains generally the northern Great Plains is thus a particular cate- meet the criteria for continuous-type accumulations gory within continuous-type accumulations. Small (Schmoker, 1996). Continuous-type accumulations sweet spots are embedded within larger volumes of

Shurr and Ridgley 1963 unconventional reservoir rock that cover wide areas. tions of biogenic gas are known in the United States However, the large areas are not continuous over the (Figure 15). Of these, the offshore Gulf of Mexico and total extent of the northern Great Plains. Although the Cook Inlet of Alaska (1 and 2 in Figure 15) are par- geologic framework constrains resource distribution, ticularly important. They have biogenic gas resources the total amount of gas available is still substantial. The estimated at about 30 and 5 tcf, respectively (Rice, most recent U.S. Geological Survey estimate of about 1993a). 10 tcf is for reserve additions in the next 30 yr (Ridgley Domestic reserves of shallow biogenic gas are ide- et al., 2001b). Eventually, areas between the sweet ally suited for exploration and development by small, spots that were developed initially may be fully ex- independent operators. Drilling and completion costs ploited. Shallow biogenic gas systems are an extremely are relatively low for these shallow accumulations, and important contribution to meeting the energy needs of lease costs also may be low. Many accumulations are the United States and the world. located in areas that are underexplored. Consequently, leases are relatively easy to acquire. Furthermore, for- Domestic and International Outlook merly stranded sweet spots will come online as the domestic infrastructure expands to meet increased de- Unconventional biogenic gas systems are estimated to mands and attendant higher prices. constitute as much as 20% of the world’s natural gas Although numerous biogenic gas accumulations resources (Rice, 1993a). Accumulations are found in are documented throughout the world (Figure 15), the deltaic, shelf, and nonmarine settings and are domi- production areas in Siberia are the most important nantly in reservoirs that are Cretaceous or Tertiary in (Rice, 1993a). The West Siberia basin (3 in Figure 15) age. In addition to the early-generation and late-gen- has extensive accumulations that were originally eration systems described in preceding sections in this thought to be dominantly biogenic but subsequently article, more than half a dozen additional accumula- have been suggested to be thermogenic. Recent studies

Figure 15. Maps showing documented accumulations of shallow biogenic gas (modiﬁed from Rice [1993a] and Martini et al. [1998]). Numbers refer to speciﬁc production areas discussed in the text.

1964 Unconventional Shallow Biogenic Gas Systems have reviewed the proposed models for gas origin in margins of several basins where production is concen- the West Siberia basin and concluded that the huge trated. Late-generation systems have a ring-shaped ge- reserves, about one-third of the world’s reserves, have ometry that corresponds to areas of production around been released from a dynamic groundwater flow sys- basin margins. Early-generation gas begins to form tem (Cramer et al., 1999; Littke et al., 1999). Al- shortly after deposition, and generation, migration, and though this work has not clearly resolved the primary accumulation may be multiphased over an extended origin of the gas, more than 85% of the gas is isotopi- period of time. As a consequence, the critical moment cally light, dry methane. This accumulation is a mix of for early systems is actually a critical time period. Late- biogenic and thermogenic gas; probably many biogenic generation gas does not form until long after deposition gas accumulations have a thermogenic component. of the reservoir and source rock; the critical moment Other important biogenic gas production in the world is much more limited in time. The early-generation includes in Europe, the Carpathian and Apennine fo- systems reviewed in this summary have unconven- redeeps of Poland and Italy, respectively, (4 and 5 in tional reservoirs that are low-permeability clastics and Figure 15); and in Asia, the Qaidam basin in China and carbonates; the late-generation systems are fractured brine gas in Japan (6 and 7 in Figure 15) (Rice, 1993a). reservoirs of black shale and coal. These differences in In an international perspective, biogenic gas sys- gas reservoirs have associated differences in the con- tems represent important and abundant energy re- comitant formation water: early-generation systems sources for emerging and developing countries. Most tend to have low volumes of producible water, and accumulations are shallow, and, consequently, rela- late-generation systems commonly have high water re- tively cheap gas can be produced. Until an infrastruc- coveries. Drilling and completion practices are dictated ture evolves, widely separated sweet spots can be used by the differences in water between the systems. Water for relatively pollution-free local consumption, includ- is avoided in early-generation systems because the low- ing generation of distributed electricity in small power permeability reservoirs are easily damaged by water plants. Natural gas is a critical component in the eco- blocks and/or swelling clays. Late-generation systems, nomic development of many countries (e.g., Ellsworth in contrast, are dewatered to facilitate production. and Wang, 1999; Badruzzaman, 2000), and shallow Natural gas is the environmental fuel of choice. biogenic gas is readily accessible for exploitation. High demands are projected for both domestic and international production. Deep, thermogenic and associated gas is widely recognized as a resource that will CONCLUSIONS help meet the high demands. However, substantial reserves are also available in shallow biogenic gas sys- Unconventional shallow biogenic gas systems fall into tems, especially where they are productive around ba- two general categories: early-generation systems ex- sin margins. 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