Shale gas, like coalbed methane, is the result of biogenic and/or thermogenic processes. Although thermogenic gas systems have had wide application in gas shale exploration, biogenic gas systems are also important. Early-generation biogenic gas that formed during deposition of the host rock is present on the eastern Williston basin margin. However, it is late-generation biogenic gas that holds the greatest resource potential.
Late-generation biogenic gas forms long after deposition of the host rock and may even be forming at the present day. On the northern margin of the Michigan basin, the Antrim Shale (Devonian) hosts an economic accumulation of late-generation biogenic gas. This area of production provides an exploration model that consists of four main components: 1) a "pasture" of organic matter, 2) a "plumbing system" of fractures, 3) favorable water chemistry, and 4) methanogenic microbes. All of these components are present in Cretaceous shales on the eastern margin of the Williston basin. However, aside from very recent limited drilling, the area is mainly untested.
Total organic carbon (TOC) measured in outcrop and core samples of Cretaceous shales from eastern South Dakota, indicate that the shales range from about 2% to 8% TOC. The Niobrara Formation is a representative unit that is a particularly attractive TOC "pasture". The fracture "plumbing system" varies in scale from regional lineament zones down to faults and fractures visible in outcrops and cores. Favorable water chemistry exists in areas where sulfate values are relatively low and bicarbonate values are relatively high.
A glacial aquifer near Dolton, South Dakota, is locally in contact with fractured Cretaceous shale and has an area of methane shows associated with favorable water chemistry. In addition to historical anecdotes, methane has been measured in headspace over water samples drawn from the aquifer. Methane is also directly detected in the air column above the water level in well bores, using equipment commonly employed in environmental monitoring.
Samples of water from the Dolton Aquifer incubated in a series of microbiology experiments, demonstrate that real-time methanogenesis does take place. The methanogens "graze" the TOC "pasture" in favorable water environments. Exploration for late-generation microbial methane must employ biological and agricultural ideas, as well as geologic and hydrologic concepts. In addition, operators willing to experiment with new drilling and completion techniques are needed. Cretaceous units on the eastern margin of the Williston basin are currently an unassessed potential gas shale resource.
MICROBIAL METHANE SYSTEMS ON SHALLOW BASIN MARGINS IN THE GREAT PLAINS
There are two separate and distinct microbial methane systems on shallow basin margins in the Great Plains. These natural accumulations include both established, commercial reserves and potential, unevaluated energy resources.
Microbial methane that forms at the time of deposition of the host rock is ancient gas trapped in a closed system. This early generation methane is found in dominantly marine rocks on the margins of depositional basins. Examples include gas in Cretaceous units deposited on the shallow-water shelves of the Western Interior Basin in Montana, Colorado, Kansas, and the Dakotas.
Microbial methane that forms in the relatively recent geologic past is young gas that may be locally trapped, but is generally part of an open, dynamic flow system. This late generation methane is found on the margins of structural basins, usually in continental environmental settings. Examples include coalbed methane in Tertiary rocks in the Powder River Basin and in Pennsylvanian rocks in the Forest City Basin. Methane in fractured Cretaceous shales and overlying glacial aquifers in the eastern Williston Basin is an additional example, though unevaluated.
In both microbial methane systems, fractures and total organic carbon sources are important. However, the microbes in early generation systems are no longer present; modern marine environments must be used as analogs. In late generation systems, the microbes are often still alive and viable. Consequently, these natural accumulations provide direct evidence for the environmental conditions needed to support the methanogens. Consortia, community structures, and metabolic pathways may not be the same for the two systems.
Traditionally, isotopic compositions have been used to distinguish the two systems, but data from Great Plains examples suggest that simple bulk composition is also diagnostic. However, exploration and evaluation of this potential domestic energy resource requires concepts from the life sciences, as well as the earth sciences.
THE DOLTON AQUIFER--A "RESEARCH FARM" FOR FIELD STUDIES OF UNCONVENTIONAL SHALLOW GAS
Water chemistry patterns in the Dolton Aquifer correspond with the area where methane is observed. Specifically, high bicarbonate values and low sulfate values outline the area of methane occurrences. Methane is documented by historic anecdotes, by headspace analyses of water samples, and by direct measurement in observation wells. Measurements using a gas detector show that methane and carbon dioxide values increase exponentially down the well bore toward the water level. Outside the area of methane occurrences, gas detector measurements do not show methane to be present.
The water chemistry reflects environmental conditions that are favorable for methanogenic microbes. Water samples from the Dolton Aquifer have been used in laboratory experiments that demonstrate two important conclusions (Patrick C. Gilcrease and Michael Green, SDSMT, personal communication, 2006): 1) microbes that produce methane are present in the water, and 2) these methanogens can effectively utilize crushed Cretaceous bedrock to generate methane.
The Dolton area provides an ideal field setting to investigate relationships between fractured bedrock, ground water, and methanogenesis. It is a "research farm" where geology, hydrology, and microbiology can all be integrated to provide useful perspectives on exploration and production in other areas. In addition to fractured shale and tight gas reservoirs, the concepts have application in areas of coal bed methane production such as the Powder River Basin in Wyoming and the Forest City and Cherokee Basins in Kansas.
THREE SHALLOW GAS SYSTEMS ON THE EASTERN MARGIN OF THE WILLISTON BASIN
Migrated thermogenic gas was probably generated in Paleozoic rocks in the deep kitchen at the center of the Williston Basin in western North Dakota. Subsequently the gas migrated, perhaps in association with oil, laterally out of the deep kitchen and onto the shallow basin margin. In addition, vertical migration took the gas to stratigraphically higher Cretaceous reservoirs, such as the Dakota Sandstone near Pierre in central South Dakota. Published gas compositions and recent headspace analyses indicate that gas in this area has a definite thermogenic component. However, the high methane content suggests that a biogenic gas system is dominant.
Early generation biogenic gas forms at the sediment-water interface during and shortly after deposition of the host rocks. This "old" biogenic gas has undergone minimum migration and consequently has remained trapped in unconventional reservoirs since the time of deposition. In the eastern Dakotas, oil tests with mud logs that start at the "grass roots" often have gas shows distributed throughout the classic Cretaceous section, viz. Graneros Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, and Pierre Shale. In addition, shows are found in water wells and in places the gas has been produced for local consumption. However, it is often difficult to distinguish this early generation biogenic gas from late generation biogenic gas.
Late generation biogenic gas is microbial methane that is generated in the relatively recent geologic past, long after deposition of the host rocks. This "young" biogenic gas is found in ultrashallow Cretaceous Niobrara and Pierre fractured shale reservoirs that subcrop beneath glacial drift. Methanogenesis probably initiated when meltwater inoculated the shale with microbes during glaciation and may even continue to the present day.
Microbial methane is an underexplored and unassessed shallow gas resource on the eastern margin of the Williston Basin. The geologic setting and ultrashallow gas accumulations are very similar to the northern margin of the Michigan Basin where trillions of cubic feet of microbial methane have been produced from the Devonian Antrim Shale. Four specific system components identified in the Antrim play can be applied to the Cretaceous of the eastern Williston Basin. There must be: 1) an adequate "pasture" of total organic carbon (TOC); 2) a "plumbing" system of natural fractures; 3) water that provides a favorable environment for the microbes; and finally 4) the methanogenic microbes must be present and thriving in the system.
Measurements of TOC range from about 1% to 10% for 80 samples collected from the Greenhorn, Carlile, and Niobrara in three cores in eastern South Dakota. This "pasture" should support "grazing" by methanogenic microbes. Lineament zones mapped on satellite images, outline Precambrian basement blocks and are corridors of natural fractures. These fracture zones constitute the "plumbing" that gives water-borne methanogens access to the TOC pasture. Ground water chemistry that provides a congenial environment for the necessary microbe consortia, helps to define more localized sweetspots within the commodity-type play. Ultrashallow gas occurrences demonstrate that methanogenic microbes are present and active in the system.
All of these exploration components are well illustrated at a microbial "research farm" near Dolton in McCook County, South Dakota. Observation wells associated with a rural water system provide data from a glacial aquifer that supplied gas for historic local consumption. Headspace analyses of water samples and direct detection of methane in the well bores demonstrate that gas is currently present. Water chemistry shows high bicarbonate and low sulfate values in the areas where methane is found. This is a characteristic pattern in other accumulations of microbial methane. The glacial aquifer is locally in contact with fractured, organic-rich Cretaceous shale. It is speculated that one microbe consortium converts the TOC to a feedstock that is transported in water and then used for methanogenesis by a separate consortium in another part of the aquifer. Preliminary laboratory data appear to demonstrate the presence of methanogenic microbes in the aquifer.
The Dolton "research farm" is a small natural research setting that provides direction for an exploration model aimed at accumulations of microbial methane on the eastern margin of the Williston Basin. However, only drilling and completion activities can evaluate the significance of the resource.
CONTRASTS IN SHALLOW BIOGENIC GAS SYSTEMS ON THE WESTERN AND EASTERN MARGINS OF THE WILLISTON BASIN
Early generation biogenic gas is produced on the western margin. This "old" biogenic gas formed by methanogenesis near the sediment-water interface during and shortly after deposition of the Cretaceous host rocks. The gas has remained trapped in unconventional reservoirs with relatively little migration since the time of generation. Production tends to be associated with paleotectonic highs where winnowing improved local reservoir quality.
Late generation biogenic gas or microbial methane dominates historic production and gas shows on the eastern margin. This "new" biogenic gas formed by microbial activity long after deposition of the Cretaceous source rocks. It probably resulted from inoculation of organic-rich bedrock by meltwater during glaciation. The geologic setting and ultrashallow gas occurrences are very similar to the northern margin of the Michigan basin where microbial methane is produced from the Devonian Antrim Shale. Microbial methane on the eastern margin of the Williston basin is an unevaluated and underexplored resource.
BUSINESS MODELS FOR SHALLOW GAS DEVELOPMENT ON THE MARGINS OF THE WILLISTON BASIN
FIRST STEPS TOWARD EVALUATION OF ULTRA-SHALLOW GAS ACCUMULATIONS IN THE NORTHERN GREAT PLAINS
A glacial aquifer on the southeastern margin of the Williston basin provides a "research farm" to investigate an ultra-shallow microbial methane system at field scales. The Dolton Aquifer in southern McCook County, South Dakota, is locally in contact with Cretaceous bedrock that furnishes the feedstock for methanogens. Analyses of TOC and micropaleontology from recently acquired bedrock cores are used to characterize this microbe pasture.
The Dolton Aquifer is penetrated by a grid of closely spaced observation wells and test holes. Gas shows are clustered within an elongate sweetspot where water compositions have high bicarbonate and low sulfate values. Concentrations of headspace gas are higher in the sweetspot than in surrounding areas. Similar patterns are documented using a gas detection meter to directly measure methane concentrations in observation wells.
On going investigations at the Dolton Aquifer "research farm" will provide the information framework needed to carry out evaluation, exploration, and development of ultra-shallow microbial methane systems.
EXPLORATION STRATEGIES FOR ULTRA-SHALLOW MICROBIAL METHANE ON THE EASTERN MARGIN OF THE WILLISTON BASIN
Ultra-shallow microbial methane on the eastern margin of the Williston Basin is an untested unconventional gas play. Resource potentials are indicated by favorable geologic and hydrologic conditions on the shallow basin margin and by historic gas shows. However, some exploration strategies for this potential resource are not in the tool box of strategies normally employed by the oil and gas industry.
Production from the Devonian Antrim Shale on the northern Michigan Basin margin provides exploration strategies that can be applied to organic rich Cretaceous shales on the eastern Williston Basin margin. The main exploration components can be cast in an agricultural grazing metaphor: 1) pastures of total organic carbon (TOC) for microbe grazing; 2) plumbing systems of fractures; 3) water quality that will support the microbes; and 4) multiple and sequential microbe consortia.
The TOC pasture is documented in outcrop samples (3 to 10%) and in cores. Sixty core samples from the Pierre, Niobrara, Carlile, and Greenhorn Formations at Iroquois and Sisseton, South Dakota, averaged 3.8% and ranged from .2 to 10%. TOC values vary near local paleogeographic features, such as Sioux Ridge, but regional variations conform with published trends for the Cretaceous Western Interior Seaway.
Fracture plumbing systems are available as regional corridors along reactivated Precambrian basement blocks. The fractured block boundaries are mapped as lineament zones on Landsat images and historical gas shows are clustered along the zones in the eastern Dakotas. In particular, ultra-shallow gas shows in glacial drift appear to be common over subcrops of fractured Niobrara Formation.
Water quality favorable for methanogenesis is generally high bicarbonate (greater than 400 mg/L) and low sulfate (less than 500 mg/L). A water quality data base maintained by the state of South Dakota contains more than 7200 analyses, mostly from shallow glacial aquifers. These data can be used for exploration because the microbial methane is in a hydrologic system that includes fractured Cretaceous subcrops as well as the glacial aquifers.
A geographic information system is used to sort and plot gas shows and water quality data. Clusters of favorable water quality values are found near gas shows; some clusters that have no documented gas show warrant specific investigation. Two excellent examples of water quality clusters near gas shows are located in McCook and Charles Mix Counties, South Dakota.
The ultra-shallow Dolton Aquifer in McCook County hosts microbes that produce methane. It is also a "research farm" that provides an opportunity to demonstrate the utility of some unconventional exploration tools. Methanogens in this glacial aquifer probably use a TOC pasture in subcropping, organic rich Cretaceous bedrock. Water quality patterns conform with regional generalizations: methane occurrences are confined to an elliptical area of high bicarbonate and low sulfate ground water. Methane is documented in headspace analyses of water samples and by direct detection in the well bore above the water. Preliminary microbial experiments with water from the aquifer show active methanogenesis in wells with methane detected by field survey and headspace analyses. No methanogenesis is observed from water in wells that had no observed methane.
Exploration strategies for ultra-shallow microbial methane that are useful on the eastern margin of the Williston Basin can also be applied in other areas. For example, the Barnett Shale play in the northern Fort Worth Basin may also have a relatively unexplored microbial methane component to the south on the Lampasas Arch.