George W. Shurr

Unconventional shallow biogenic gas systems

Unconventional shallow biogenic gas falls into two distinct systems that have different attributes. Early-generation systems have blanketlike geometries, and gas generation begins soon after deposition of reservoir and source rocks. Late-generation systems have ringlike geometries, and long time intervals separate deposition of reservoir and source rocks from gas generation. For both types of systems, the gas is dominantly methane and is associated with source rocks that are not thermally mature. Early-generation biogenic gas systems are typified by production from low-permeability Cretaceous rocks in the northern Great Plains of Alberta, Saskatchewan, and Montana. The main area of production is on the southeastern margin of the Alberta basin and the northwestern margin of the Williston basin. The huge volume of Cretaceous rocks has a generalized regional pattern of thick, nonmarine, coarse clastics to the west and thinner, finer grained marine lithologies to the east. Reservoir rocks in the lower part tend to be finer grained and have lower porosity and permeability than those in the upper part. Similarly, source beds in the lower units have higher values of total organic carbon. Patterns of erosion, deposition, deformation, and production in both the upper and lower units are related to the geometry of lineament-bounded basement blocks. Geochemical studies show that gas and coproduced water are in equilibrium and that the fluids are relatively old, namely, as much as 66 Ma. Other examples of early-generation systems include Cretaceous clastic reservoirs on the southwestern margin of Williston basin and chalks on the eastern margin of the Denver basin. Late-generation biogenic gas systems have as an archetype the Devonian Antrim Shale on the northern margin of the Michigan basin. Reservoir rocks are fractured, organic-rich black shales that also serve as source rocks. Although fractures are important for production, the relationships to specific geologic structures are not clear. Large quantities of water are coproduced with the gas, and geochemical data indicate that the water is fairly fresh and relatively young. Current thinking holds that biogenic gas was generated, and perhaps continues to be, when glacial meltwater descended into the plumbing system provided by fractures. Other examples of late-generation systems include the Devonian New Albany Shale on the eastern margin of the Illinois basin and the Tertiary coalbed methane production on the northwestern margin of the Powder River basin. Both types of biogenic gas systems have a similar resource development history. Initially, little technology is used, and gas is consumed locally; eventually, sweet spots are exploited, widespread unconventional reservoirs are developed, and transport of gas is interstate or international. However, drilling and completion techniques are very different between the two types of systems. Early-generation systems have water-sensitive reservoir rocks, and consequently water is avoided or minimized in drilling and completion. In contrast, water is an important constituent of late-generation systems; gas production is closely tied to dewatering the system during production. Existing production and resource estimates generally range from 10 to 100 tcf for both types of biogenic gas systems. Although both system types are examples of relatively continuous accumulations, the geologic frameworks constrain most-economic production to large geologic structures on the margins of basins. Shallow biogenic gas systems hold important resources to meet the increased domestic and international demands for natural gas.

Regional Setting of Niobrara Formation in Northern Great Plains

Natural gas is currently produced from the Upper Cretaceous Niobrara Formation in northeastern Colorado, northwestern Kansas, and several small fields in Nebraska. As a part of studies of low-permeability gas reservoirs in the northern Great Plains, the regional geologic setting of the Niobrara has been investigated in North Dakota, South Dakota, and Nebraska. In North Dakota and South Dakota, 2 Niobrara chalk tongues grade into calcareous shales, which in turn intertongue northward and westward with thick noncalcareous shales of the Pierre Shale. In Nebraska, 3 Niobrara chalk tongues and intervening calcareous shales thicken southward as a unit. An isopach map of the interval from the Niobrara base to the Ardmore Bentonite Bed in the overlying Pierre Shale shows an oblate area of thin sediments extending from southwest to northeast out of the Nebraska Panhandle and into central South Dakota. Erosion along the basal Niobrara unconformity is greatest in the area of thin strata and an unconformity is locally developed on the top of the Niobrara in that same area. Structural contours of the Ardmore Bentonite Bed suggest that the area of thin N obrara strata presently approximates the south flank of the Williston basin and north flank of the Denver and Kennedy basins. Chalk tongues are interpreted as low-angle shelf surfaces, known as carbonate ramps, which sloped gently to the northwest and southeast off a paleotectonic high. The paleotectonic high cut obliquely across the seaway and was close to the position of the Transcontinental arch that influenced Paleozoic sedimentation. As a result, the present-day stratigraphy and structural setting of the Niobrara are different north and south of the arch crest.

Zuni Sequence in Williston Basin--Evidence for Mesozoic Paleotectonism

The Zuni sequence in the Williston basin is a large-scale lithogenetic package bounded by interregional unconformities. Within the sequence, three major subdivisions are separated by unconformities or marker beds and correspond with chronostratigraphic units: (1) Middle and Upper Jurassic, (2) Lower Cretaceous, and (3) Upper Cretaceous and Paleocene. The basin has clear expression in the Jurassic subdivision, poor expression in the Lower Cretaceous, and good expression in the Upper Cretaceous. Jurassic units are thick in the basin center, thin in central Montana and northeastern Wyoming, and thick in southern Montana and western South Dakota. Jurassic marine carbonates are found along the western basin margin and marine sandstones mark the southern and eastern margins. Lower Cretaceous rocks display a regional thinning from west to east with little expression of the basin center and margins. Lower Cretaceous marine and nonmarine sandstones form blanket deposits. Upper Cretaceous units preserved below the Paleocene show a clearly defined depocenter at basin center. Upper Cretaceous shales characterize the depocenter; facies patterns of marine sandstones on the west and south and carbonates on the east correspond with paleotectonic elements distributed around the basin margin . A series of seven marginal paleotectonic elements surround the basin center on the west, south, and east in the United States. Five more marginal elements have been described in Canada. Occurrences of oil in the Jurassic and Lower Cretaceous and of natural gas in the Upper Cretaceous are broadly related to the pattern of marginal paleotectonic elements.

Exposures of Greenhorn Formation and Carlile Shale (Upper Cretaceous) at Lake Traverse, western Minnesota

The Greenhorn Formation and Carlile Shale (Cenomanian and Turonian) crop out in western Minnesota near the town of Browns Valley. The locality (NE¼, NE¼, Sec. 2, T125N, R49W) provides important lithologic and biostratigraphic data because it is situated in an area of sparse outcrop. About 3 m of chalk and limestone are assigned to the Greenhorn; Mytiloides labiatus (Schlotheim) and Watinoceras coloradoense (Henderson) are present in the unit. The Carlile Shale is approximately 35 m thick and includes the Fairport, Blue Hill, Codell Sandstone, and upper unnamed members. Collignoniceras woollgari (Mantel) is present in the Blue Hill Member. The Greenhorn Formation and Carlile Shale were deposited in shallow marine environments on the eastern margin of the Late Cretaceous seaway. Postdepositional tectonism is suggested by an observed dip of 27°. The units are potential shallow, low-permeability gas reservoirs.

Basement Control on Laramide Deformation in the Powder River and Williston Basins and Adjacent Areas of the Northern Midcontinent—Tectonic Hierarchy in Continental Lithosphere

Continental lithosphere is subdivided into a tectonic hierarchy in the Powder River and Williston Basins and in adjacent areas of the northern midcontinent of North America. Small units less than about 50 sq mi (130 sq km) are termed “basement blocks”; larger elements of approximately 1000 to 1500 sq mi (18,000 sq km) are designated “lithosphere blocks.” At each scale in the hierarchy, elements consist of a mosaic of smaller units. This tectonic hierarchy in continental lithosphere controlled Laramide deformation as well as earlier episodes of epeirogenic activity.

Paleotectonic Controls on Deposition of Niobrara Formation, Eagle Sandstone, and Equivalent Rocks (Upper Cretaceous), Montana and South Dakota: ABSTRACT

The deposition of the Niobrara Formation, Eagle Sandstone, and equivalent Upper Cretaceous rocks was controlled by paleotectonic activity on lineament-bound basement blocks in Montana and South Dakota. Linear features observed on Landsat images provide an interpretation of lineament geometry that is independent of stratigraphic data. Paleotectonism on lineament-bound blocks is documented in three areas that were located in distinctly different depositional environments. In central Montana, coastal and inner-shelf sandstones and nonmarine coastal-plain and wave-dominated delta deposits reflect paleotectonic control by lineaments trending north-south, east-west, northwest, and northeast. In the northern Black Hills, chalks and outer-shelf sandstones End_Page 866------------------------------ reflect control by lineaments trending north-south, northwest, and northeast. In central South Dakota, erosion and deposition of chalk and calcareous shale on a west-sloping carbonate ramp were controlled by lineaments that generally trend northeast and northwest. Paleotectonism on lineament-bound blocks characterized four tectonic zones located in the Late Cretaceous seaway: the western foredeep, the west-median trough, the east-median hinge, and the eastern platform. The regional geometry of all four tectonic zones appears to be related to the geometry of the convergent plate margin on the west. Paleotectonic activity on lineament-bound blocks may have been the result of horizontal forces related to the convergent margin and to vertical forces related to the movement of the North American plate. End_of_Article - Last_Page 867------------

Seismic Expression of Structural Features on Landsat Lineaments: an Example from Denver Basin: ABSTRACT

Lineaments interpreted from Landsat images mark the location and trend of basement faults observed on seismic lines in the eastern Denver basin. Linear features mapped as tone and texture patterns on multispectral scanner images in northeastern Colorado and southwestern Nebraska are used to interpret regional lineaments. Individual linear features up to 25 mi (40 km) long and visible on both bands 5 and 7 define a grid of regional lineaments trending northeast and northwest. Comparisons of lineaments with aeromagnetic and gravity maps and with interpretations of basement geology suggest that lineaments are the boundaries of basement blocks with areas of about 1,000 mi2 (2,590 km2). Constituent linear features within the lineament zone probably mark boundaries of smaller blocks of about 50 mi2 (130 km2). Seismic lines in northeastern Sedgwick County in extreme northeastern Colorado cross linear features that are components of a broad regional lineament that trends northeast and parallels the South Platte River. Seismic data consist of a grid of about 100 mi (160 km) of multifold Vibroseis lines. Basement faults, generally with offsets of less than 100 ft (30 m), are observed in seismic lines that cross some individual linear features. Monoclines and faults are present in Pennsylvanian to Tertiary strata that overlie basement. A time-structure map on the Precambrian and an isochron map of Wolfcamp (Lower Permian) to Precambrian show that the lineament is a 7-mi (11-km) wide zone of small, downdropped basement blocks. Thickening of Permian-Pennsylvanian strata on the down-thrown side of faults suggests tectonic activity. Further tectonic activity is indicated by listric faulting in the Cretaceous Niobrara Formation. End_of_Article - Last_Page 307------------

Geologic Setting and Natural Gas Potential of Niobrara Formation, Williston Basin: ABSTRACT

Chalk units in the Niobrara Formation (Upper Cretaceous) have potential for generation and accumulation of shallow, biogenic gas in the central and eastern Williston basin. Similar to areas of Niobrara gas production in the eastern Denver basin, Niobrara chalks in South and North Dakota were deposited on carbonate ramps sloping westward off the stable eastern platform of the Western Interior seaway. Within the Williston basin, the Niobrara of the western Dakotas, eastern North Dakota, and central South Dakota has different stratigraphic relationships. These three areas can be further subdivided and ranked into six areas that have different exploration potential. The south margin of the Williston basin in central South Dakota is the most attractive exploration area. Niobrara chalk reservoirs, source rocks, and structural traps in the southern Williston basin are similar to those in the eastern Denver basin. Chalk porosities are probably adequate for gas production, although porosity is controlled by burial depth. Organic carbon content of the chalk is high and shows of biogenic gas are reported. Large, low-relief structural features, which could serve as traps, are present. End_of_Article - Last_Page 867------------

Landsat Linear Features in Montana Plains: ABSTRACT

Multispectral scanner images obtained from satellites provide a unique regional perspective of geologic features on the earths surface. Linear features observed on Landsat images are particularly conspicuous and can be mapped easily. In Montana, east of long. 110°W and in adjoining parts of Canada, the Dakotas, and Wyoming, linear features have been mapped on 14 images. Black and white film products in bands 5 and 7 at a scale of 1:1,000,000 were employed. Specific linear features observed on both bands were compiled on a mosaic covering more than 90,000 mi2 (233,000 km2). Trends to the northwest and northeast are most common, but north-south and east-west linear features are also observed. Four separate tectonic regions of the Montana plains seem to be characterized by different populations of linear features. In an area 100 mi (160 km) wide along the Canadian border, linear features trending northwest are common, and only a few local structures, such as Poplar and Bowdoin domes, are present. In the vicinity of the Central Montana uplift, east-west linear features are associated with features trending northwest and northeast. An area 80 mi (129 km) wide along the Wyoming border has linear features which trend dominantly north-south and east-west, although northeast and northwest trends are also present. This part of southern Montana includes the northern flanks of the Big Horn uplift, Powder River basin, and Black Hills uplift. In eastern Montana the western margin of t e Williston basin has linear features which trend mainly northeast and northwest; north-south and east-west trends are rare. Published syntheses of geophysical, structural, and stratigraphic data can be used to establish the geologic significance of specific linear features. Magnetic, gravity, and seismic data suggest that linear features may reflect basement structural elements such as fault-bounded blocks. Some specific geologic structures shown on structure contour maps are marked by linear features. Examples include Bowdoin dome, portions of Cat Creek, Lake basin, and Nye-Bowler fault zones, Cedar Creek anticline, and the Brockton-Froid fault zone. Paleotectonic features interpreted from stratigraphic maps have surface expression on Landsat that have not been recognized previously. For example, the southern margin of the Alberta shelf (Mississippian) appears to correspond with a zone of concentrated eas -west linear features in north-central Montana. End_of_Article - Last_Page 1355------------

No Title Provided: ABSTRACT

Late Cretaceous marine deposition in the western interior of the United States occurred in an epicontinental seaway elongate in a north-south direction. In central Montana, the western side of the seaway was characterized by a broad, tectonically active shelf. In eastern Montana and the western Dakotas, an actively subsiding basin was located in the central part of the seaway. In western and central South Dakota, the eastern side of the seaway was a more stable west-sloping ramp. Distinctive facies belts in the Eagle Sandstone and equivalent rocks are found in each of these tectonic settings, and some specific tectonic features have expression in the facies patterns. However, paleotectonism was even more important than suggested by these regional patterns. Selected study reas End_Page 1355------------------------------ show that subtle tectonic features have influenced deposition within each of the regional facies belts. On the western shelf, coastal sandstones of the Eagle-Sandstone near the Bearpaw Mountains show facies and isopach variations which are controlled by linear features visible on satellite images; the linear features generally trend north-south and east-west. Inner shelf sandstones of the Eagle thin and pass laterally northeastward into marine siltstones and shales across the Cat Creek fault zone near Winnett, Montana. Sandstone lenses in the lower Eagle, which are interpreted to be sand ridges, prograde south and west at approximately right angles to the fault zone. Farther east on the outer margin of the western shelf, areas of sand ridge fields in the Shannon Sandstone Member of the Gammon Shale are delimited by northeast and northwest linear features observed on satellite images nea the northern Black Hills. Within the basin, thick areas of Gammon Shale are delimited by northeast and northwest lineaments interpreted from Landsat linear features. On the eastern ramp, noncalcareous shales of the Gammon Member of the Pierre Shale thin and intertongue eastward with chalks in the upper part of the Niobrara Formation. This facies change occurs across linear features visible on Landsat images in western South Dakota. To the east at the inner margin of the ramp, the degree of erosion on the unconformity between the Niobrara Formation and the overlying Pierre Shale changes systematically across northeast-trending Landsat linear features observed near the Missouri River in central South Dakota. Based on these studies, we interpret the stratigraphic variations to be the expression of paleotectonism on discrete basement blocks bounded by fault zones which are observed on Landsat images as linear features. On the western shelf, elevated blocks controlled the sites of the winnowing and deposition of sandstones. Within the basins, subsiding basement blocks were filled by deposition of shales. These basin blocks acted as sediment sinks which inhibited the eastward dispersal of terrigenous materials from the west. On the eastern ramp, chalks were deposited and locally eroded on slightly elevated blocks which were relatively free of terrigenous material. Paleotectonism, therefore, influenced deposition not only on the active western shelf and in the basin, but also on the more stabl eastern ramp. End_of_Article - Last_Page 1356------------