Jennie L. Ridgley

Ancient Microbial Gas in the Upper Cretaceous Milk River Formation, Alberta and Saskatchewan: A Large Continuous Accumulation in Fine-grained Rocks

The Upper Cretaceous Milk River Formation in southeastern Alberta and southwestern Saskatchewan has produced more than 2 tcf of dry (99% methane) microbial gas (65 to 71) that was internally sourced. Production is from underpressured fine-grained sandstone and siltstone reservoirs, whereas the gas was generated in interbedded organic-bearing mudstones with low organic carbon contents (0.5–1.50%). The formation experienced a shallow burial history (maximum burial, 1.3 km [0.8 mi]) and cool formation temperatures (50C [122F]). Petrologic and isotopic studies suggest that methanogenesis began shortly after deposition and continued for at least 20 to 25 m.y. Mercury injection capillary pressure data from the Milk River Formation and the overlying Upper Cretaceous Pakowki Formation, which contains numerous regionally extensive bentonitic claystones, reveal a strong lithologic control on pore apertures and calculated permeabilities. Pore apertures and calculated permeabilities in Milk River mudstones range from 0.0255 to 0.169 m and less than 0.002 to 0.414 md, respectively, and claystones from the overlying Pakowki Formation have pore apertures from 0.011 to 0.0338 m and calculated permeabilities of 0.0017 to 0.0065 md. The small pore apertures and low permeabilities indicate that claystones and mudstones served as seals for microbial Milk River gas, thereby permitting gas to accumulate in economic quantities and be preserved for millions of years. Based on the timing of gas generation, the gas system of the Milk River Formation can be considered an ancient microbial gas system, which is one of several ways it differs from that of the Devonian Antrim Shale, Michigan Basin, where microbial gas generation is a geologically young (Pleistocene and younger) phenomenon. The difference in timing of gas generation between the Milk River and Antrim systems implies that gases in the two formations represent end members of a spectrum of microbial gas accumulations in fine-grained rocks, with the Milk River Formation being an excellent example on which to base a paradigm for an ancient microbial gas system.

Assessment of undiscovered biogenic gas resources, North-central Montana Province

In 2000 the U.S. Geological Survey (USGS) assessed the undiscovered biogenic (also known as microbial) continuous gas resource potential of the North-Central Montana Province in eastern Montana (fig. 1) as part of a national oil and gas assessment project. The assessment was based on the general geologic elements used to define a total petroleum system (TPS), including hydrocarbon source rocks (hydrocarbon generation and migra tion), reservoir rocks (sequence stratigraphy and petrophysical properties), and hydrocarbon traps (trap formation and timing). Using this geologic framework, the USGS defined the Cretaceous Judith River through Belle Fourche Biogenic Gas TPS and seven assessment units (AUs) within it, and quantitatively estimated the undiscovered continuous gas resources within each AU. Resource Summary

ABSTRACT: Timing of Biogenic Methane Generation in Cretaceous Rocks of the Northern Great Plains, Southeastern Alberta and Southwestern Saskatchewan: Petrologic and Fluid Inclusion Evidence

Petrologic studies reveal that both units have experienced a similar paragenetic sequence of early authigenic minerals. Of particular interest are early-formed carbonates (Milk River, siderite and calcite; Belle Fourche, ferroan dolomite and calcite) because of their presumed association with methanogenesis. Isotopic values for Milk River siderite and calcite overlap and vary broadly, but fall within the range of values that could be expected for carbonates associated with methanogenesis (δCPDB from -1.99 to -14.23 permil, δOPDB from -0.63 to -19.41 permil). Preliminary fluid inclusion petrographic and volatile (FIS) analysis suggest that methane and other bacterial species occur within primary inclusions in both carbonate phases, which implies that biogenic methane was present in pore fluids during carbonate precipitation.

Paleontology and sedimentology of upper clastic member of Wanakah Formation, Chama basin, New Mexico: Lacustrine paleoenvironmental implications

Lacustrine strata of the upper part of the Jurassic Wanakah Formation were restricted to the Chama basin of north-central New Mexico by mid-Jurassic tectonic activity in the Brazos and Nacimiento uplifts and along the Gallina-Archuleta anticlinorium. Lateral and vertical facies of the upper Wanakah exposed around the southern margin of the Chama basin indicate that the deeper part of the lake was north of the outcrop belt. The upper 3-5 m of the Wanakah consists of thin-bedded rippled sandstone, interbedded mudstone, and limestone containing trace fossils and freshwater mollusks characteristic of marginal lacustrine facies. Taphonomic studies of mollusks in the Wanakah Formation have been combined with application of ecophenotypic variation documented in extant unionid bivalves to produce paleoenvironmental interpretations of these lacustrine rocks.

Distribution and Stratigraphic Correlation of Burro Canyon(?) Formation, Chama and Northern San Juan Basins, New Mexico: ABSTRACT

The Lower Cretaceous conglomerate sandstone, sandstone, and mudstone interval stratigraphically between the Upper Cretaceous Dakota Sandstone and the Upper Jurassic Brushy Basin Member of the Morrison Formation in the Chama and northern San Juan basins, New Mexico and Colorado, is tentatively called the Burro Canyon(?) Formation. In this area the Burro Canyon(?) consists of a basal fluvial sequence deposited in braided-stream environments and an upper fluvial sequence deposited in braided to meandering-stream environments. The Burro Canyon(?) is unconformably overlain by the Dakota Sandstone and may disconformably overlie the Brushy Basin Member of the Morrison Formation. The Burro Canyon(?) thins southward from the Chama and northern San Juan basins because of truncation of the formation under the End_Page 567------------------------------ pre-Dakota erosion surface. The relation of the Burro Canyon(?) to the underlying Morrison Formation is less clear and has been the subject of controversy; the Burro Canyon(?) has at times been equated with the Jackpile-type sandstone at the top of the Brushy Basin Member in the San Ysidro area and along the east side of the San Juan basin. However, several lines of evidence, including facies relations, differing styles of sedimentation and sedimentary structures, and differences in composition of sandstone and mudstone of the upper part of the Brushy Basin Member and of the Burro Canyon(?) Formation in the San Juan and Chama basins, indicate that the Burro Canyon(?) Formation occurs stratigraphically above similar-appearing sandstones at the top of the Brushy Basin along the east side of the San Juan basin. End_of_Article - Last_Page 568------------

Roll-Type Uranium Occurrence at Dennison-Bunn Claim and Possibility of Uranium Deposits in Eastern Part of San Juan Basin, New Mexico: ABSTRACT

Uranium at the Dennison-Bunn claim, south of Cuba, New Mexico, along the east margin of the San Juan basin, occurs in stacked fluvial-channel sandstones interbedded with gray-green mudstones of the Westwater Canyon Sandstone Member of the Morrison Formation of Jurassic age. Although all the sandstone units are mineralized, the greatest concentration of uranium occurs in the uppermost sandstone unit. The uranium deposits are low to medium grade, range from 0.001 to 0.07% U3O8, and are irregularly distributed along the margins of intertonguing oxidized and unoxidized sandstone. The configuration indicates that these are roll-type uranium deposits and that they formed at the interface between oxidizing and reducing solutions. The host rocks dip 45° west into the basin. Reconstruction of the tectonic and sedimentologic history along the eastern margin of the basin suggests that conditions favorable for the solution, transportation, and deposition of uranium probably occurred from Late Cretaceous into Eocene time. Uranium in the mineralizing solutions may have originated from within the Morrison Formation or may have been leached from the Paleocene Ojo Alamo Sandstone or Nacimiento Formation, or from the Eocene San Jose Formation which once covered the area. Similar uranium deposits occur in the Morrison Formation at the Goodner lease, north of Cachana Spring. The presence of oxidized sandstone in the Morrison End_Page 692------------------------------ Formation along the outcrop belt from Cachana Spring north to Cuba suggests that additional uranium deposits may exist at the boundaries of oxidized and unoxidized sandstone in the subsurface along the eastern margin of the San Juan basin. End_of_Article - Last_Page 693------------