VOLUME 43 NUMBER 2
Revisions to stratigraphic nomenclature of the Upper Triassic Chinle Group in New Mexico: New insights from geologic mapping, sedimentology, and magnetostratigraphic/paleomagnetic data
Kate E. Zeigler1,* , Shari Kelley2
and John W. Geissman1
1 Department of Earth and Planetary Sciences,
Northrop Hall MSC 03-2040, University of New Mexico, Albuquerque, New Mexico
87131, U.S.A.
2 New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech
University, 801 Leroy Place, Socorro, New Mexico 87801, U.S.A.
* Correspondence should be addressed to: bludragon@gmail.com
Upper Triassic strata, termed the Chinle Formation or Group, reveal a complex sedimentologic history, which has led to numerous modifications to the lithostratigraphic nomenclature for this unit. Recent revisions to Chinle nomenclature in New Mexico are based on local stratigraphic section inspection. The utility of some stratigraphic subdivisions, especially at formation rank, are problematic when mapping units away from type areas. New magnetostratigraphic records for the Chinle Group, as well as 1:24,000-scale mapping, stratigraphic section logs, and sedimentologic analyses support the following revisions to current nomenclature of the Chinle: (1) formally raising Chinle strata to group status in New Mexico; (2) abandoning the term Zuni Mountains Formation for lower Chinle strata in the Chama Basin; (3) including the lower member of the Salitral Formation in the Shinarump Formation; and (4) reassigning strata termed Rock Point Formation in the Chama Basin to the Moenave Formation. Sedimentologic and basin-analysis interpretations utilized in earlier studies of the Chinle should be resurrected. The revised stratigraphic nomenclature for the Chinle Group and sedimentologic analyses, both old and new, greatly expands our understanding of depositional changes in the Chinle Basin of northern New Mexico. The trunk stream for Chinle deposition in New Mexico coursed westward through the site of the modern Sierra Nacimiento of north-central New Mexico during Shinarump time (?late Carnian or early Norian), then moved northward to flow to the west in the area of the modern Chama Basin. In addition, north-central New Mexico was probably a topographic high during the time of Owl Rock and Rock Point deposition in the late Norian, and thus no uppermost Triassic (late Norian to early Rhaetian) strata were deposited in the area of the modern Chama Basin.
Key Words: Chinle Formation • Upper Triassic • stratigraphy • New Mexico • Chama Basin • magnetostratigraphy
Stenomylus (Stenomylus) taylori, sp. nov. (Stenomylinae, Camelidae), from the early Miocene (Hemingfordian) Blick Quarry in New Mexico
Michael Cassiliano
Department of Geology and Geophysics, The University of Wyoming, Laramie, Wyoming 82071, U.S.A.
email: mcassil@uwyo.edu
Stenomylus (Stenomylus) taylori is a new species of stenomyline camel from the early Hemingfordian (early Miocene) of New Mexico. The holotype, F:AM 50858, a palate with left and right P2 –M3, was collected from the Blick Quarry, which is in the Chamisa Mesa Member of the Zia Formation. Stenomylus (Stenomylus) taylori is a derived species diagnosed by greatly reduced premolars, length of P2 –P4 shorter than in any other species of Stenomylus (Stenomylus), less well-developed parastyles and metastyles on P4 –M2, M3 with weak parastyle and well-developed metastyle forming a heel, M3 extremely hypsodont with open roots, and anteroposteriorly short internal nares. Stenomylus (Stenomylus) taylori is the sixth named species in the genus Stenomylus and shows that the locus of stenomyline evolution apparently shifted southward during the early Miocene (late Arikareean or earliest Hemingfordian).
Key Words: Blick Quarry • Camelidae • Chamisa Mesa Member • Hemingfordian • New Mexico • Stenomylinae • Stenomylus • Zia Formation
Sub-hydrostatic pore pressure in coalbed and sand aquifers of the Powder River Basin, Wyoming and Montana, and implications for disposal of coalbed-methane-produced water through injection
Hannah E. Ross* and Mark D. Zoback
Department of Geophysics, Stanford University, Stanford, CA 94305-2215, U.S.A.
* Correspondence should be addressed to: heross@stanfordalumni.org
Coalbed methane (CBM) production in the Powder River Basin (PRB), Wyoming, is associated with the production of large volumes of water. Locally, water in coalbeds from the PRB has high saline and sodium contents, making it unsuitable for agricultural use and potentially environmentally damaging if discharged at the surface. One option for the disposal of CBM-produced water is injection into aquifers. For injection to be feasible, however, the porosity and permeability of the sands need to be high, the pore pressure ideally needs to be sub-hydrostatic, and the aquifer cannot be in hydraulic communication with coalbeds or aquifers used for irrigation use. In order to determine if pore pressures in the aquifers are low enough to allow for significant water injection (and to determine whether the coals and nearby sands are in hydraulic communication), we have compiled pore pressures in 250 wells that monitor water levels in coalbeds and adjacent sands within the PRB.
All 250 wells have pore pressures below hydrostatic pressure, suggesting that injection of produced water should be feasible. Through the analysis of pore pressure changes with time for both the coals and their overlying/underlying sands, we find after 8 to 13 years of water-level monitoring that none of the sands more than 200 ft (61 m) vertically from producing coals appear to be in hydraulic communication with the coalbeds. Therefore, injection of CBM-produced water should be carried out in sands at least 200 ft (61 m) from adjacent coalbeds to be sure that the disposed water does not rapidly migrate back into the coalbeds.
In addition, we constructed two 3D stochastic reservoir models of
conceptualized sand units to determine the rates at which water can
be injected into shallow (
300 ft
(91 m)) and deep (1000
ft (305 m)) sub-hydrostatic aquifers. We find that for shallow sands
we can inject water at a rate of 160
bbl/day, whereas for deeper sands, whose pore pressures are lower
than the shallower sands, the rate is
435 bbl/day. Both of these
rates are higher than the average water production rate from
CBM wells in the PRB of 100
bbl/day. This implies that for deep aquifer injection sites, it would
take only one injection well to dispose of the water production from
approximately four CBM wells.
Key Words: Powder River Basin • coalbed methane • coalbed methane-produced water • pore pressure • sub-hydrostatic • hydraulic communication • coalbed methane-produced water disposal • injection
Strontium isotopes as indicators of aquifer communication in an area of coal-bed natural gas production, Powder River Basin, Wyoming and Montana
Catherine E. Campbell1,2 , Benjamin N.
Pearson1,3 and Carol D. Frost1,*
1 Department of Geology and Geophysics, The
University of Wyoming, Dept. 3006, 1000 East University Avenue, Laramie, Wyoming
82071, U.S.A.
2 EnCana Oil and Gas (USA) Inc., 370 17th Street, Suite
1700, Denver, Colorado 80202
3 Crimson Exploration, 717 Texas Avenue, Suite 2900, Houston, Texas
77002
* Correspondence should be addressed to: frost@uwyo.edu.
Development of the coal-bed natural gas resource of the Powder River Basin of Wyoming and Montana has proceeded rapidly, from fewer than 200 wells in 1995 to more than 22,000 wells in 2007. Continued development of this resource will depend on minimization of water production during gas recovery as well as responsible use of the produced water. Ideally, water should be withdrawn only from isolated coal aquifers to prevent any unnecessary water withdrawal from overlying or underlying aquifers. This study uses the ratio of 87Sr/86Sr of ground water to identify hydraulically isolated coal seams. The ratio of 87Sr/86Sr of ground water represents a time-integrated record of water–rock interaction, such that water from aquifers composed of different rocks may acquire different Sr isotopic ratios.
Sr isotopic data are presented for 145 samples of ground water co-produced with coal-bed natural gas and 14 water samples from wells completed in sandstone aquifers in the Powder River Basin. The coal zone from which each sample was collected was determined by analysis of gamma logs and correlation with the Wyoming State Geological Survey database.
The Sr isotopic ratios and geochemical compositions of ground waters from coal in the Powder River Basin of Wyoming are influenced by a number of factors, including the coal zone from which ground waters are produced, their residence time, the degree to which coal aquifers are confined, and geographic location. The data indicate that the Upper Wyodak coal-zone aquifer in the Gillette and Schoonover areas in the eastern Powder River Basin appears to be a well-confined, combined sand and coal aquifer unit. In contrast, the Wyodak Rider coal zone aquifer may be only partially confined, allowing interactions between sandstone and possibly other coal aquifers. Wells in this area exhibit highly variable Sr isotope ratios and total dissolved solids, and they also are characterized by greater than average water/gas production ratios, consistent with incomplete isolation of the Wyodak Rider coal zone. Faults in the northeastern part of the Powder River Basin may affect aquifer connectivity, either by acting as seals or conduits. Higher gas production correlates with lower Sr isotopic ratios in this part of the basin. Although a correlation between Sr isotopic ratios of produced water with fracture pattern developed during the well enhancement process might be expected, no strong relationship was observed. Evidently there are many factors in addition to fracture pattern that control interactions between aquifers.
Key Words: aquifer communication • coal-bed natural gas • Powder River Basin • produced water • sodium-adsorption ratio • strontium isotopes • water quality
Israel Cook Russell—explorer, geomorphologist, geographer, educator
K. R. Aalto
Department of Geology, Humboldt State University, Arcata, CA 95521, U.S.A.
email: kra1@humboldt.edu
Key Words: Israel Cook Russell • Lake Lahontan • Mono Lake • Sierra Nevada • Cascades • St. Elias Range