Codell carrier-bed play, Denver Basin

Carrier-bed plays are an emerging type of unconventional oil play in which reservoirs are generally of low quality because they are characterized by: 1) thinly bedded heterolithic strata; 2) significant compaction and/or diagenesis; and 3) burrowing that has mixed sandstones and mudstone lithologies (i.e., heterogeneous lithologies). In this type of play, the carrier beds are pervasively hydrocarbon saturated and can be areally extensive (>50 mi2 or 130 km2). These low-quality reservoirs generally do not meet traditional petrophysical cutoffs and because of their high clay contents can have low resistivity, low contrast pays. The reservoirs may be composed of siliciclastics or carbonates or both. Due to reservoir quality and degree of oil migration, carrier-bed plays like the Codell are being developed with horizontal drilling and multistage hydraulic fracturing. Traditional vertical drilling yields marginal to uneconomic wells that can provide a clue to the existence of a carrier-bed play. The Codell Sandstone is a low-resistivity, low-contrast pay in parts of the northern Denver Basin. The area of oil and gas production is in the deeper part of the basin between and including Silo and Wattenberg fields of Wyoming and Colorado, respectively. The thickness of the Codell in this part of the Denver Basin ranges from 15 to 25 ft (4.5 to 7.6 m). Keys to Codell production are source rock maturity, and oil entrapment in the carrier bed. Oil in the Codell carrier-bed traps was generated in various intervals including the Niobrara (mainly the “B” marl), Sharon Springs Member of the Pierre Shale, Greenhorn/Carlile, and, rarely, the Mowry Shale.

Geochronology of late Albian−Cenomanian strata in the U.S. Western Interior

Since the publication of 40Ar/39Ar dates from Cretaceous bentonites in the Western Interior Basin by J.D. Obradovich in 1993 and in Japan by J.D. Obradovich and colleagues in 2002, improvements in the 40Ar/39Ar method have included a shift to astronomically calibrated ages for standard minerals and development of a new generation of multi-collector mass spectrometers. Thus, the 40Ar/39Ar chronometer can yield results that are synchronous with U-Pb zircon dates and astrochronologic age models for Cretaceous strata. Ages determined by Obradovich have ± 2σ analytical uncertainties of ± 400 ka (excluding J value or systematic contributions) that have been used to discriminate stratigraphic events at ca. 1 Ma resolution. From among several dozen sanidine samples, 32 of which were dated by Obradovich in 1993, we present new multi-collector 40Ar/39Ar ages that reduce the average analytical uncertainties by nearly an order of magnitude. These new ages (where the uncertainties also include the contribution of the neutron fluence J value) include: • Topmost Bentonite, Mowry Shale, Kaycee, Wyoming, USA, 97.52 ± 0.09 Ma • Clay Spur Bentonite, Mowry Shale, Casper, Wyoming, 98.17 ± 0.11 Ma • Arrow Creek Bentonite, Colorado Shale, Montana, USA, 99.12 ± 0.14 Ma • Upper Newcastle Sandstone, Black Hills, Wyoming, 99.49 ± 0.07 Ma • Middle Newcastle Sandstone, Black Hills, Wyoming, 99.58 ± 0.12 Ma • Shell Creek Shale, Bighorn Basin, Crow Reservation, Wyoming, 99.62 ± 0.07 Ma • Shell Creek Shale, Bighorn Basin, Greybull, Wyoming, 99.67 ± 0.13 Ma • Shell Creek Shale, Bighorn Basin, Lander, Montana, 100.07 ± 0.07 Ma • Muddy Sandstone, Wind River Basin, Wyoming, 101.23 ± 0.09 Ma • Thermopolis Shale, Bighorn Basin, Wyoming, 101.36 ± 0.11 Ma • Vaughn Member, Blackleaf Formation, Sweetgrass Arch, Montana, 102.68 ± 0.07 Ma • Taft Hill Member, Blackleaf Formation, Sweetgrass Arch, Montana, 103.08 ± 0.11 Ma • Base of the Skull Creek Shale, Black Hills, Wyoming, 104.87 ± 0.10 Ma • Thermopolis Shale, Bighorn Basin, Wyoming, 106.37 ± 0.11 Ma A new U-Pb zircon age of 104.69 ± 0.07 Ma from the Skull Creek Shale at Dinosaur Ridge, Colorado, USA, is close to the new 40Ar/39Ar age of the Skull Creek Shale in the Black Hills, Wyoming, but 5 m.y. is missing in the unconformity between the Skull Creek Shale of the Black Hills and the overlying Newcastle Sandstone. Considering the average total uncertainties that include decay constant and standard age or tracer composition for the 40Ar/39Ar (± 0.19 Ma) and the U-Pb (± 0.13 Ma) ages does not alter this finding. Moreover, the lower Thermopolis Shale in the Bighorn Basin is 1.5 Ma older than the Skull Creek Shale in the Black Hills. The 100.07 ± 0.07 Ma Shell Creek Bentonite in Montana is close to the Albian−Cenomanian boundary age of 100.2 ± 0.2 Ma of Obradovich and colleagues from Hokkaido, Japan, and 100.5 ± 0.5 Ma adopted in the 2012 geological time scale of J.G. Ogg and L.A. Hinnov. Our findings indicate that correlations based on similarity of lithology, without independent radioisotopic ages or detailed biostratigraphic constraints, can be problematic or invalid. There is much more time missing in unconformities than has been previously recognized in these important, petroleum-bearing reservoir strata.