2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 2
Presentation Time: 1:45 PM

PALEOGEOGRAPHIC EVOLUTION OF THE CRETACEOUS IN SE COLORADO


CORNELIUS, Chris1, CLARKE, Paul1 and TURNER, Peter2, (1)Evergreen Rscs Inc, 1401 17th Street, Suite 1200, Denver, CO 80202, (2)Petroleum Geoscience Unit, School of Geography, Earth and Environmental Sciences, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom, ChrisC@EvergreenGas.com

Cretaceous strata of SE Colorado record a largely uninterrupted sedimentary succession, deposited under a range of depositional environments, occurring between the well-defined Jurassic-Cretaceous (J-K) and Cretaceous-Tertiary (K-T) boundary layers. This section is typically 1200m thick, defined by a basal unconformity, where Jurassic redbeds are incised by Cretaceous mixed clastics; while the uppermost Cretaceous is defined by air-fall deposits associated with the Chicxulub impactor. Using outcrop (sedimentology and ichnology), core, and subsurface petrophysical log datasets a complete high-resolution Cretaceous ‘strato-type’ section is constructed.

Cretaceous sequences afford an unparalleled insight into basin scale controls, both allogenic and autogenic in origin. Five major depositional systems are identified: a) the Western Interior Seaway with deposition of cyclic marginal marine, coastal plain and fluvio-lacustrine sandstones and associated organic rich shales (Dakota Group); b) shallow marine epeiric sheet-form carbonates, marls and organic shales (Greenhorn and Niobrara Groups) with localized storm-deposits, shelly coquinas and near-shore transgressive sands (e.g. Codell Sandstone Member); c) restricted basin mudstones and shales (Pierre Group) with regional “hot-shale” highstand markers and localized thin siliciclastics reflecting detached lowstand shoreface deposits; d) establishment of the Rocky Mountain retroarc foreland basin and progradation of extensive linear clastic shoreline sandstones (Trinidad Sandstone Formation) and trailing alluvial floodbasins that accommodated extensive coal swamps (Vermejo Formation); e) initiation of Laramide orogenesis marked by an erosive, valley-fill sequence (Raton Conglomerate) composed of extraformational clasts reflecting basement unroofing and a switch to an intramontane setting.

Collectively, this Cretaceous lithostratigraphic framework and the paleogeographic interpretations allow a comprehensive evaluation of the total petroleum system of SE Colorado, enabling a more predictive method for reservoir distribution patterns.