Integrated geochemical and paleoecological approach to petroleum source rock evaluation, Cretaceous Niobrara formation, Lyons, Colorado

Conference · Wed May 01 04:00:00 EDT 1985 · Am. Assoc. Pet. Geol., Bull.; (United States)

OSTI ID:5969555

Barlow, L K; Pratt, L M

Studies of the upper Turonian to upper Coniacian or lower part of Niobrara Formation (in ascending order, the Forty Hays Limestone Member and the marlstone, shale, and limestone of the lower part of the Smoky Hill Shale Member) reveal a significant relationship between petroleum source rock potential and paleoclimate. Trends in bioturbation, sigmaO/sup 18/ (oxygen isotopic ratio), and C/sub org/ (organic carbon content) during lower Niobrara deposition suggest that paleoclimatic factors limited bioturbation of the sediment, favored high C/sub org/ contents, and resulted in excellent source rock potential in the shale unit of the lower Smokey Hill Member. From the Fort Hays through the overlying malstone unit, sigmaO/sup 19/ in inoceramid shells shows a gradual shift from nearly normal marine values (-2 to -4%) to lighter values (-4 to -6%); over the same interval, C/sub org/ changes from relatively low values (0.1-1.4%) to moderate values (0.5-2.5%). Bioturbation throughout these units is high. A marked shift toward heavier sigmaO/sup 18/ (-6.0 to -9.1%) occurs upward into the shale unit and coincides with increase in C/sub org/ (2.3-4.9%) and an absence of bioturbation. In the limestone unit, sigmaO/sup 18/ shifts to less negative values (-6.2 to -6.5%), C/sub org/ decreases (average 1.5%), and bioturbation returns. The trend toward more negative sigmaO/sup 18/ and higher C/sub org/ values in the shale unit is inferred to reflect a lowering of surface-water salinity in the Western Interior seaway due to climatic warming and increased freshwater input. Resultant salinity stratification of the water column apparently inhibited vertical mixing and oxygenation of the bottom waters, resulting in limited benthic activity and enhanced preservation of organic matter.

• Research Organization: Univ. of Colorado, Boulder
• OSTI ID: 5969555
• Report Number(s): CONF-8506201-
• Conference Information: Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Journal Volume: 69:5
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
020200* -- Petroleum-- Reserves
Geology
& Exploration
CARBONATE ROCKS
COLORADO
EVALUATION
EXPLORATION
FEDERAL REGION VIII
GEOLOGIC DEPOSITS
GEOLOGIC FORMATIONS
LIMESTONE
MINERAL RESOURCES
NORTH AMERICA
PETROLEUM DEPOSITS
RESOURCES
ROCKS
SEDIMENTARY ROCKS
SOURCE ROCKS
USA