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Integrated geochemical and paleoecological approach to petroleum source rock evalution, Cretaceous Niobrara Formation, Lyons, Colorado

L. K. Barlow and Lisa M. Pratt
Integrated geochemical and paleoecological approach to petroleum source rock evalution, Cretaceous Niobrara Formation, Lyons, Colorado (in AAPG Rocky Mountain Section meeting, Anonymous)
AAPG Bulletin (May 1985) 69 (5): 842

Abstract

Studies of the upper Turonian to upper Coniacian or lower part of Niobrara Formation (in ascending order, the Fort 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, delta O18 (oxygen isotopic ratio), and Corg (organic carbon content) during lower Niobrara deposition suggest that paleoclimatic factors limited bioturbation of the sediment, favored high Corg contents, and resulted in excellent source rock potential in the shale unit of the lower Smoky Hill Member. From the Fort Hays through the overlying marlstone unit, delta O18 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, Corg 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 delta O18 (-6.0 to -9.1 %) occurs upward into the shale unit and coincides with increase in Corg (2.3-4.9%) and an absence of bioturbation. In the limestone unit, delta O18 shifts to less negative values (-6.2 to -6.5%), Corg decreases (average 1.5%), and bioturbation returns. The trend toward more negative delta O18 and higher Corg 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 imput. Resultant salinity stratification of the water column apparently inhibited vertical mixing and oxgenation of the bottom waters, resulting in limited benthic activity and enhanced preservation of organic matter.


ISSN: 0149-1423
EISSN: 1558-9153
Coden: AABUD2
Serial Title: AAPG Bulletin
Serial Volume: 69
Serial Issue: 5
Title: Integrated geochemical and paleoecological approach to petroleum source rock evalution, Cretaceous Niobrara Formation, Lyons, Colorado
Title: AAPG Rocky Mountain Section meeting
Author(s): Barlow, L. K.Pratt, Lisa M.
Author(s): Anonymous
Affiliation: Univ. Colo., Boulder, CO, United States
Pages: 842
Published: 198505
Text Language: English
Publisher: American Association of Petroleum Geologists, Tulsa, OK, United States
Meeting name: AAPG Rocky Mountain Section meeting
Meeting location: Denver, CO, USA, United States
Meeting date: 19850602June 2-5, 1985
Summary: Y
Accession Number: 1986-035321
Categories: Economic geology, geology of energy sourcesStratigraphy
Document Type: Serial Conference document
Bibliographic Level: Analytic
N40°15'00" - N40°19'60", W105°19'60" - W105°15'00"
Secondary Affiliation: U. S. Geol. Surv., USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute.
Update Code: 1986
Program Name: USGSOPNon-USGS publications with USGS authors
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