An Integrated Geochemical and Paleoecological Approach to Petroleum Source Rock Evaluation, Lower Niobrara Formation (Cretaceous), Lyons, Colorado

A detailed study of paleoecological, geochemical, and stable isotopic properties of the lower Niobrara Formation (upper Turonian to lower Coniacian) was undertaken in order to evaluate petroleum source rock potential and to gain an understanding of the processes affecting variation in organic carbon content. The highest organic carbon contents In the lower Niobrara Formation occur in the lower shale unit of the Smoky Hill Shale Membel Trends in extent of bioturbation, organic carbon contents, and oxygen isotopic ratios of carbonates suggest that paleoclimatic factors influenced bottom water environments during deposition of this unit. A shift toward a more negative oxygen isotopic ratio in the lower shale unit is interpreted to be a result of decreased surface water salinity due to increased fresh water input and possibly to climatic warming. Resultant stratification of the water column limited benthic oxygenation thereby limiting benthic activity, enhancing the preservation of marine organic matter, and increasing source rock potential for petroleum. Data from underlying and overlying units in the lower Niobrara Formation suggest more normal marine conditions with well-oxygenated bottom waters, normal levels of bioturbation, and relatively low organic carbon contents. Pyrolysis data are interpreted to reflect a principally marine source of organic matter with substantial alteration due to bioturbation and thermal evolution. Elevated thermal maturity of the section at Lyons is inferred to be a local feature caused by local heating associated with fluid movement along fault zones or with emplacement of Tertiary sills.