Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Source Rock Characteristics in the Green River Oil Shale, Piceance Creek Basin, Colorado – An Integrated Geochemical and Stratigraphic Analysis
(1) Colorado School of Mines, Golden, CO.
The Green River Oil Shale in the Piceance Creek Basin (PCB) is an early Eocene lacustrine source rock with fine-scale variability in organic content. Previous characterization of organic matter suggests algal as primary source. Based on sedimentologic and stratigraphic study of cores, outcrops, and well logs of the PCB, six lake evolutionary stages have been defined: S1 Freshwater lake, S2 High siliciclastic input, S3 Rapidly fluctuating lake, S4 Rising lake, S5 Stable lake, S6 Closing lake. It is important to correlate organic matter changes with lake evolution to understand controls on source rock deposition in the region.
Two reference sections respectively from the basin center and margin have been chosen for detailed studies of organic matter content and type. Weatherford’s Source Rock AnalyzerTM(SRA) was used to analyze TOC, hydrogen index and oxygen index. In addition, analysis of Fischer Assay and geochemical open file data from a well USBM-01A in the center of the PCB was used. Factor analyses of whole rock X-ray fluorescence and ICP-AES data of USBM-01A, indicates three main sources of basin center sediments: terrestrial input, carbonate deposition, and apatite related processes. Elements showing high positive correlation with terrestrial input factor, namely Al2O3, SiO2, and Fe2O3, are used as terrestrial input proxies. Salinity proxies like Sr/Ba and Sr/Cu are applied to track climate change during Eocene.
Profiles of terrestrial input and climate proxies vs. depth show general consistency with the stages of lake evolution. During S1 and S4, high terrestrial input corresponds to warm and wet climate. SRA analyses show moderate TOC, high OI, low HI and type II kerogen. During stages S2 and S3, moderate siliciclastic input is accompanied by an overall dry climate. High oil yield in the S3 corresponds to oil shale breccias and high shale oil specific gravity. SRA analyses show high TOC, high OI and type II kerogen. The organic-rich Mahogany zone in Stage 5 is dominated by low siliciclastic input and a dryer climate, and has high TOC and type I kerogen.
Organic richness variation within lake evolution suggests terrestrial input as another important source of organic matter. Both mineral dilution and redox condition play significant roles in organic matter deposition. Organic matter is suggested to be deposited in the stratified lake, where organic matter is preserved under reducing condition in the bottom of the lake.