GEOCHEMICAL COMPOSITION AND ORGANIC FACIES CHARACTERIZATION OF CRETACEOUS SOURCE ROCKS AT THE JAGO RIVER OUTCROP IN ANWR, ARCTIC ALASKA

Cretaceous Brookian strata comprise stacked clinothems deposited by eastward, longitudinal filling of the Colville Foreland Basin. Distal toes of the clinothems comprise deep-water, condensed mudstone of the Hue Shale. Along the Jago River on the coastal plain of the Arctic National Wildlife Refuge (ANWR), a structural imbricate exposes Albian–Campanian Hue Shale source rocks. Samples from a 113 m-thick measured section of the outcrop were analyzed using programmed pyrolysis, organic petrology, X-ray diffraction, elemental analyses, and spectral gamma ray.

Programmed pyrolysis results indicate organic-rich, oil-prone intervals with total organic carbon (TOC) values of 0.4-26.3 wt.% (avg. 6.9 wt.%, n=94) and hydrogen index values of 27-661 mg HC/g TOC (avg. 409 mg HC/g TOC, n=94). Sedimentary organic matter comprises mainly fluorescent amorphous organic matter and other algal-derived macerals, with smaller amounts of terrigenous (vitrinite) and liptinite (sporinite) macerals. Vitrinite reflectance of 0.49-0.52% R_o and an average T_max value of 408^oC indicate the section is thermally immature. Evidence of migrated oil is indicated by bi-modal S2 peaks, elevated production index values >0.2, and clouding of immersion oil. The source of this oil is hypothesized to be coeval source rocks in more deeply buried thrust imbricates. Mineralogical data indicate the Hue Shale consists mainly of clay-rich siliceous mudstone, but also mixed siliceous and carbonate/silicious mudstones, highlighting heterogeneity throughout the formation. Elemental data indicate a euxinic paleodepositional environment for the basal, highly radioactive gamma ray zone of the Hue Shale, with fluctuations in redox conditions and organic matter preservation throughout younger facies. A subset of Hue Shale facies (organic, lithological, and depositional) is undergoing hydrous pyrolysis and advanced molecular analyses of biomarkers and diamondoids. Oil-saturated turbidites from the overlying Paleocene Canning Formation also are being extracted to determine the source of migrated oil. This integrated approach will contribute to a comprehensive reconstruction of paleoenvironments and paleoecology of Brookian source rocks, marine productivity cycles, source rock heterogeneity, and oil migration pathways.