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Integration of microfacies analysis, inorganic geochemical data and hyperspectral imaging to unravel mudstone depositional and diagenetic processes in two cores from the Triassic Shublik Formation, northern Alaska

Abstract

The Middle - Late Triassic Shublik Formation is an organic-rich heterogeneous carbonate-siliciclastic-phosphatic unit that generated much of the oil in the Prudhoe Bay field and other hydrocarbon accumulations in northern Alaska. A large dataset, including total organic carbon, x-ray diffraction, x-ray fluorescence and inductively coupled plasma - mass spectrometry measurements, have been built from core and outcrop samples of the Shublik, with a focus on the organic-rich intervals. In addition, two core intervals from the Shublik were analyzed using a hyperspectral imaging system in the visible, near-infrared and shortwave-infrared range. Integration of the hyperspectral results with core descriptions, microfacies interpretations, and analytical data is being used to decipher mudstone depositional and diagenetic processes. Petrographic analysis of Late Triassic organic-rich intervals within the Shublik suggests that the main microfacies is a laminated bioclastic wackestone/packstone that was episodically disrupted by energetic events (EEs) of variable intensity. These EEs produced transitional and chaotic bioclastic wackestone/packstone intervals, depending on the depth of sediment column disturbance. By using the hyperspectral imaging data, individual distribution maps for minerals of interest have been generated and corroborate the microfacies interpretations. These maps also illustrate small-scale vertical changes in mineralogy. The laminated bioclastic wackestone/packstone intervals contain less calcite than in adjacent chaotic bioclastic wackestone/packstone intervals. The calcite in these laminated intervals is more iron-rich. This interpretation is substantiated by ICP-MS data that show lower iron values (in parts per million) in the disrupted intervals than in nearby laminated intervals. Textural features are also enhanced in the hyperspectral images relative to visual description of the cores by combining the extraction of the average reflectance in the visible part of the electromagnetic spectrum and the depth of the main carbonate-related feature belonging to calcite. Examples noted in the enhanced imagery include low-angle features, calcite grain-size, and the size, shape and orientation of phosphatic nodules. This enhancement is being used to differentiate laminated from chaotic bioclastic wackestone/packstone-rich intervals and provides a more comprehensive assessment of the microfacies than is practical by thin-section analysis.