| Summary: | Mudstone is vertically and laterally heterogeneous in terms of variations in mineral composition, organic carbon content, and petrophysical properties. Although it was previously viewed as a homogeneous rock, with the advent of mudstone exploration techniques, it has been revealed to be more heterogeneous at multiple scales of resolution (nanometer to meter scales). The purpose of this study is to classify mudstone lithofacies within the Hue and High Radioactive Zone (HRZ) Shale on the North Slope, Alaska based upon mineralogy, total organic carbon (TOC), and petrophysical properties to determine their depths, distributions, and depositional environments. The goal is to analyze well log and core data to better understand the mudstone lithofacies variation across the North Slope of Alaska and provide insights into the depositional and diagenetic conditions. Core data from six wells and well logs from eighteen wells are integrated to analyze the variations in rock properties in the shale, and interpret the depositional/diagenetic environments. Analysis of cores at multiple scales of resolution by techniques including X-ray diffraction, X-ray fluorescence, TOC, and scanning electron microscopy are integrated with well log data comprising the standard well log suite and special logs (including pulsed neutron spectroscopy) to map vertical and lateral variation in mudstone lithofacies across the study area. Results show that the Hue and HRZ Shale both exhibit high degrees of heterogeneity in terms of mineralogy, TOC, and petrophysical properties. These shale formations are composed of twelve lithofacies: pyritic organic mixed shale, organic mixed shale, pyritic mixed shale, mixed shale, pyritic organic mudstone, organic mudstone, pyritic grey mudstone, grey mudstone, pyritic organic siliceous shale, organic siliceous shale, pyritic siliceous shale, and siliceous shale. Quartz abundance is generally higher in the Hue than in the HRZ Shale across the North Slope. Smectite and illite clay proportions increase from the southwest to northeast. The lithofacies are influenced by depositional conditions, diagenesis, redox conditions, organic matter productivity, and preservation. The TOC content is high across the HRZ Shale, ranging from 1.5–15%. Northern regions of the North Slope show very high proportions of pyrite compared to the overall study area. Well logs show a ~40 ft highly siliceous layer at the bases of both formations, with a slightly higher quartz percentage in the base of the Hue Shale. This may be a potential region for hydraulic stimulation, although it has lower TOC than the HRZ Shale. In terms of implications, this study provides an understanding of potentially productive lithofacies, as very little research has been done on the potential of shale as an unconventional play on the North Slope. The quantitative methodology for mudstone lithofacies classification will be directly applicable to other basins worldwide, which may be useful as unconventional shale exploration continues to increase.
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