Preservation of organic carbon in the Cretaceous Hue Shale on the North Slope of Alaska: Insights from pyrite morphology

Abstract

The Cretaceous Hue Shale is a condensed mudstone section of the Brookian sequence with type II and type III kerogen, and it is one of the major source rocks that fill the world-class conventional oil fields on the North Slope, Alaska. The bottom section of the Hue Shale is referred to as Gamma Ray Zone (GRZ) or Highly Radioactive Zone (HRZ) and believed to be the most organic-rich section. The thermal maturity increases from immature to over oil window towards the southwest. Although, the research on the Hue Shale as an unconventional shale reservoir is still at an early stage, where exploration activities are very limited. The Hue Shale thickens to the east in general. However, regional variations of thickness and more importantly total organic carbon (TOC) have been reported.In this research, a suite of nineteen core samples of the Hue Shale are selected. These cores are collected from six wells covering a wide spectrum of thermal maturity with four wells in the National Petroleum Reserve Alaska and two wells in the state land. A combined geochemical and petrographic methodology is utilized to address the regional redox conditions and the preservation of organic carbon. TOC, Tmax, hydrogen index, and oxygen index are acquired through pyrolysis to characterize the abundance, thermal maturity, the origin, and the evolution of organic matter. Scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) is used to investigate the mineralogy and diagenesis of the Hue Shale. Pyrite crystal forms, and pyrite framboid size distributions are used as an indicator of the redox conditions during deposition. The results show substantial variance in oxygen levels in the water column. High abundance and small sizes of pyrite framboids, which indicates a depleted oxygen environment, show better preservation potential of organic matter.