Lateral Lithofacies Variability in the Shublik Formation Across the Triassic Arctic Alaska Basin

Abstract

The Triassic Shublik Formation, a major source rock interval for hydrocarbons in many Alaska North Slope fields, has been described and interpreted mainly from cores and well logs in the Prudhoe Bay region, and from outcrops in the northeastern Brooks Range. Recent fieldwork has focused on additional Shublik outcrops across the Brooks Range, which allows for a more complete understanding of lateral lithofacies variability across the Triassic Arctic Alaska Basin (TAAB). We have divided the Shublik Formation and adjacent formations in the eastern TAAB into three large-scale stratal packages. The lower stratal unit, which includes the underlying Fire Creek Siltstone Member of the Ivishak Formation, consists of a lower siltstone with sandstone and mudstone intervals overlain by a phosphatic sandstone that marks the base of the Shublik Formation. The middle unit consists of phosphatic siltstone, marl and limestone. The upper unit is a black, fissile mudstone with intercalated carbonate, overlain by and including the Karen Creek Sandstone and Sag River Formation. The middle unit contains the most organic-rich strata, although all units have intervals with total organic carbon (TOC) values greater than 2%. Two Shublik outcrops occur in the southern portion of the study area and adjacent to the north-central fringes of the Brooks Range. These sections are predominantly black, slightly calcareous mudstone, with thinner and less common resistant intervals than those observed in more northerly and easterly Shublik outcrops. Both outcrops also contain phosphatic and flat clam-rich intervals. The thick intervals of black mudstone at these two southern localities suggest that the net:gross of organic-rich intervals increases in deeper, more distal parts of the TAAB, which could be explained by a decrease in dilution by event beds. In the western Brooks Range, the Surprise Creek outcrop consists of mainly siliceous to calcareous mudstone, with phosphatic mudstone and radiolarite also present. This Shublik occurrence is unique in that it contains siliceous mudstone and radiolarite as well as phosphate and abundant mega-fossils. Surprise Creek may represent an outcrop analog for the main source rock in the Chukchi shelf exploration play. Flat clam-rich facies occur in each of the studied Shublik occurrences, and most contain TOC values greater than 2%. Thus, this organo-facies may represent the most widespread source interval within the Shublik Formation.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016