Stratigraphic framework of the Middle—Upper Triassic Shublik Formation, Arctic Alaska: Insights from regional outcrop, core, and well log analyses

Abstract

The Middle—Upper Triassic Shublik Formation in Arctic Alaska has long been recognized as a world-class source rock that charged many conventional oil and gas accumulations, including part of the super-giant Prudhoe Bay field. The Shublik Formation has also recently become an exploration target as a shale-oil play. It is a vertically and laterally heterogeneous formation comprising organic-rich mudstone, bioclastic wackestone and packstone, nodular and pebbly phosphorite, and sandstone that is locally phosphatic and glauconitic. The formation has historically been subdivided into four distinct lithologic and petrophysical zones established during equity negotiations in the Prudhoe Bay field. Recent USGS work on cores and outcrops of the Shublik has facilitated regional surface to subsurface correlations across the Alaskan North Slope. We have integrated vertical facies successions observed in core and outcrop, as well as gamma ray and sonic log patterns, and have interpreted five transgressive-regressive (T-R) sequences. Isochore maps constructed for each sequence illustrate the influence of paleohighs on Shublik Formation depositional patterns, and suggest the timing of reactivation of several older tectonic elements during Shublik deposition. Middle Triassic isochore maps show the development of localized depositional thickening within an embayment bounded by the Barrow and Colville highs, along with southward thickening off of the Mikkelsen High. This depositional pattern persisted until the middle of the Upper Triassic, during which accommodation decreased in the embayment and large amounts of sediment were routed to and accommodated in the Ikpikpuk and Meade basins, and between the Fish Creek Platform and Mikkelsen High. These depocenters suggest reactivation of the Colville High during the middle Triassic, and reactivation of the Fish Creek Platform, Ikpikpuk Basin, and Meade Basin during the late Triassic. The reactivation of these older tectonic elements may signal initial pulses of extension related to opening of the Canada Basin, which occurred during the Jurassic to Early Cretaceous.

AAPG Datapages/Search and Discovery Article #90266 © 2016 AAPG Pacific Section and Rocky Mountain Section Joint Meeting, Las Vegas, Nevada, October 2-5, 2016