Unconventional Oil Exploration in Wyoming Using Regional Chronostratigraphic Correlation

Abstract

For more than a decade, the formal definition of an “unconventional” oil reservoir has proven elusive and variable, prompting adoption of a working definition throughout much of Wyoming: a reservoir in which production is improved through horizontal drilling and enhanced stimulation techniques. Practically speaking, Wyoming’s unconventional oil reservoirs consist of horizontal wells with lengthy laterals and large multi-stage completions. Technically speaking, Wyoming’s unconventional oil reservoirs are low permeability sandstones and siltstones that were deposited in marine to transitional marine settings of the Western Interior Foreland Basin (WIFB) during the Late Cretaceous and are now part of the Laramide Powder River and Denver basins. Seven of Wyoming’s top ten oil-producing formations were deposited as part of the WIFB, and account for 59% of 2016 production. Reservoir facies are commonly highly bioturbated with pore space occlusion caused by clays and multi-phase porosity and permeability development. These reservoirs were produced historically with conventional vertical wells, yet recent unconventional exploration of these same reservoirs prompted a significant increase in production from each well drilled.

To assess potential correlative unconventional strata for equivalent depositional conditions, including time and environment of deposition, we correlated 25 Upper Cretaceous sections in Wyoming to U.S. Western Interior biostratigraphic zonation (molluscan, palynostratigraphic, and land vertebrate), radiometric age control, and polarity chrons. Results demonstrate that (1) second-order sea level cycle maximum flooding surfaces occur at approximately 10 to 18 Myr intervals, (2) the bulk of Wyoming’s current unconventional oil production occurs as part of two different retrogradational transgressive sequence sets deposited in shallow marine environments during the upper Turonian and middle Campanian, which account for 29% and 9% of 2016 oil production, respectively, (3) the larger Niobrara play is part of the Coniacian to Santonian highstand sequence set, with 5% of 2016 oil production, and (4) exploration of unconventional plays in other basins may benefit from consideration of the regional chrono-stratigraphic setting. This correlation provides a statewide baseline for future assessment of Laramide basin-specific hydrocarbon systems.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018