Redeveloping the Almond Formation in the Vermillion Basin of southwestern Wyoming: structural and stratigraphic complexity in tight-gas development

Abstract

Laramide-age anticlines in the Vermillion Basin in southwestern Wyoming provide structural traps for oil and gas accumulations. Productivity of the Upper Cretaceous Almond Formation is closely tied to these Laramide-age anticlines. Historic drilling programs have focused on the modern configuration of these anticlines as the primary predictor of well performance. Data collected from a 40-acre drilling program for the Almond Formation in the Vermillion Basin between 2009-2013 reveals patterns of well productivity that have significant deviations from a simple structural accumulation model. Detailed parasequence-scale correlations of Almond reservoirs indicate that depositional facies has a strong correlation with well productivity, although the productive facies within each parasequence varies. Reservoir quality, inferred from IP and production log data, is highly variable between the Almond Formation parasequences, but internally consistent within each parasequence. These parasequence-scale variations in reservoir quality are interpreted to be related to original depositional fabric, mineralogy, diagenetic history, and structural position. Additionally, recovery factor maps, calculated using decline curve EURs, and OGIP values, exhibit productivity patterns that appear to be structurally controlled, but are offset from the present-day structural configuration. The recovery factor map patterns are interpreted to reflect early Laramide paleostructure at the onset of gas migration out of the deeper Washakie Basin. These early structures were the first to be filled with hydrocarbons, which preserved reservoir quality in their updip paleostructural positions. In the period between early hydrocarbon charge and subsequent structural reorganization, diagenesis degraded the paleo-downdip reservoir quality, effectively trapping the gas in the old structural highs, despite subsequent tectonic movement. Simplistic models of stacked sands draped over an anticline worked in the early exploration phase of the Vermillion Basin. However, as the recent data demonstrate, thoroughly understanding the elements of source, migration, trap, reservoir, timing, and seal is critical to effectively developing tight gas accumulations in the Vermillion Basin. Maintaining an active development program in this area requires attention to all of the aspects of hydrocarbon accumulations. Work is ongoing to further understand the timing and location of early Laramide structures throughout the Vermillion Basin and to better understand the reservoir quality and diagenetic history of the Almond Formation to aid in further exploitation of this tight-gas resource.

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