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Abstract

Three-dimensional outcrop exposures of the Ferron Sandstone in the Willow Springs Wash area of east-central Utah illustrate the importance of recognizing allocyclic and autocyclic processes in marginal-marine settings. Three chronostratigraphically distinct wave-dominated shoreline sandstone successions display vertical offset at their landward pinchouts. These coarsening upward successions are interpreted to be parasequences produced by allocyclic processes. Fluids trapped within these sandstones may be compartmentalized by this vertical offset. In contrast, a fluvial-dominated coarsening upward succession, interpreted to have been produced by autocyclic delta lobe-switching processes, displays no vertical offset between its landward pinchout and an underlying wave-dominated parasequence. In a landward direction, these two distinct sandstone bodies merge into one and fluids migrating updip would not be compartmentalized. Reservoir continuity is further complicated by younger multi-lateral and multi-story distributary channels that can incise into previously deposited marine parasequences, thereby creating a fluid pathway between these otherwise isolated sandstone bodies.

Within the area of Willow Springs Wash, exposures of the Ferron Sandstone permit detailed examination of both macro-and mega-scale features in three dimensions. Observations of lithology, primary sedimentary structures, bedding, bioturbation, and internal architectural elements allow identification of eleven facies. These facies are grouped into wave-dominated shoreline and fluvial-dominated deltaic depositional systems.

The deposits exposed along Indian Canyon in the Willow Springs Wash area consist of four distinct, mappable depositional units; three deposited along wave-dominated shorelines (depositional units A, B, and C) and one deposited in a lobe of a fluvial-dominated delta (depositional unit D). Vertical offset (or the lack of it) among the landward pinchouts of these individual depositional units helps distinguish between allocyclic and autocyclic parasequences. Although autocyclic deposits produced by deltaic lobe switching are recognized by many as parasequences, their macro- and mega-scale features are very distinct relative to marine-dominated allocyclic parasequences.

Misidentification of parasequence type and failure to distinguish between autocyclic and allocyclic changes involved in a rock succession can result in misunderstanding the depositional history, the distribution of related facies, and the quality and extent of potential hydrocarbon reservoirs. Indeed, without recognizing the parasequence type, the utility of parasequence correlation is diminished and optimal reservoir development strategies cannot be accurately formulated. Models based on outcrop analogs such as the Ferron Sandstone in the Willow Springs Wash area can provide insight into the complexities of these reservoir rocks.

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