Quantitative characterization of shales within tidally influenced fluvial valley fill deposits of the Ferron Sandstone, eastern Utah: Implications for hydrocarbon exploration

Abstract

This study evaluates the proportion, length, and effective properties of thin (0.003 - 0.7 m [0.01 – 2.3 ft.]) shale beds and drapes in tidally-influenced channels within a compound valley fill with a focus on estimating geologically-based effective rock properties. The Cretaceous Ferron Sandstone is an outcrop analog for fluvial-tidal systems with primary reservoirs being deposited as tidally-influenced valley filling point bars. The study outcrops expose three valley systems in Neilson Wash of Utah. Light detection and ranging derived digital outcrop models have been used to characterize shale length, width, thickness and frequency of each valley fill succession. Long, uncommon, and anisotropic shales in Valley-1 (V1) were deposited in a braided setting with little tidal influence. In contrast, shales in Valley-2 (V2) were abundant, short, common, and equidimensional suggesting deposition by more tidally-influenced meandering rivers. Short, frequent, and equidimensional shales in Valley-3 (V3) were deposited in single-thread meandering rivers with less tidal influence. A sandstone-shale model was utilized to estimate the effects of shales on vertical to horizontal permeability ratio (kv/kh). The unique character of each depositional unit was reflected in resultant kv/kh distributions. The valley fill deposits, V1, V2, and V3 had an average kv/kh ratio of 0.11, 0.09 and 0.17, respectively. More tidally-influenced reservoirs such as the studied V2 had short, but frequent shales which resulted in low kv/kh estimates. Estimates of kv/kh for valleys that predominantly contained fluvial point bar deposits with lesser tidal influence (V1 and V3) were higher. The results of this study highlight the link between shale heterogeneity, reservoir architecture, and inferred flow parameters.