Architectural Analysis, Hierarchical Framework, and Cyclicity of Floodplain Strata in a Low Net-Sand Content Fluvial Succession, Lower Wasatch Formation, Uinta Basin, Utah

Fluvial successions are important hydrocarbon-bearing reservoirs worldwide. Although studies of fluvial successions are abundant, few outcrop studies focus on floodplain-dominated or low net-sand content fluvial successions, as they are not commonly well exposed. Outcrops of the lower Wasatch Formation in the southern Uinta Basin, Utah offer exceptionally well-exposed successions of fine-grained, low-net sand content floodplain strata. This study focuses on a 3 kilometer-wide outcrop along the Green River, which contains the best exposures of low-net sand content fluvial strata available. The goal of this study is to quantitatively document the stratigraphic architecture of this exposure using a hierarchical framework.

Data used to address the goals of this study are 410 meters of stratigraphic columns and several interpreted photo-panels, which document lithology, grain-size, sedimentary and biogenic structures, and the size and shape of fluvial architecture. The architecture is organized in a hierarchical framework that consists of stories, elements, and archetypes. Two architectural elements are recognized: (1) channel belt and (2) floodplain. Within the floodplain element, three floodplain stories are recognized: (1) splays, (2) levees, and (3) overbank fines. Characteristics of splays change with distance from their associated channel belt. In proximal locations, splays display abrupt changes in thickness and lack well-developed paleosols and desiccation cracks. In distal positions, splays have consistent thickness and contain well developed paleosols and large-scale desiccation cracks.

An archetype is defined as a floodplain element overlain by a genetically related, coeval channel belt element. Six archetype cycles are documented in this study. Upward patterns are identified using statistical analysis on floodplain characteristics such as net-sand content, paleosol presence within overbank fines, and element abundance and thickness. Each archetype cycle has the following patterns: upward increases in (1) net-sand content (2) the occurrence of paleosols (3) splay abundance, and (4) splay thickness.

The results of this study are useful for constraining reservoir models in low net-sand content fluvial systems. Furthermore, these data improve characterization of sand connectivity, via floodplain deposits, between isolated channel bodies.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California