Scaling of Fluvial Sand Body Lateral Dispersal Patterns From Normal Flow to Backwater Hydraulic Conditions: Implications for the Cretaceous McMurray Formation, Alberta, Canada

Abstract

In modern fluvial systems, channel-belt sand body scales and lateral relationships tend to reorganize through the normal flow to the marine-influenced backwater reach. Geometries and scales of transformations in individual channel-belt characteristics across this transition are becoming well-known, and include laterally-amalgamated channel-belt sand bodies with high width-to-thickness ratios in upstream normal flow reaches, to narrow width-to-thickness sand bodies encased in flood- and delta-plain muds within the backwater reach. This pattern is potentially related to relatively low tidal-energy input. However, larger valley-scale patterns of sand-body distribution remain poorly understood. This presentation is suspected to have occurred in the sedimentary rock record. Therefore, identifying the specific mechanisms that explain the change in lateral-dispersal of sandstone bodies through this hydraulic transition and how they relate to one another is essential towards developing more robust sedimentological models of fluvial systems experiencing varying degrees of tidal-energy at their downstream boundaries. Furthermore, the scaling of such criteria is critical to the development of petroleum system plays interpreted as fluvial-tidal in origin.

The enormous subsurface dataset for the Early Cretaceous McMurray Formation in the Alberta foreland basin, Canada, provides an opportunity to test these relationships to help interpret an ancient fluvial system. The high McMurray data density per area allows for the construction of a high-resolution cross-valley sedimentologic framework for strata that have been interpreted to be of both fluvial and tidally-influenced origin. The objective of this study is to develop a new dimensional relationship that explains the downstream transition in geometric scales and ratios within McMurray Formation strata to then compare against observed ones from modern systems. Herein, borehole well-log data, covering hundreds of square-kilometers, is used to characterize McMurray sandstone body distributions as well as their lateral and downstream connectivity through the normal flow/backwater hydraulic transition. The results of this study will provide important insights towards: i) the degree of potential tidal-influence experienced by McMurray deposits, and ii) unravelling the physical parameters dictating preserved sandstone body distributions and patterns across normal flow/backwater transitions in both modern and ancient systems.

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