Application of System Scale Fluvial Models to the Basin Scale

Abstract

Understanding the variation in fluvial facies and architecture is key to understanding reservoir heterogeneity and extent. Distributive fluvial systems (DFS) have been shown in recent literature to be key components of modern sedimentary basins, with important implications with regards to the predictability of fluvial systems in the subsurface. Recent rock record work has quantitatively shown DFS to have predictive downstream trends, with a systematic downstream decrease in channel dimension, amalgamation, occurrence and grain size while floodplain facies become increasingly more preserved. However, at a basin scale such predictive models are scarce, particularly in a quantified manner.

This study aims to firstly test whether DFS, as recent literature suggests, do form a considerable portion of ancient continental sedimentary basin fills and secondly understand whether the DFS model can be up scaled to better understand heterogeneity at the basin scale. The Palaeogene Fort Union and Willwood Formations of the Bighorn Basin, Wyoming, were selected as units to study due to excellent outcrop exposure over a wide area of the basin and a considerable degree of chronostratigraphic control. A total of 4000 m of succession over 8,700 km² was quantitatively analysed.

Analysis at the lithostratigraphic scale showed the fill of the Bighorn Basin to be highly heterogeneous in nature. Channel presence within measured successions varied from 12 - 81 %, average channel body thickness 4 – 20 m and average weighted channel grain size from 59 – 0.001 mm. Additionally the channel geometry varied considerably across the basin. Establishing predictive trends across the basin within the lithostratigraphic units were difficult, however, by applying a systems approach to the units predictable trends were apparent. The heterogeneous nature of the fill is considered to be due to the presence of multiple, different sized alluvial fan and DFS, with an important axial DFS component also recognised. Downstream trends within the individual systems were apparent once the systems had been defined.

This study outlines a methodology that allows petroleum geologists to discern fluvial systems and predictive trends within lithostratigaphic units, with important implications for reservoir identification and predictability at the exploration and production stages.

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