Evaluating Process Interpretations of Multistory Fluvial Sand Bodies

Chamberlin, Ellen P.; Hajek, Elizabeth

Sand-body architecture is a key control on reservoir quality and connectivity in fluvial deposits. Multistory channel-belt sand bodies are loosely defined as amalgamated channel complexes, but because multiple processes can produce stories, uniquely interpreting multistory deposits and predicting connectivity within subsurface sand bodies is difficult. In order to determine uncertainties associated with interpreting the origins of multistory sand bodies, and to improve predictions of sand-body connectivity within these deposits, we review and categorize published descriptions and interpretations of multistory sand bodies and use simple geometric models to generate multistory sand bodies through channel migration, avulsion, and incision.

Published interpretations of processes that control multistory sand-body formation can be classified into three end-member cases: 1) intra-channel-belt processes such as channel and channel-thread migration and avulsion, 2) channel-belt avulsion reoccupation, and 3) cut-and-fill cycles related to discharge variation, uplift, and or base-level changes. Intra-channel belt processes generate multistory bodies with smooth, concave up basal erosion surfaces, limited preservation of floodplain deposits within the sand body, and fully preserved channel fills only in the uppermost stories. Avulsion reoccupation builds multistory sand bodies with irregular or step-like basal scour surfaces, fully preserved channel fills throughout the sand body, and stories that are conformable with lateral floodplain deposits. Multistory sand bodies formed through cut-and-fill cycles have irregular basal scour surfaces that clearly truncate older floodplain and channel deposits, and lack conformable aggradational features like channel levees and floodplain deposits.

To evaluate the confidence with which we can make interpretations and predict connectivity within this framework, we use a simple object-based geometric model to explore the probability of channel-fill preservation in net-aggradational settings with differing degrees of channel migration, avulsion, and incision. We compare these results to field examples including the Williams Fork Formation (Upper Cretaceous, western Colorado) and the Ferris Formation (Cretaceous/Paleogene, Wyoming), where we evaluate how uncertainty in process interpretations can be constrained in outcrop settings.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013