Scaling Relationships of Channel-Lobe Deposits in Large Fluvial Fan Systems: Implications for Subsurface Reservoir Prediction

Abstract

Large fluvial fan systems can cover areas of over 10³ km², and have significant hydrocarbon reservoir potential. Although morphologically similar to alluvial fans, fluvial fans are an order of magnitude larger and are characterized by lacking sediment gravity flow processes. Fluvial fans are unconfined river systems, where a river builds a fan-shaped sedimentary body through multiple avulsions. Studies of modern systems show that the successive avulsions tend to cluster within lobes rather than being randomly distributed across the fan. Lobe shifts also occur such that each lobe is characterized by an independent radial drainage pattern and is clearly distinguishable from channel patterns in adjacent lobes. While there are studies that establish lobe morphologies and the characteristic depositional features of large fluvial fans, such as decreasing channel dimensions and grain size downfan, quantitative scaling relationships among fluvial fan elements are lacking.

Modern large fluvial fans are typically formed in front of mountain ranges and vary in sediment budget, water discharge, and catchment dimensions. While modern systems have been a focus of research, few outcropping large fluvial fans have been documented. Comparing modern to ancient fluvial fans enables us to understand the channel-lobe dynamics. Using high-resolution satellite and topography data, this study analyzes quantitative scaling relationships from modern fluvial fans, and examines whether avulsion locations relate to lobe development. We compare fan and fan-apex channel dimensions, the downfan decay of channel dimensions, and lobe dimensions. We document these relationships in the context of terminal fans vs. fans that feed into other rivers or basins. We use these data to establish the range of quantitative relationships between lobe/fan geometries and the genetically related channel dimensions. These data are then compared to ancient (outcropping) fluvial-fan datasets to test the robustness of the scaling relationships and whether a channel-lobe-fan hierarchy can be established. The normalized fan-apex channel dimension is used to infer a hierarchy in the ancient. These scaling relationships help to improve predictions of the evolution and facies architecture of subsurface and outcropping large fluvial fan systems.

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