AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Downstream Variations in Distributive Fluvial System Characteristics, Huesca System, Ebro Basin, Spain
(1) Department of Earth Sciences, Royal Holloway University of London, Egham, United Kingdom.
(2) Department of Geology & Petroleum Geology, University of Aberdeen, Aberdeen, United Kingdom.
(3) Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM.
The Huesca system is one of the best documented examples of an ancient (Cenozoic) distributive fluvial system that has been shown to have a radial paleocurrent pattern and a fan planform shape.
The purpose of this study is to document the variation of the sandbody architecture through the system and determine the controls on these variations. Three outcrops have been documented in proximal, medial and distal parts of the system and sandbodies formed by channelised flow (sheet-like/ribbon) and by unconfined flow (thin sheet-like) were distinguished.
Downstream variations have been established: a decrease in the grain size and dimensions of sandbodies; an increase in fine-grained deposit proportion; an increase in proportion of ribbon sandbodies compared to sheet-like sandbodies; an increase in the proportion of thin sheet-like sandbodies.
These variations are thought to be controlled by decrease in flow energy downstream due to the decrease in surface gradient and water discharge. The flow strength decrease results in downstream decrease of sediment grain size causing an increase in substrate stability and formation of smaller and more laterally stable channels. Both flow energy decrease and increase in substrate cohesion limits the ability of flow to incise into floodplain deposits and consequently produces poorly channelised flow in the distal part.
The sandbody distribution across the system was controlled by lateral migration and avulsion frequency of the channels. The overbank packages intercalating with channel complexes indicate periods of channel avulsion when the channel was diverted to a new location and overbank deposition took place. The sheet-like sandbody shape suggests an occurrence of longer-lived laterally unstable channels. The presence of amalgamated and vertically aligned sandbody stacks indicates channel re-occupation as an avulsion mechanism. Avulsion by crevasse splay progradation was also an important process, producing continuous sandbodies on the floodplain which connect channel sandbodies.
Lateral migration of depositional lobes across the system results in lateral and vertical variations in sandbody distribution that result in a complex three-dimensional heterogeneity of the succession. This complexity and the factors that control the sandbody architecture in a distributive system need to be considered when predicting reservoir characteristics.