On the Nature of Delta Distributary Channel Fill

Edmonds, Douglas¹, Benjamin Sheets², David Hoyal², Roger Bloch³ (1) The Pennsylvania State University, University Park, PA (2) ExxonMobil Upstream Research Company, Houston, TX (3) ExxonMobil Upstream Research Company, Houston,

During the evolution of a deltaic distributary channel network a significant number of channels are abandoned. The caliber of sediment that fills an abandoned channel could have a significant effect on the reservoir properties of the deltaic sandbody. Here we have used experimental flume data to define a process-based model of channel filling and the resulting sedimentology. Results show that distributary channels evolve through a cycle of channel lengthening, backstepping, and abandonment. During the lengthening stage, knickpoints within the channel migrate upstream and capture additional discharge. Interestingly, during this process the width remains constant because as channels lengthen (or as time progresses) the channel banks become more cohesive and force the channel to scour deeper instead of widening. Thus, channel scour depth is directly proportional to channel length (R2 = 0.52). During the lengthening stage a complex, heterolithic cut and fill facies is deposited in the proximal channel portions. At maximum channel length, overbank flow upstream of the shoreline increases, which decreases discharge in the main channel and signals the start of the backstepping stage. During backstepping, sediment deposition fills the most distal portions of the channel, burying the levees, and shortening the active channel length. Backstepping is characterized by deposition of two semi-contemporaneous facies. The distal facies is sandy and characterized by upstream dipping sediment packages, while the proximal facies is sandy with vertically aggrading packages. The channel filling is complete when all the discharge has been routed down another course and the old channel is abandoned.