AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Revisiting the Subsurface Classification of Fluvial Sandbodies
(1) Chevron ETC, Perth, WA, Australia.
(2) Chevron ETC, Houston, TX.
(3) Chevron ETC, San Ramon, CA.
Subsurface characterization of fluvial sandbodies is inherently challenging at least in part due to the fact that fluvial classification schemes are generally based on observations of modern river planview patterns without consideration of deposit preservation. Fluvial deposit classifications that distinguish end-member “meandering” and “braided” types seldom provide adequate evidence to constrain either river channel sinuosity or braiding. Rather such classes are generally assigned based on a host of circumstantial evidence like interval net/gross, 2D sandbody shape, or the degree of internal heterogeneity. Direct evidence for channel sinuosity and braiding in the subsurface is unusual. A useful subsurface classification of fluvial reservoir sandbody types needs to be based on objective observations that can be made with conventional industry data.
The basic reservoir element of most fluvial deposits is a channel belt (CB), composed of an amalgamation of channel bar and abandonment fill deposits formed between episodes of river avulsion. A “storey” defines a channel bar and adjacent channel abandonment fill formed during migration of a single channel segment. The geometry of storeys and textural contrasts between bar and abandonment fill deposits have major impacts on reservoir connectivity along a CB. Well penetrations of individual CBs often show blocky or fining-upwards grain size trends that reflect a single to multi-lateral storey CB. Channel belts can cluster to form CB complexes, which define larger-scale reservoir bodies. CB character varies for different fluvial systems, which can change spatially and with time due to allocyclic controls such as changing slope, water discharge, and sediment load.
Channel belts are typically the scale of fluvial sandbody defined in the subsurface and can often be directly imaged in seismic. In seismic amplitude horizon maps of reservoir intervals it is common to observe CB orientation and changes in width, but less common to observe variations within channel belts due to limits of seismic resolution. In cases where well logs or core are available through specific channel belts, it is common to infer dimensions of intra-CB variations from the sandbody thickness and from the degree of internal heterogeneity. We suggest that CB rugosity, the un-parallel nature of opposing CB margins, may provide another measure of channel segment sinuosity, which may be linked to the degree of intra-CB heterogeneity.