Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Facies Modeling of Fluvial Systems, Fifty Years On
(1) Geology, University of Toronto, Toronto, ON, Canada.
Facies models, which first emerged in the early 1960s, have served as essential education tools, and have formed the basis for much practical mapping work in clastic systems, but have reached their limit of usefulness for subsurface exploration and production. In the case of fluvial systems, a continuing focus on the supposed differences between reservoir bodies formed by (for example) braided versus meandering systems distracts attention from a consideration of the various controls that influence the way in which conglomerates and sandstones accumulate.
Application of the principle of uniformitarianism led geologists to base depositional interpretations of the ancient record on observations of modern environments. In the case of rivers systems, some ancient environments (e.g., pre-Silurian humid systems lacking in vegetation) have no modern analogs and, in any case, surface processes and subsequent burial can lead to selective preservation and removal that do not necessarily reflect faithfully the surface model. Large bar forms in active rivers may be very ephemeral; scour hollows, barely visible at the surface of modern rivers, tend to have high preservation potential; floodplain preservation is highly variable and not indicative of fluvial style.
Numerical modeling of fluvial systems, a popular method of assessing fluvial reservoir for production purposes, has the disadvantageous need to assume statistical uniformity within a reservoir volume, contrary to the reality of natural systems, which may include rivers of contrasting size and style.
Developments in sequence stratigraphy have opened up the possibility for much more sophisticated analysis of the origins of clastic successions, and have clarified the nature of upstream and downstream controls, including source-area uplift and climate, basin subsidence rate, and sea-level change, that determine the architecture of the final preserved deposit. Empirical classifications of the shape and stacking patterns of reservoir bodies, and of the architectural elements of which they are composed are much more useful for practical exploration and production purposes, and may be explored using a variety of practical tools, including large-scale architectural work on outcrop analogs, pressure testing, 3-D seismic, and surveillance monitoring of producing reservoirs.