The Importance of Fluvial Processes and Related Reservoir Deposits
Authors: Swanson Donald C. 
Published: December 2007

Fluvial/deltaic deposits may be the most important hydrocarbon reservoirs in the world. Many large fields have reservoirs of varying combinations of braided-stream, point-bar, distributary-fill, and valley-fill deposits. Adeterministic relationship exists between natural phenomena and the nature of these deposits. Their location, size, shape, and internal porosity distribution are functions of stream processes. Reservoir geologists and engineers should use data that correlate related natural processes, stream variables, and fluvial facies to improve predictions in all aspects of reservoir description. predictions in all aspects of reservoir description.

Introduction

For a 1991 program to encourage production of 300 billion bbl of unrecovered oil, the U.S. DOE screened more than 2,000 U.S. oil fields and identified fluvial/deltaic reservoirs as the highest-priority targets. This is notsurprising. For 30 years, geoscientists have become increasingly aware of the importance of fluvial reservoir deposits. Since the advent of environmental facies analysis, studies have identified almost all major clastic reservoirs as fluvial or fluvial/deltaic in origin. If the immense quantities of heavy oil in fluvial/deltaic deposits of Venezuela and Canada are included, land-derived clastic reservoirs may surpass Middle East carbonate reservoirs in importance.

Basic Assumptions. Sedimentologists and facies geologists have acknowledged the close relationship among depositional environments, the processes(physical, chemical, and biological) operating in them, and the facies (aspect)of the resulting deposits. It is generally accepted that deposition environment and processes are the primary factors influencing location, size, shape, and internal porosity distribution in many reservoirs, especially in land-derived elastic deposits. The most important characteristics of elastic reservoirs result from the combination of processes operating in a particular deposition environment. For fluvial/deltaic reservoirs, the tie between process and faciesis especially strong. The reliable causal relationships among gravity, flowing water, stream load, and reservoir deposits offer the promise of improved reservoir description.

Domination of Fluvial/Deltaic Reservoirs. Most major hydrocarbon reservoirs are fluvial or fluvial/deltaic. Table 1 is a list of significant fluvial/deltaic fields that emphasizes the potential economic benefit that can result from the use of the potential economic benefit that can result from the use of the close process/facies relationship that molded their reservoirs. The list also calls attention to the circumstances and phenomena that concentrate and preserve large volumes of coarse, porous, and permeable debris in fluvial/deltaic reservoirs, and the reasons permeable debris in fluvial/deltaic reservoirs, and the reasons that tremendous volumes of hydrocarbons become concentrated in them. The close relationships among stream load, flow velocity, and stream gradient are important in molding fluvial/deltaic reservoirs. Whether reservoir-size elastic particles are transported or deposited is very sensitive to flow velocity. Fig. 1 clearly shows how water velocity determines whether a particle is deposited or transported. Stream velocity, although affected by discharge, nature of load, channel geometry, and texture of channel material, depends mostly on stream gradient. Stream and distributary gradients become very low in the lower alluvial valley and on coastal and deltaic plains. Coarse elastic particles are not readily transported under low-velocity conditions, so large amounts of reservoir material are deposited and eventually preserved in coastal- and deltaic-plain fluvial or fluvial/deltaic environments. The contribution of fluvial/deltaic deposits to major elastic sequences is important. Thick elastic deposits that fill a mega-sequence or thick wedge of stratigraphy must be composed of regressive prograding elasticdebris carried to the receiving basin by streams. With this premise, it is obvious that the terrigenous sediment usually would be introduced into the basin through fluvial-dominated deltas, where fluvial processes concentrate and preserve much of the coarse elastic debris in specific fluvial and closely associated environments.

Fluvial Deposits

Fluvial processes involve streams and stream deposits; however, many important factors affecting streams (gravity, gradient, discharge, load, and channel geometry) affect any unidirectional flow, including runoff from melting glaciers or density flows along deepwater channels. From high mountain valleys to deepwater fans, moving fluid can build levees, meander, branch, shift courses, and adjust channel geometry to discharge, all in response to the same causes. By understanding and using the influence of these natural processes on the behavior of streams and their facies, we can processes on the behavior of streams and their facies, we can improve reservoir description.

JPT

P. 368