The Importance of Fluvial Processes and Related Reservoir Deposits
- Donald C. Swanson (Swanson Consulting Services)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- April 1993
- Document Type
- Journal Paper
- 368 - 377
- 1993. Society of Petroleum Engineers
- 1.2.3 Rock properties, 5.8.5 Oil Sand, Oil Shale, Bitumen, 4.2 Pipelines, Flowlines and Risers, 6.5.7 Climate Change, 4.3.4 Scale, 1.6.9 Coring, Fishing, 2.4.3 Sand/Solids Control, 5.6.1 Open hole/cased hole log analysis, 5.1.5 Geologic Modeling, 4.1.2 Separation and Treating, 5.1.1 Exploration, Development, Structural Geology, 5.8.7 Carbonate Reservoir, 4.1.5 Processing Equipment, 5.1 Reservoir Characterisation
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Fluvial/deltaic deposits may be the most important hydrocarbon reservoirs inthe world. Many large fields have reservoirs of varying combinations ofbraided-stream, point-bar, distributary-fill, and valley-fill deposits. Adeterministic relationship exists between natural phenomena and the nature ofthese deposits. Their location, size, shape, and internal porosity distributionare functions of stream processes. Reservoir geologists and engineers shoulduse data that correlate related natural processes, stream variables, andfluvial 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 unrecoveredoil, the U.S. DOE screened more than 2,000 U.S. oil fields and identifiedfluvial/deltaic reservoirs as the highest-priority targets. This is notsurprising. For 30 years, geoscientists have become increasingly aware of theimportance of fluvial reservoir deposits. Since the advent of environmentalfacies analysis, studies have identified almost all major clastic reservoirs asfluvial or fluvial/deltaic in origin. If the immense quantities of heavy oil influvial/deltaic deposits of Venezuela and Canada are included, land-derivedclastic reservoirs may surpass Middle East carbonate reservoirs inimportance.
Basic Assumptions. Sedimentologists and facies geologists have acknowledgedthe 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 environmentand processes are the primary factors influencing location, size, shape, andinternal porosity distribution in many reservoirs, especially in land-derivedelastic deposits. The most important characteristics of elastic reservoirsresult from the combination of processes operating in a particular depositionenvironment. For fluvial/deltaic reservoirs, the tie between process and faciesis especially strong. The reliable causal relationships among gravity, flowingwater, stream load, and reservoir deposits offer the promise of improvedreservoir description.
Domination of Fluvial/Deltaic Reservoirs. Most major hydrocarbon reservoirsare fluvial or fluvial/deltaic. Table 1 is a list of significantfluvial/deltaic fields that emphasizes the potential economic benefit that canresult from the use of the potential economic benefit that can result from theuse of the close process/facies relationship that molded their reservoirs. Thelist also calls attention to the circumstances and phenomena that concentrateand preserve large volumes of coarse, porous, and permeable debris influvial/deltaic reservoirs, and the reasons permeable debris in fluvial/deltaicreservoirs, and the reasons that tremendous volumes of hydrocarbons becomeconcentrated 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 verysensitive to flow velocity. Fig. 1 clearly shows how water velocity determineswhether a particle is deposited or transported. Stream velocity, althoughaffected by discharge, nature of load, channel geometry, and texture of channelmaterial, depends mostly on stream gradient. Stream and distributary gradientsbecome very low in the lower alluvial valley and on coastal and deltaic plains.Coarse elastic particles are not readily transported under low-velocityconditions, so large amounts of reservoir material are deposited and eventuallypreserved in coastal- and deltaic-plain fluvial or fluvial/deltaicenvironments. The contribution of fluvial/deltaic deposits to major elasticsequences is important. Thick elastic deposits that fill a mega-sequence orthick wedge of stratigraphy must be composed of regressive prograding elasticdebris carried to the receiving basin by streams. With this premise, it isobvious that the terrigenous sediment usually would be introduced into thebasin through fluvial-dominated deltas, where fluvial processes concentrate andpreserve much of the coarse elastic debris in specific fluvial and closelyassociated environments.
Fluvial processes involve streams and stream deposits; however, manyimportant factors affecting streams (gravity, gradient, discharge, load, andchannel geometry) affect any unidirectional flow, including runoff from meltingglaciers or density flows along deepwater channels. From high mountain valleysto deepwater fans, moving fluid can build levees, meander, branch, shiftcourses, and adjust channel geometry to discharge, all in response to the samecauses. By understanding and using the influence of these natural processes onthe behavior of streams and their facies, we can processes on the behavior ofstreams and their facies, we can improve reservoir description.
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