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Revista de la Asociación Argentina de Sedimentología

Print version ISSN 1853-6360

Abstract

BRIDGE, John S. Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions. Rev. Asoc. Argent. Sedimentol. [online]. 2001, vol.8, n.2, pp.87-114. ISSN 1853-6360.

Fluvial deposits are important hydrocarbon reservoirs and aquifers in many parts of the world. In order to assess, develop and manage these resources, it is necessary to determine the three-dimensional geometry, orientation, spatial distribution and total volume of the reservoirs/aquifers. Such characterization of reservoirs/aquifers normally involves (1) analysis of well logs, cuttings, cores and seismic data (2) correlation of strata between wells, and (3) modeling of the three-dimensional volume between wells. These procedures require a great deal of geological knowledge. Unfortunately, current approaches to reservoir/aquifer characterization are problematical in many cases. Specific fluvial depositional forms (e.g., channel bars, crevasse splays, lacustrine deltas) are commonly interpreted from well logs and cores, because it is thought that deposits associated with particular depositional forms will have particular geometries and reservoir quality. However, it is very difficult to unambiguously interpret fluvial depositional forms from such one-dimensional data, and it is questionable that a particular depositional form will have a distinctive stratal geometry. In particular, it is not possible to distinguish braided river deposits from meandering river deposits based on their vertical facies sequences. Also, it is a common myth that there is a relationship between channel-belt width/thickness, channel pattern, grain size of sediment load, and bank stability. However, with very careful analysis, the lateral extent of subsurface channel-belt deposits and overbank deposits can be estimated from their facies, thickness, and proportion observed in well logs and cores. Correlation of fluvial lithostratigraphic units (e.g., channel-belt sandstone bodies) between wells is commonly based on untenable assumptions. The result is normally that stratigraphic units appear much more laterally continuous than they really are. Before attempting correlation, it is desirable to estimate the likely lateral extent and thickness variation of stratigraphic units, based on the interpretation of available well logs and cores mentioned above. Then, it is wise to set up multiple working hypotheses for the correlation. Estimation of the geometry of stratigraphic units is aided by use of modern analogs, ancient analogs, and depositional models. Modern analogs are superior aids because it is possible to describe fluvial morphology and deposits directly and unambiguously. Ancient analogs are less reliable, because their origin must be interpreted, and because outcrops are commonly not large enough to allow definition of the facies geometry (e.g., channel-belt width). Depositional models can be used to help estimate the lateral extent of stratigraphic units (e.g., zones of high or low net-to-gross). However, such models may be only two-dimensional, not quantitative, or do not represent depositional processes and products correctly. Sequence stratigraphic models are particularly problematical in these regards. In order to characterize the three-dimensional volume between wells, it is common to make use of ancient analogs in combination with object-based stochastic models. The geometry and proportion of stratigraphic objects such as channel-belt sandstone bodies are determined statistically from well data and ancient analogs, and these objects are located in space using their known positions in wells and using quasi-random placement between wells. Although this method honors well data, the shapes and locations of objects can be very unrealistic. Process-based (forward) models are capable of producing much more realistic stratigraphy, and have much greater predictive value, but it is very difficult to fit such models to well data exactly. For this reason, process-based models are not routinely used for reservoir/aquifer modeling. However, it has been shown recently that hybrid process-based and stochastic models can potentially be fitted exactly to well data using a trial-and-error approach with new optimization procedures (genetic algorithms).

Keywords : Fluvial reservoirs and aquifers; Well logs and cores; Stratigraphic correlation; 3-D alluvial architecture models.

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