Characterization of fluvial hydrocarbon reservoirs and aquifers: problems and solutions

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