Quantitatively Modeling Alluvial Strata for Reservoir Development with Examples from Krasnoleninskoye Field, Russia

ABSTRACT Tye, R.S. , 2013. Quantitatively modeling alluvial strata for reservoir development with examples from Krasnoleninskoye field, Russia. Industry-accepted methods for estimating the subsurface dimensions of fluvial channel and channel-belt bodies were evaluated on their conceptual and physical bases. Geocellular models of fluvial reservoirs were built using several of these methods in conjunction with core and wireline-log data from the Krasnoleninskoye field, Russia. The objective was to use typical oil-field data and existing modeling technology to build geologically accurate geocellular models for field-development planning. Uncertainty in stratigraphic interpretation and geocellular modeling of fluvial reservoirs can be reduced by using floodplain deposits as pseudo-chronostratigraphic horizons to limit the miscorrelation of sandstone bodies, identifying and mapping paleovalleys constituting sandstone-prone fairways, and applying the physical relationships among maximum bankfull channel depth, channel width, and channel-belt width. A drawback to existing reservoir-modeling software is that it builds statistically based models conditioned to well data, but it does not incorporate physical-sedimentary laws. Therefore, reservoir zones in geocellular models must mimic paleovalley trends to prevent channel-body placement in a geologically inappropriate setting. Channels constrained by a geologically reasonable range of estimated dimensions (e.g., width, thickness, sinuosity) are distributed within the alluvial valleys along with the correct proportions of overbank-sandstone and floodplain deposits. Subsurface data interpreted from the standpoint of the physical-depositional processes they record, and with the recognition of the large- and small-scale sedimentation units comprising channel-bar and channel-fill strata, reveal how these strata vary from bar head to bar tail, and how the channel's geometry and migration style influenced the resultant vertical profile. Recognition of upper-bar deposits and their transition into natural-levee and/or floodplain deposits defines the maximum bankfull channel depth from which channel width and channel-belt width are estimated. Maximum bankfull channel depths of two Krasnoleninskoye fluvial reservoirs were estimated using core data in conjunction with thickness measurements based on wireline-log data. As a comparison, estimates of maximum bankfull channel depth were calculated based upon the relationships of cross-set thickness to dune height and dune height to flow depth using cross-set thicknesses measured in the cores. Estimated channel widths and channel-belt widths range by a factor of 2 to 5, depending upon the calculation method. Realizations of the Krasnoleninskoye fluvial strata show channel-body dimensions and channel-body morphologies comparable to the Santee River, South Carolina, alluvial valley, in which straight, meandering, and anastomosed channel reaches occur within a 4 by 11 km area. Thus, stochastic, but geologically realistic geocellular models of fluvial reservoirs are achievable if the model structure accurately defines alluvial valleys, valleys are populated with channel bodies appropriately sized by maximum bankfull channel-depth estimates, and one abandons the traditionally held belief that alluvial stratigraphy varies due to, and can be predicted from, the planform morphology of the river system from which it formed.