ABSTRACT

The Frio Fluvial-Deltaic Sandstone along the Vicksburg Fault Zone play of South Texas has produced nearly 1 billion barrels (Bbbl) of oil from fluvial-deltaic sandstones since field development began in the 1940's. More than half the reservoirs in this depositionally complex play have been abandoned, even though large volumes of oil remain. Current efforts integrating geological and engineering reservoir characterization are being used to identify the location of unrecovered mobile oil, estimated at more than 1 Bbbl, that remains in unproduced reservoir zones in fields within this very mature play.

Geologic and engineering data from Rincon field are being evaluated to identify interwell stratigraphic heterogeneity and potential for compartmentalization of significant volumes of unrecovered oil. The producing Frio reservoir interval, representative of other fields within the play, consists of a 1,000-ft-thick mixed aggradational and progradational sequence of fluvial channel-fill and delta-plain distributary-channel sandstones interbedded with low-permeability overbank, floodplain, and interdistributary mudstones. Oil reservoirs consist of multiple thin (0-40 ft) dip-elongate sandstone units that stack to form gross thicknesses of 50 to 100 ft. They occur both as narrow channel fills isolated vertically and laterally by low-permeability overbank and floodplain facies and as large channel complexes with multiple laterally coalescing sandstone lobes. These large channel-sandstone complexes provide ideal conditions for the isolation of oil accumulations in multiple reservoir compartments, many of which are now incompletely drained or completely untapped.

The stratigraphic position of individual reservoir sandstones in Rincon field within the overall genetic stacking sequence of the Frio depositional episode provides an important control on hydrocarbon producibility. Reservoir attributes characterized from core analysis data from more than 100 wells reveal distinct distributions of porosity and permeability values for upper Vicksburg progradational units, lower Frio mixed progradational/aggradational units, and middle Frio aggradational units. Reservoir architectural mapping and facies studies are being used in conjunction with petrophysical data to identify heterogeneity style and degree of channel connectedness in each type of reservoir and to identify zones with high potential for recovering additional reserves.