Datapages, Inc.Print this page

Abstract: Integrated Reservoir Characterization of Mature Oil Reservoirs: An Example from Oligocene Frio Fluvial/Deltaic Sandstones, Rincon Field, South Texas

L. E. McRae, M. H. Holtz

The Frio fluvial/deltaic sandstone along the Vicksburg fault zone play of south Texas has produced nearly 1 billion bbl of oil from fluvial/deltaic sandstones since field development began in the 1940s. More than half of 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 billion bbl, that remains in unproduced reservoir zones in fields within this very mature play.

Engineering data from Frio reservoirs in Rincon field were used to assess past production behavior, determine completion density, and prioritize zones for incremental reserve growth opportunities. Geologic data have been evaluated to identify interwell stratigraphic heterogeneity and potential for compartmentalization of significant volumes of unrecovered oil. The producing reservoir interval, representative of other fields within the play, consists of a 1000-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. Major oil reservoirs represent deposition in broad, dip-elongate fluvial systems. Individual zones consist of mul iple thin (0-40 ft) 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 very low-permeability overbank facies and flood-plain mudstones and as large channel complexes with multiple laterally coalescing sand lobes. Large dip-elongate 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.

Reservoir architectural mapping and core analysis data from more than 100 wells are being combined with reserve volumetrics to describe heterogeneity and to identify possible locations of additional reserves within a 100-ft reservoir interval that has already produced more than 15 million bbl of oil. Trends in petrophysical attributes within reservoir subzones, illustrated by a variability in core porosity of 22 to 29% and in geometric mean permeability of 5 to 91 md, have been used in conjunction with facies studies to characterize reservoir architecture, delineate possible flow barriers, and identify zones with significant undeveloped potential.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994