Abstract: Targeting the Unrecovered Oil Resource in Mature Field Areas: An Example from Oligocene Frio Fluvial-Deltaic Reservoirs of South Texas

L. E. McRae, M. H. Holtz

Fluvial-deltaic sandstone reservoirs represent significant opportunities for redevelopment in mature fields throughout the world. The stratigraphic complexity inherent in these deposits is responsible for isolating significant volumes of mobile oil in undeveloped reservoir sand bodies that can be identified by integrated geological and engineering reservoir characterization and targeted for incremental recovery by recompletions and infill drilling. The Frio Fluvial-Deltaic Sandstone Play of South Texas is one example of a mature play where reservoirs are being abandoned at high rates, potentially leaving behind significant unrecovered resources in untapped and incompletely drained reservoirs. Nearly 1 billion barrels of oil have been produced from Frio reservoirs since the 1940s, yet ore than 1.6 BSTB of unrecovered mobile oil is estimated to remain in the play.

Reservoirs from Rincon field in South Texas are typical examples of maturely developed fluvial-deltaic sandstones with significant additional recovery potential. More than 30 productive reservoir zones are present in a 1000-ft stacked sequence of fluvial channel-fill and delta-plain distributary-channel sandstones and low-permeability overbank, floodplain, and interdistributary mudstones. Large reservoir complexes composed of laterally coalescing, dip-elongate channel sandstones provide ideal conditions for the isolation of oil that is presently incompletely drained or untapped. Data from core analyses, reservoir mapping, and stratigrapic log correlations are being used to describe heterogeneity and identify potential locations of additional reserves within a 100-ft reservoir interval that has already produced more than 15 MMSTB of oil. Porosity ranges of 22 to 29% and geometric mean permeability ranges of 5 to 91 md determined using core data from >100 wells illustrate the variability of reservoir sandstone fades present within this interval. Trends in porosity, permeability, and Sw exhibited within reservoir sub-zones have been used in conjunction with lateral facies studies to delineate reservoir architecture identify possible flow barriers, and target zones with undeveloped potential.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994