--> ABSTRACT: Reservoir Architecture and Permeability Characteristics of Fluvial-Deltaic Sandstone Reservoirs, Frio Formation, Rincon Field, South Texas, by L. E. McRae, M. H. Holtz; #91020 (1995).

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Reservoir Architecture and Permeability Characteristics of Fluvial-Deltaic Sandstone Reservoirs, Frio Formation, Rincon Field, South Texas

L. E. McRae, M. H. Holtz

Stratigraphically complex fluvial-deltaic sandstone reservoirs possess excellent potential for incremental recovery of large volumes of additional mobile oil now isolated in undeveloped reservoir sandstone bodies. Frio reservoirs of the South Texas Gulf Coast are being studied to better characterize interwell stratigraphic heterogeneity in fluvial-deltaic depositional systems and determine controls on locations and volumes of unrecovered oil.

Major oil reservoirs in Rincon field represent deposition in broad, dip-elongate fluvial channel systems. Comparison of net-sandstone patterns, facies distributions, core descriptions, and petrophysical data with existing oil production trends from two highly prolific reservoirs reveals details of the permeability structure of these reservoirs and shows how different architectural styles impact recovery efficiency. Porosity averages for three sandstone facies are 19%-22%. Mean permeability values are more diagnostic of specific lateral channel facies than are porosity values and vary from 47 md (range 4-1,649 md) in channel point bars to 38 md (range 0.1-1,253 md) in the channel thalweg, 10 md (range 0.1-250 md) for channel margins, 25 md (range 0.3-199 md) for crevasse-splays, and 5 d (range 0.1-54 md) in non-reservoir overbank levee and floodplain sediments. Permeability also exhibits a consistent vertical trend for the various channel facies with lowest values at channel bases (mean: 6 md, range: 0.1-157 md), where development of a mud-chip lag is common. Permeability increases upward through mid-channel (mean: 60 md, range: 0.1-1,530 md) and then decreases at channel tops (mean: 14 md, range: 0.1-185 md) where grain size decreases.

The Frio D and E reservoir zones have similar proportions of channel, overbank, and floodplain facies but different recovery efficiencies, from 38% in the B sandstone channel system to 29% for the D sandstone interval. Relative broadness (3,500-7,000 ft), lower sinuosity, and greater interconnectedness of channels in the E reservoir caused increased flow communication between individual channels, resulting in higher recovery efficiency. The narrower (<4,000 ft), less interconnected channels of the D reservoir, in contrast, provide ideal conditions for isolation of oil accumulations in multiple reservoir compartments. Interwell-scale petrophysical models are being developed to further characterize D reservoir flow-unit boundaries and identify locations of untapped and incompletely d ained zones.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995