--> ABSTRACT: Modeling Reservoir Attributes and Estimating Additional Hydrocarbon Potential for Redevelopment in Fluvial-Deltaic Reservoirs: An Example from the Frio Fluvial-Deltaic Sandstone Play in South Texas, by M. H. Holtz, L. E. McRae; #91020 (1995).

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Modeling Reservoir Attributes and Estimating Additional Hydrocarbon Potential for Redevelopment in Fluvial-Deltaic Reservoirs: An Example from the Frio Fluvial-Deltaic Sandstone Play in South Texas

M. H. Holtz, L. E. McRae

Fluvial-deltaic sandstones contain >30 Bbbl of unrecovered oil resources in the U.S. and are major targets for redevelopment and reexploration in mature field areas worldwide. Proper assessment of additional oil and gas potential in these fields requires risk-adjusted estimates of the remaining available resource. In fields with incomplete or suspect reservoir data, evaluation must rely on statistical models developed from analogous, better known reservoirs.

The Frio Fluvial-Deltaic Sandstone Play of South Texas, which has produced 1 Bbbl of oil since the 1940's, exemplifies a mature play where reservoirs are being routinely abandoned, even though large volumes of oil may remain in untapped and incompletely drained reservoir zones. To estimate remaining oil resources and the potential for unproduced reservoir zones in this depositionally complex play, geologic and petrophysical characteristics of reservoirs in fields playwide were evaluated. Typical Frio oil reservoirs are dip-oriented, 10-ft-thick fluvial- or distributary-channel sandstones characterized by 25%^phgr, 31% Sw, 27% S0, and a reservoir area of 2,100 acres. Porosity, net pay, and residual S0 exhibit narrow variability; 68% of all reservoirs ha e porosity values between 22% and 26%, and 85% of reservoirs have net-pay values <20 ft. Reservoir initial Sw ranges from 18% to 54% (mean of 30.6% and median of 23%).

Probability distributions were generated for each reservoir attribute, distributions were best-fit to probability functions, and distributions of original (OOIP), mobile (OMOIP), and residual oil-in-place (OROIP) were calculated. Oil volumes from individual reservoirs were summed to produce a significant risk-adjusted total resource for the play. Volumetric probability distribution between 5% and 95% probability for OOIP ranges from 3.8 to 5.6 BSTB, OMOIP ranges from 2.5 to 3.6 BSTB, and OROIP ranges from 1.5 to 2.3 BSTB. Remaining mobile oil, at 95% probability, is >=1.2 BSTB. The untapped oil resource (sandstone bodies unconnected to a wellbore) may be 10% of OOIP, or 380 MMSTB. Reservoir characterization coupled with proper reservoir management techniques can be applied to recov r additional mobile oil in this play and in other analogous reservoirs.

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