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