Ra. Eisenberg et al., MODELING RESERVOIR HETEROGENEITY WITHIN OUTER RAMP CARBONATE FACIES USING AN OUTCROP ANALOG, SAN-ANDRES FORMATION OF THE PERMIAN BASIN, AAPG bulletin, 78(9), 1994, pp. 1337-1359
Variably cyclic, fusulinid-rich, outer ramp facies of the Permian San
Andres Formation are exposed along the Algerita escarpment, Guadalupe
Mountains, New Mexico. We have used the outcrop exposures and cored we
lls drilled adjacent to the outcrop to assess reservoir- and interwell
-scale variability of permeability as a potential analog for carbonate
reservoirs in the Permian basin and elsewhere. Permeability distribut
ion was evaluated using a field permeameter and conventional measureme
nts on small core plugs taken along vertical and horizontal outcrop tr
averses and from the slim-hole cores. Permeability is related to the v
ariable development of depositional cycles within a small-scale sequen
ce and also to diagenetic overprinting. Geostatistical models of perme
ability variation, honoring the geologic and petrophysical data, were
constructed and input into a waterflood simulator to understand the in
teractions between heterogeneity and flow. Variograms constructed from
conventional plug analyses of permeability fog a vertical outcrop tra
nsect contain a significant small-scale signal (''noise''), as seen in
the high nugget effect (approximately 50% of the overall sample varia
nce), and have correlation ranges of less than 4.5 m (15 ft). Differen
t vertical variogram characteristics are displayed by cyclic and less
distinctly cyclic parts of the San Andres. Variograms constructed for
horizontal transect data from three distinct stratigraphic units have
nearly identical properties. Overall, the ranges of correlation are sh
ort (3-3.5 m; 10-12 ft) when compared to typical interwell distances,
supporting a nearly uncorrelated and highly variable permeability mode
l. Using observed short ranges of vertical and horizontal correlation
and honoring the vertical transect data, cross sectional, conditionall
y simulated permeability fields were generated and used in simulated w
aterfloods to investigate the sensitivities to an oil recovery model a
nd overall fluid injection rate for this style of stratigraphy and cyc
licity. Cyclic parts of the section are characterized by a potential f
or early water breakthrough and relatively high vertical sweep efficie
ncies. Within the less distinctly cyclic section, waterflood fronts ha
ve a fingerlike profile and vertical sweep efficiency is generally poo
rer.