Impact of Archie "m
" on Evaluating Conventional and Low
Resistivity Pay
R. M
. Sneider, H. R. King, K. K. Stolper, J. T. Kulha
Correct interpretation of water saturation of many clastic reservoirs
requires reliable estimates of Archie's lithology exponent "m
". Archie "
m
" is
related to formation factor (F), the ratio of the electrical resistivity of a
100% water saturated porous rock (R0) to the resistivity of the
formation water (Rw). Archie "
m
" values are a function of (1)
mineralogy including clay type, amount and distribution, (2) cementation and
compaction, and (3) pore geometry (grain roughness, surface area, vugs, and
pore-size distribution).
Petrophysical and petrological studies of thousands of reservoir rocks in
many basins throughout the world show that "m
" varies typically from 1.4 to 2.9.
"
m
" is assumed to be 2 in many equations and log evaluation tables. If "
m
" is
assumed to be 2, but the actual value is lower, hydrocarbon saturation
calculates too low. Conversely, if "
m
" is assumed to be 2, but the actual value
is higher than 2, the value of the hydrocarbon saturation is calculated too
high.
The "m
" value of many low resistivity, low contrast reservoir pays range from
1.5 to 1.8. Using "
m
" equal to 2, many pay zones appear "wet" or to have very
high water saturation.
Log-derived water saturations using the correct "m
" values versus "
m
" equal 2
are compared for several conventional and low resistivity pay zones. The use of
rock comparators to estimate Archie "
m
" is illustrated.
AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995