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Impact of Archie "Previous HitmNext Hit" on Evaluating Conventional and Low Resistivity Pay

R. Previous HitMNext Hit. 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 "Previous HitmNext Hit". Archie "Previous HitmNext Hit" 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 "Previous HitmNext Hit" 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 "Previous HitmNext Hit" varies typically from 1.4 to 2.9. "Previous HitmNext Hit" is assumed to be 2 in many equations and log evaluation tables. If "Previous HitmNext Hit" is assumed to be 2, but the actual value is lower, hydrocarbon saturation calculates too low. Conversely, if "Previous HitmNext Hit" is assumed to be 2, but the actual value is higher than 2, the value of the hydrocarbon saturation is calculated too high.

The "Previous HitmNext Hit" value of many low resistivity, low contrast reservoir pays range from 1.5 to 1.8. Using "Previous HitmNext Hit" equal to 2, many pay zones appear "wet" or to have very high water saturation.

Log-derived water saturations using the correct "Previous HitmNext Hit" values versus "Previous HitmNext Hit" equal 2 are compared for several conventional and low resistivity pay zones. The use of rock comparators to estimate Archie "Previous HitmTop" is illustrated.

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