--> Smectite Grain Coating and Low-Resistivity Pay Sands, Santa Margarita Sand, Southern San Joaquin Valley, by Robert C. Orlando, James J. Hickey, and R. Wydrinski; #91024 (1989)

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Smectite Grain Coating and Low-Resistivity Pay Sands, Santa Margarita Sand, Southern San Joaquin Valley

Robert C. Orlando, James J. Hickey, R. Wydrinski

Log evaluations of the Santa Margarita Sand (upper Miocene "Delmontian") from the ARCO Tejon A 579-35 well, Kern County, California, calculate Sw of up to 100%. Fluid resistivity (Rw) measured 5.5-6.0 ohm-m; yet, when tested, the initial production was 33 BOPD with 8 BWPD. Deep resistivity (RILD) ranged between 5.7-9.5 ohm-m with corresponding LDT-CNL porosities of less than 30%. Initial analyses suggested that high clay content was influencing resistivity.

The Santa Margarita was extensively cored in the Tejon A 510-36 that was drilled approximately 850 ft east of the 579-35 well. A nearly continuous 228-ft conventional core recovered sediments from the Santa Margarita and lower Chanac (lower Pliocene) Sands. The Santa Margarita consists of massive poorly consolidated coarse-grained sands, arkosic with a significant volcanogenic component. Selected log calculations (using an adjusted Rw of 0.55 ohm-m and the modified Simandoux equation) indicate Sw from 70-73% with corresponding LDT-CNL porosities from 22-25%. Measured PKS data indicate Sw from 48-51% with porosities from 32-35% over the same interval. Even with the lower Rw, calculated water saturations were at least 20% higher than the measu ed values.

Grain-coating authigenic clay is extensively developed in the sands throughout the interval of interest. Average clay content of the sands is about 26 wt. %. The clay fraction is dominated by highly swelling smectitic clay (90% expandable illite/smectite) that occurs as a thin honey-comblike grain-surface coating. The smectite grain coatings, with a high specific surface area and high cation exchange capacity, form a physically continuous network through the sands. This network contributes both surface (matrix) and bound-water conductivity, generating a low-resistivity pathway for induced currents.

Well logs from the field are being reevaluated to determine if Santa Margarita Sands are productive in areas where the sands calculate wet. Additional petrophysical data, including electrical properties (Archie equation parameters, i.e., a, m, and n), are needed to assess quantitatively the influence of the smectite-clay network on log response and to determine appropriate formation evaluation parameters.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.