Geochemical Signatures of Rapid Carbonate Crystallization in Well Scales

Giles, Grace F.¹, James R. Boles¹, Hilario Camacho² (1) University of California, Santa Barbara, Santa Barbara, CA (2) Signal Hill Petroleum, Inc, Long Beach, CA

Los Angeles Basin and San Joaquin basin carbonate well scales show isotopic disequilibrium and highly variable magnesium content, apparently due to rapid CO₂ degassing and crystallization. The effect of rapid crystallization on carbonate geochemistry is studied with well scales, because they form with known fluid composition, temperature, and pressure changes. Calcite scales and their respective waters have been analyzed for geochemical and isotopic composition and incorporated with reservoir data.

The δ¹⁸O of scales is up to 10 ‰ more positive than expected for equilibrium with pore water. This deviation from equilibrium temperature-dependent fractionation is linear, suggesting that greater precipitation rates (i.e. larger pressure differentials) increase isotopic disequilibrium. Rapid CO₂ degassing preferentially strips the lighter isotopes from the water, leaving precipitated calcite enriched in ¹³C. Most of the scale samples have positive δ¹³C values (+6.83 – +28.7 ‰) indicating rapid CO₂ degassing. Assuming a constant input fluid composition, a positive correlation of carbon and oxygen isotopes is evidence of rapid crystallization. Vertical sample sets from different depths of the tubing and horizontal sample sets from transects of thick samples all show positive δ¹³C/δ¹⁸O slopes (typically +0.9 to +5.5), which is intermediate of Hendy's (1971) proposed rapid degassing range for δ¹³C/δ¹⁸O co-variation of +0.6 to +8.3.

Analyzed calcite scales from waters with low [Mg²⁺]/[Ca²⁺] ratios and wells with minimal temperature and fluid composition change show 4 to 16 mole percent Mg content. The wellbore conditions suggest that precipitation rate, rather than temperature or saturation, is controlling the Mg content of the scales.