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
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.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California