SMITH, L. K., T. L. DUNN, and R. C. SURDAM, Department of Geology, Univ. of Wyoming, Laramie, WY

ABSTRACT: Cation Geothermometry and the Effect of Organic-Inorganic Diagenetic Reactions

Predicted cation ratio geothermometry temperatures, using equations of Na/K, Na-K-Ca, Mg-Na-K-Ca, and Mg/Li, were compared between oilfield and geothermal waters. Geothermometers in oilfield waters yielded much less consistent temperature predictions compared to geothermal waters in the same temperature range. Scatter of predicted temperature in oilfield waters is greatest in the temperature interval where carboxylic acid anions

(CAAs) are in greatest concentration. CAAs are not present in geothermal systems, and temperature prediction greatly improves in oilfield waters when CAAs are less than 80% of total alkalinity.

The assumptions of cation ratio geothermometry are invalid in oilfield waters where CAAs are abundant. These are: (1) Cation ratios are controlled by exchange between solid aluminosilicates. However, CAAs affect mineral solubility by forming complexes with the cations. Therefore, the ratios of cations in solution change from values expected when cation ratios are controlled solely by cation exchange between aluminosilicate minerals.Furthermore, concentrations of Ca and Mg are strongly controlled by carbonate equilibria; (2) Aluminum is conserved in solid phases. However, CAAs form stable complexes with Al increasing Al-silicate solubility and mobilizing Al, thus Al is not conserved in solid phases; (3) neither H+ nor CO2 enter into the net reactions (ie. pH is buffered by aluminosilic te hydrolysis). But acetate is an effective buffer of pH in feldspar-rich systems, and at higher temperatures, decarboxylation of CAAs increases P<CO2>.

Behavior of these geothermometers indicates that organic-inorganic diagenesis is an important control on oilfield water chemistry.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.