Going Beyond Br/Cl Ratios to Distinguish Mid-Continent Oil Field Brines

Banaszak, Konrad J.
Keramida Environmental, Indianapolis, IN 

The well-used method for identifying oil-field brines that may have sullied a fresh groundwater is the bromide to chloride (Br/Cl) ratio. In the Mid-Continent, this method has been used to distinguish brines and to define mixing ratios. The method may be insufficient or provide equivocal answers. It also may not be sufficient to scientifically support a particular conclusion. One reason for these short comings can be an insufficient knowledge of the Br/Cl in the potential source brines because samples are often limited to brines in tanks after the major production of petroleum. In some cases, no brine characteristic of that used in stimulating or primarily produced may be left.

In the Mid-Continent, there are several other methods to add to knowledge base. A simple anion that can add to chemical understanding is iodate. The stable isotopic composition of oxygen and hydrogen of water in Mid-Continent brines is different from geologically young waters in drinking water aquifers. Isotopes of chloride are another means of adding equations of mixing. The oxygen and sulfur isotopic composition of sulfate may add another independent test. Finally, the isotopic composition of iron, because of the high likelihood that black shales are involved in the origin of petroleum in Mid-Continent, may add yet another independent test.

Distinguishing and getting to mixing, especially in legal proceedings, should rely on more than a single line of evidence.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004