WATER CHEMISTRIES ASSOCIATED WITH LIGHT AND HEAVY OILS IN THE POWDER RIVER BASIN AND LARAMIE BASIN, WYOMING

We identify two water types within the same structure of the Laramie Basin. These waters are co-produced with heavy oil and come from wells inside a 1mile-diameter circle. For this investigation these two types are called Type S waters (n=6), and Type C waters (n=5). All samples were collected and analyzed by the same protocols. The Laramie Basin samples were collected on one day. The samples for the Powder River Basin (PRB) (treated as one group of similar chemistry, n=21) were collected on two sequential days. This control yields a pair of data-sets with high control over introduced error, allowing description of nuanced processes.

Type S waters have two orders of magnitude more sulfate than Type C waters. Type S waters are characterized by a more reducing environment (ORP averages -269mV) and a more saline chemistry averaging 4172ppm total dissolved solids. Type S waters have higher concentrations of most analytes, especially (ordered from greatest significance to least): strontium, calcium, magnesium, potassium, ammonia, chloride, sodium, bromide, boron and silicon.

Type C waters are more dilute, and contain at most 18ppm Sulfate, resulting in them appearing as Na-CO₃ waters despite having only slightly greater alkalinity than Type S waters in absolute terms. The type C waters are not only more acidic with an average pH of 5.6 units but also more oxidizing with an ORP of -139mV. Type C waters have significantly more barium and also often more fluoride. The low sulfate content of Type C waters is almost certainly responsible for the increase in observed barium as BaSO4 is a common irreversible precipitate.

PRB waters are associated with light oils and oil condensates. These produced waters are universally more saline than either type of Laramie Basin water. The roughly one order of magnitude increase in their salinity mostly results from chloride and sodium, but extends over almost all analytes. This makes the PRB waters, in terms of proportion, intermediate between the Type S and Type C waters, with limited exceptions. Of particular note are the bromide concentrations which are anomalously high in the PRB samples.

This work uses stable isotope data and shows that deeper Tensleep waters match Type S, shallower Sundance and Muddy waters match the Type C, and allows detection of a Muddy well that was re-completed in the Tensleep.