ORGANIC COMPOUNDS RELATED TO PRODUCED-WATER RELEASES: RESULTS FROM THE OSPER "A" SITE, OSAGE COUNTY, OKLAHOMA

We are investigating the fate and transport of produced water and associated hydrocarbon releases at the Osage-Skiatook Petroleum Environmental Research (OSPER) site “A”, an inactive oilfield within the larger Lester lease .The site is located adjacent to Skiatook Lake, a 4250-hectare potable water reservoir in Osage County, OK. Produced water and associated hydrocarbon releases occurred primarily from 1912 to 1937, when most of the total ~100,000 barrel (bbl) of oil were produced. Oil production was from the Bartlesville sandstone (Pennsylvanian age). Stripper wells in nearby areas currently average ~2.8 bbl/d oil and >30 bbl/d brine. Data from nearby oil wells show that source of the produced water is a Na-Ca-Cl brine, with relatively high concentrations of Mg, NH₄, but low HCO₃ and very low SO₄ and H₂S; the DOC concentrations range from 2 to 7 mg/L, and carboxylic acid anions, BTEX, phenols and PAHs are relatively low. The source oil is paraffinic-naphthenic light crude (API gravity ~35°), containing n-alkanes as the dominant components. Biodegradation of the oil is likely limited by the high salinity of the associated produced water. The asphaltic and weathered oil persisting on the site are from the same source, but have been subjected to various biodegradation stages. Extensive and repeated sampling of 58 wells (1 to 36 m deep) from this site show a plume of high salinity water (2,000-30,000 mg/L TDS), generally at intermediate depths, that extends beyond the visibly impacted areas and intersects Skiatook Lake. Minor amounts of oil were observed only in water samples from a well drilled 0.6 km northwest of the visibly impacted area, but high DOC (up to 500 mg/L), organic acid anions (mainly acetate and propionate, and to a lesser extent oxalate), hydrocarbon gases, and relatively low concentrations of BTEX and other VOCs, were measured in the water samples from the main plume. Reducing conditions in parts of the plume are indicated by relatively higher Fe, Mn and DOC, and lower D.O. concentrations. Results from this site show that in addition to inorganic salts, oil degradation products, including small amounts of BTEX and organic acid anions from produced water and petroleum releases remain in the rocks and water of the impacted area after more than 65 years of natural attenuation.