Abstract

Use of gas chromatography (GC) as a tool for characterizing source rocks and understanding hydrocarbon migration and emplacement is well established in petroleum geology. Petroleum geologists experienced in GC and source-rock characterization, and in other concepts such as structural and stratigraphic trapping, reservoir compartmentalization, emplacement theory, sequence stratigraphy, and diagenesis, can establish themselves as key team players in environmental investigations, especially those involving hydrocarbons. Environmental professionals lacking petroleum geology backgrounds can possibly bias field sampling and analysis of hydrocarbons because they may not consider natural hydrocarbon charging at sites. This problem was recognized recently at the Lexington-Bluegrass Army Depot in Kentucky, where the occurrence of hydrocarbons upgradient from underground storage tanks initially was not adequately deciphered.

The nearly 3.25-km² (800-acre) site, situated near the crest of the Cincinnati Arch, is located on the Middle to Late Ordovician Lexington Limestone. This shaly limestone displays epikarst features and contains two contaminated aquifers traditionally subdivided as the Tanglewood Member (upper aquifer) and the Grier Member (lower aquifer). Original remediation plans included pump and treat, based on the belief that hydrocarbons were anthropogenic. Expensive remedial efforts, however, are being avoided because it has been suggested that a significant portion of the site may be “contaminated” by natural sources. Site re-evaluation should consider (1) retention times required to detect natural hydrocarbons versus refined product; (2) regional structural position of the site; (3) sequence stratigraphic concepts; (4) chemical variations in formation waters versus compartmentalization of aquifers; and (5) expected characteristic GC signatures associated with source rocks in adjacent basins.