Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, March 21, 2003
Since the early 1950s, crude oil has been seeping into the Colorado River, which is adjacent to an oil field in Wharton County, Texas, U. S. A. The volume of oil discharging to the river has changed over time; increases in discharge apparently correlate with increased rainfall. River levels have also changed significantly over time as a result of drought or flood conditions. Multiple seeps occur over 182 m along the east bank of the river. Since 1936, oil-field features in the area immediately adjacent to the east bank have included (1) numerous oil wells, one of which blew out in 1959; (2) six surface tanks that contained oil or saltwater; and (3) as many as four former earthen pits reported to have contained crude oil, saltwater, or drilling mud.
In August 1999 we assessed the possible sources of oil by collecting continuous cores and installing groundwater-monitoring wells at 12 locations. Strata are deltaic clays interspersed with thin channel sand deposits of the Beaumont Formation. The clay is highly fractured most likely as a result of rapid removal of overlying sediments when the river was established in its present day location. During drought conditions the fractures in the clay that has a high shrink-swell potential are most likely open.
The measured plan-view hydraulic gradient is toward the river. We found hydrocarbon impacts (elevated concentrations of ionizable compounds and TPH) in six of the cores as deep as 14 m and free-phase product as thick as 0.4 m in two monitoring wells downgradient of the largest earthen pit and upgradient of the seeps. We hypothesize that oil from the pit migrated downward through fractures in the clay, collected at the interface with the underlying water-bearing sand at a depth of ~15-m, and is moving laterally toward the river.