SMYTH, REBECCA C.
The University of Texas at Austin, Bureau of Economic Geology, Austin, Texas

Abstract: Mn-oxide Concentration as Evidence of a Pathway for Infiltration of Crude Oil into a Shallow Aquifer, West Texas

In November 1991, land owners near Abilene, Texas, found crude oil in their water well. Subsequent drilling (4 cores and 30 borings) defined a plume of crude oil (approximately 300 bbl) floating on shallow, perched ground water. Data suggest that the oil came from a near surface leak associated with oil-production activities. Crude oil is present in a thin (0.5 ft), silty sand layer 17.7 to 19 ft below the surface. Because of water level fluctuation, traces of oil also occur along fractures as deep as 35 ft in two cores collected within the crude-oil plume.

The presence of manganese (Mn) oxide coatings along fracture surfaces might prove to be a record of the path of oil as it infiltrated the subsurface. Mn oxide minerals are concentrated along fracture surfaces to depths of 20 ft in two cores located nearest the suspected crude-oil source. Changes in redox conditions and increased microbial activity associated with the crude oil probably caused dissolution, followed by reprecipitation and concentration of Mn oxides.

Other effects of crude-oil degradation include high unsaturated zone methane concentrations in a halo around the oil plume. Methane was measured in boreholes at concentrations mainly between 5 and 50 percent, but locally as high as 98 percent, at depths of 8 to 10 ft. The methane is most likely a result of both volatilization and biodegradation of the crude oil. Coincident with the methane plume are zones of high carbon dioxide (as much as 10 percent) and low oxygen (as little as 1.9 percent) content.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas