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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 52 (2002), Pages 207-216

Effect of Pleistocene Depositional Heterogeneity on Movement of a Subsurface Crude-Oil Spill, San Patricio County, South Texas

Dutton, Alan R., Hovorka, Susan D., Bennett, Philip C.,

ABSTRACT

Crude oil discharging into a coastal creek at a South Texas abandoned oil field was tracked through Pleistocene (Lissie Formation) fluvial channel deposits using a soil-vapor survey and analyses of UV fluorescence and total petroleum hydrocarbons (TPH) in core. The source of the subsurface oil spill was an uncapped gathering line. The Pleistocene deposits include northeast-oriented channel sandstone bodies (150 times.gif (834 bytes)10 ft), clayey abandoned channel fill, and overbank deposits. Associated floodplain facies are characterized by clay beds and thin crevasse splay sands. A fairly continuous, 1-to 7-ft-thick silty clay bed records channel abandonment and separates an upper from two lower stacked channel sands.

Oil in the creek bed is seeping from crevasse splay deposits oblique and adjacent to the lower channel sands. The abandoned channel clays trap volatile petroleum vapors, making the plume easy to map by soilvapor survey. A map of C1-C8 hydrocarbon vapors shows that oil is more degraded along the flanks than on the axis of the plume. Distribution of oil shows, reduction-discolored sediments, and gas chromatogram "fingerprints" suggest that fluctuation in the water table sweeps the oil vertically between stratigraphic units and laterally within the channel sand. Degraded oil in upper channel-sand and floodplain deposits records the position of the plume during an intermittently high water table. Anaerobic biodegradation of the oil and of organics dissolved in the groundwater reduces the oxidation potential of groundwater, which is reflected in discolored sediments.


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