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ABSTRACT: Sr Isotopic Composition of Oil-Field Brines in the Wilcox Group of Eastern Central Louisiana: Identification of Sources of Saline Contamination

HUFF, G. F. and J. S. HANOR

Saline water has been found locally in shallow otherwise freshwater aquifers in eastern-central Louisiana. Effective remediation of these locally-saline areas requires determining whether salinity originates from natural upwelling of saline waters from moderate depth or from introduction of oil-field brines from plugged and abandoned oil wells. To aid in determining sources of saline contamination, we have made a study of oil-field brines from the Wilcox Group, the principal hydrocarbon-bearing unit in eastern central Louisiana.

Regional ground-water flow patterns and spatial variations in brine chemistry are consistent with the hypothesis that Wilcox brines acquired salinity by subsurface dissolution of salt along the northern margin of the Gulf Coast Salt Dome Basin and in the Mississippi Salt Dome Basin. The 87Sr/86Sr ratio of Wilcox brines increases systematically with depth in eastern-central Louisiana. This trend may reflect mixing of more radiogenic brines derived from dissolution of Gulf Coast salt domes with brine derived from dissolution of salt in the Mississippi Salt Dome Basin, whose evaporites have a characteristic mid-Jurassic signature.

Where oil-field brine contamination is suspected in the shallow subsurface, the87Sr/86Sr ratios of shallow saline samples could be used to infer the 87Sr/86Sr ratio of a potential parent brine and thereby constrain the depth from which brine contamination could originate. Depth of origin of a parent brine could, in turn, serve as a constraint on potential brine migration pathways, such as plugged and abandoned oil wells.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana