Sediments in predominantly sand-shale basins have abnormally high salinity near faults and in the edgewater surrounding hydrocarbons. Anomalies can be seen readily when countywide maps are prepared on brine concentration at the top of the hydrocarbon-bearing zone. Faulting tends to yield a linear zonation of strong brines, whereas hydrocarbon anomalies appear similar to structural contours. About 80 percent of Gulf Coast fields have anomalous salinity waters in edge wells which are no more than 2,000 m from commercial production. Productive wells have redox potential which reduces SP and prevents accurate analysis.

Water composition is just one of the major variables used to evaluate a subsurface system. When combined with pressure gradients, temperature gradients, and redox-potential data, the hydrocarbon environment may be located within 2,000 m and classified with a rough production-probability index. Temperature gradients are influenced strongly by faulting and geopressure, whereas pressure anomalies correlate with the occurrence of gas and condensate production. For example, some counties in the Wilcox trend have most productive fields located over geopressure anomalies. The brine anomalies are on the edge of the abnormal pressure-gradient anomaly, which is opposite to the source of migration.

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