A summary of hydrologic testing in an offshore oil-test well (LB427) drilled for Tenneco, Inc., 55 miles east of Fernandina Beach, Florida, is presented. The interval tested (1,050 to 1,070 feet below sea level) is in a calcarenite that is equivalent to the Ocala Limestone (late Eocene) of onshore Florida and South Georgia. At this site the Ocala forms the highly productive Tertiary limestone aquifer system of the southeastern United States. Pressure-head measurements indicate an equivalent freshwater head of 24 to 29 feet above sea level. These pressure-head measurements and an earlier one made in the nearby JOIDES J- I hole are the only hydraulic head determinations to date in the offshore extensions of any of the aquifers underlying the Atlantic coastal plain.
A drill-stem test recovered water samples containing about 7,000 milligrams per liter chloride. However, seawater used in the drilling process apparently contaminated the samples and the formation water is considered slightly fresher.
The head and salinity data from the Tenneco well suggest that the sampled interval lies in the transition zone between fresh and seawater in the limestone aquifer. These data, when viewed with similar data from JOIDES J-I, show the transition zone to slope very slightly landward. The interface position is probably intermediate between a position compatible with present-day heads and a position compatible with predevelopment heads.