ABSTRACT

Apatite and zircon mineral separates were extracted from cores from near the bottom of two geopressured-geothermal wells in Vermilion Parish, southern Louisiana and dated by the fission track method. Samples were taken in the sandstone units of the Oligocene age Frio Formation. The purpose of the study was to determine if fission track clocks had been affected by long term heating within the zone. Downhole temperature measurements indicate that the samples are currently at 277°F (136°C) and 338° (169°C). Fission track clocks such as apatite and zircon lose their tracks when subjected to temperatures of 212°F (100°C) and 347°F (175°C), respectively, for geologically significant periods of time (one million years).

Results show that apatite clocks were reset to 0 m.y. whereas zircon yielded ages of 62 and 68 m.y. (Cretaceous). If bottom-hole temperatures are reliable, then the data suggest the following:

1) Zircon ages are relict, reflecting times of cooling of the volcanic, plutonic, or metamorphic source. The Frio Formation in southern Louisiana was at least in part derived from a Cretaceous or older source. Such cooling ages are common in the Ouachitas, southern Appalachians, and the Gulf Coast Plain.

2) Reset apatite and relict zircon ages suggest that temperatures within the geopressured zone have probably not been any higher than they are today <347°F (175°C).