INTEGRATED GEOLOGIC MAPPING, GEOCHRONOLOGY, BOREHOLE GEOPHYSICS, AND GROUND WATER ISOTOPE CHEMISTRY IN THE NASHUA SOUTH QUADRANGLE, NEW HAMPSHIRE AND MASSACHUSETTS, AND APPLICATIONS TO A METHANE-YIELDING WATER WELL IN CRYSTALLINE ROCK

Bedrock geologic mapping of the 7.5-minute Nashua South, NH-MA quadrangle shows that the area is underlain by Silurian metasedimentary rocks of the Merrimack terrane, which largely consist of biotite-plagioclase-quartz granofels and calc-silicate rocks of the Berwick Formation. These rocks were deformed, metamorphosed to the garnet zone, and cut by gabbro, diorite, granodiorite, and granite in the Acadian orogeny. U-Pb zircon geochronology by SHRIMP on two syn-tectonic intrusions indicates a Lower Devonian age of 407 ± 4 Ma for a granodiorite phase of the Ayer Granite, and an Upper Devonian age of 376 ± 4 Ma for the Chelmsford Granite. Mapping, borehole geophysics, and hydrocarbon isotope analyses from an abandoned methane-yielding domestic water well in Tyngsborough, MA reveal a complex history for the origin of the methane.

The well, located in the dated phase of the Ayer Granite, contains abundant xenoliths of Berwick Formation and has many fractures, but no clearly discernible fracture at the bottom of the well. The deepest fracture occurs parallel to the foliation near the bottom of the well (294 m). This fracture occurs at the base of a 10-cm-thick zone of reflective minerals that are interpreted as sulfides from optical televiewer data. Water was not encountered during drilling. Currently the well produces very little water, recharging at approximately only 3 cm per day. The original methane was probably stored in the deepest fracture or an undisclosed fracture at the bottom of the well (305 m), and explosively released during drilling in August 2004. Though carbon isotope data suggest that the original gas was in the range of thermogenic, petroleum gas, low ethane amounts indicate that the flammable gas was most likely of microbial origin, probably by CO₂-reduction, using H₂ from redox reactions with water in the granite. The exact composition and origin of the original gas could not be determined because once the system was opened, microbial oxidizers consumed the remaining methane in the well bore to current levels below detection.