WELL LOCATION AT SAN PEDRO
NEW MEXICO, U.S.A.
William M. Turner, Ph.D.
We, at American Ground Water Consultants (AGW), were retained to locate water in an
area of New Mexico where no water wells had ever been successful. Figure
1 shows the location of the study area.
We decided on a combination of exploratory methods using down-the-hole geophysical logs
from nearby oil exploration wells, geological reconnaissance, and Thermonics.
The area of exploration comprises about 2,000 acres (810 Ha) situated on the eastern
edge of the San Juan Basin of northwestern New Mexico in the United States. The
eastern edge of the San Juan Basin at Regina, New Mexico is the Nacimiento Fault where
Tertiary marine sediment of the San Juan Basin is in structural juxtaposition against
Precambrian granite of the San Pedro Wilderness to the east. Along the Nacimiento
Fault, interbedded Tertiary sandstone and shale units stand nearly vertical.
Within a short distance west of the Nacimiento Fault, the Tertiary sediments are nearly
flat-lying. They dip at a low angle toward the center of the Tertiary marine basin
50 miles to the west.
Geophysical logs from a nearby oil exploration well indicated a thick sand bed at a
depth of about 1500 feet (450 m). The sand unit had not been described in the
literature and had never been mapped. AGW scientists named it the Cuba Mesa Member
of the Nacimiento Formation. The Cuba Mesa Member is sandwiched between thick
sequences of green unctuous shale of the Nacimiento Formation that contains abundant
bentonitic clay from the major stratovolcanic center of the Jemez Mountains farther east
beyond the Precambrian granite of the San Pedro Wilderness.
We believed that recharge to the Tertiary sandstone units must take place along the
western face of the Precambrian basement where the sandstone units could accept
precipitation over their outcrops.
No water wells existed in the area from which water levels and directions of ground
water flow could be obtained.
We drilled two fences of Thermonic observation holes perpendicular to the assumed
westward direction of ground-water flow. One fence of observation holes was situated
in a north-to-south direction through the middle of the client's land about 1.5 miles (2.4
km) west of the Nacimiento Fault. The second north-to-south line of Thermonic
observation holes was constructed one-half mile (0.8 km) farther west along the western
boundary of the property.
The Thermonic observation holes were 30 feet (10 m) deep. We measured
temperatures in each of these holes. We corrected the data for variable thermal soil
properties and solar heat input from the land surface using our proprietary data
processing methods. The data was plotted and contoured using our proprietary
Our Analysis of the data revealed two zones of rapidly moving ground water along each
fence of Thermonic observation holes. We noticed that lines connecting the zones of
rapid ground-water flow from once fence of holes to the other and extended eastward,
converged to a point east of the client's property. We discovered that the point of
convergence was the point where a surface stream passed over the upturned beds of the Cuba
Mesa Member of the Nacimiento Formation. This established, in a rather forceful
manner, that recharge, in fact, occurred along the mountain front of the San Pedro
Wilderness and that the recharge was particularly strong where streams debouched from the
high Precambrian basement massif and flowed westward across upturned sandstone beds.
We subsequently drilled and constructed a very successful, 2,000-foot water well on one
of the target zones of high ground-water flow.
Drilling conditions were very difficult because of the heaving, bentonite-rich, shale
beds above the Cuba Mesa Member of the Nacimiento Formation. These conditions necessitated
24-hour per day drilling and a highly engineered, low-filtrate loss drilling mud.