WELL LOCATION AT SAN PEDRO ESTATES, REGINA, NEW MEXICO, U.S.A.
William M. Turner, Ph.D.
INTRODUCTION
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.
HYDROGEOLOGIC SETTING
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.
PROCEDURE
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 "valley-mapping function."
RESULTS
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.
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