geothermal resource potential of the safford-san simon basin, arizona

of the Buena Vista geothermal system.
Drillers' logs of area wells deeper
than 60 m show the lithology of the shallow geothermal reservoir at Buena
Vista.
Figure 11 is a fence diagram constructed from geologic interpretation
of drillers' logs listed in Appendix 1.
Well D-7-27-llBBB is in mostly sandy
clay, clay and sand to 105 m depth.
These deposits are correlated with the
delta facies of Harbour (1966), which intertongues with the green clay facies
three miles west of Buena Vista in outcrops north of the Gila River (Harbour,
1966). The mostly fine-grained delta facies provides an aquiclude over permeable
gravels that contain confined thermal water.
East of well D-7-27-llBBB,
in wells D-7-27-2DBB and D-7-27-2AAA, the delta facies is thinned by 50 m;
and it is more coarse grained and is interbedded with 3 to 6 m thick gravel
tongues or lenses.
The delta facies is not present in wells D-7-27-lBBA,
D-7-27-2AAD and D-6-27-36CCC;
instead, these wells penetrate permeable gravel
and "caliche".
The absence of the delta facies is probably caused by pinchout
and not by faulting because, north and south of the lithology change in wells
D-6-27-35CBB and D-7-27-2CDB, bedrock depths are interpreted from drillers'
logs at approximately 213 m.
The shallow geothermal reservoir at Buena Vista
is in permeable gravel below the delta facies.
Top of the reservoir ranges
from 105 m depth at the Sanchez Monument to less than 46 m depth 1.4 miles
northeast of Sanchez Monument (Figure 12). The base of the shallow reservoir
is 213 m depth.
The permeable gravels comprising the reservoir are tentatively
correlated with the lower basin fill, red facies or basal conglomerate
facies, of Harbour (1966).
Thermal water contained in the shallow reservoir may leak upward along
buried structure from a deep reservoir.
Figure 12 is a reconnaissance geologic
map of the area compiled from available published maps and from aerial
U-2 photos and field reconnaissance. A major fault, the Butte fault, crosses
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