what deep drilling reveals about groundwater salinity in the picacho basin
south-central arizona

Lee Allison noted in his Arizona Geology blog of 20 February 2009 that “Companies have been looking at the Picacho basin between Tucson and Phoenix for a number of years as an attractive place to develop giant caverns in underground salt deposits to store natural gas for use in Arizona during peak demand periods.” (continue reading below)

Picacho Basin Salinity (download 11x17 poster)

Creating caverns in salt deposits involves a two-step process of pumping in fresh water to dissolve the salt and then drawing the resulting saltwater out.  The saltwater is then held in large holding ponds at the surface or re-injected into strata below the salt deposit. Contamination of groundwater is a major concern when the salt water is re-injected back into the ground. That is why the salt water would be re-injected into deeply buried permeable rocks below the salt deposit where the ground water is far too salty to be used as drinking water.

Deep wells drilled in the Picacho Basin indicate water salinities several times that of seawater (see the block diagram to the right).  In 1972, Humble Oil drilled a deep well (Permit 583) about 4 miles east of Eloy in Section 2, Township 8 South, Range 8 East (2-8s-8e) to a total depth of 10,179 ft. Humble reported evaporite deposits, mostly anhydrite, from 1700-8500 ft. Humble tested conglomerates from 8640-8840 ft and recovered salty water with chlorides ranging from 76,000-113,000 ppm, which is 2 to 3 times as salty as sea water.

In 1974, Geothermal Kinetics drilled a well (Permit 622) about 4.5 miles north of Eloy in 8-7s-8e to a total depth of 8024 ft. Geothermal Kinetics reported about 820 ft of salt from 1640-2950 ft.

In 2005, Unocal and El Paso Natural Gas drilled three wells northeast of Eloy to investigate the feasibility of storing natural gas in deep subsurface salt. Unocal drilled the 1-27 Mesa (Permit 923) in 27-7s-8e to a total depth of 4895 ft in February. El Paso drilled the 1-20 State (Permit 926) in 20-7s-8e to a total depth of 3316 ft in August and the 1-11 State (Permit 925) in 11-7s-8e to a total depth of 3170 ft in September. All three stratigraphic wells were plugged. The three wells indicated a sufficient volume of subsurface salt for storing natural gas.

In October 2006, El Paso drilled a third stratigraphic well, the 1-21 AGS (Permit 933) in 21-7s-8e, to a total depth of 8784 ft to collect information about subsurface units below the salt. The 1-21 AGS is temporarily abandoned pending additional analysis.

These deep wells reveal an extensive mass of bedded salt below a depth of about 1600 ft in this part of the Picacho Basin. Analysis of waters drawn from the conglomerate in the Humble well (Permit 583) shows highly saline groundwater below the salt deposits that has a salinity 2 to 3 times the salinity of sea water.


Steve Rauzi
Arizona Geological Survey

Tucson, AZ




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