Nov. 6--Millions of years ago, sand and gravel that washed down from the Sierra Nevada created layers of porous rock that eventually trapped a large reserve of natural gas on the Valley's west side.

Since the Gill Ranch gas field was discovered in the early 1940s, more than a mile below the San Joaquin River and modern-day farmland, it's been tapped for its fuel, and some parts have been drained.

Now California's rising demand for natural gas -- and places to store it for periods of peak use -- are driving plans to use the depleted pockets as a subterranean gas tank.

"We're really taking what Mother Nature has provided and using it for another purpose," said Charlie Stinson, director of project development for Gill Ranch Storage LLC.

Gill Ranch Storage, an Oregon company, and Pacific Gas & Electric Co. won approval last week from the California Public Utilities Commission to develop the old gas fields for storing up to 20 billion cubic feet of natural gas -- enough to supply Fresno County's gas needs for eight or nine months.

The gas field is embedded in sand-rich layers of rock under about 5,000 acres east of Mendota, in Madera and Fresno counties. Stinson said his company hopes to begin construction of a compressor station, pipelines and power lines later this year and start injecting gas underground by sometime next summer.

Construction is expected to cost $200 million to $225 million, according to documents submitted to the Public Utilities Commission. Stinson said Gill Ranch Storage's share of the cost is estimated at between $160 million and $180 million. Gill Ranch Storage will own 75% of the storage capacity and operate the site; PG&E will own 25% of the capacity.

The project includes:

- New injection/withdrawal wells drilled into the depleted gas-bearing layers about 6,000 feet underground.

- A central compressor station and pipelines connecting it to the wells.

- A 27-mile line of 30-inch pipe to deliver natural gas from a PG&E main supply line near Interstate 5 in western Fresno County.

- A 9.5-mile power line for electrical service.

Some old wells that mined gas from the field between the 1950s and 1990s may be re-used as monitoring wells, Stinson said.

At its peak, construction of the project could put as many as 350 people to work. Once it's operational, the facility will have about 10 employees.

Tens of millions of years of geological forces enable the Gill Ranch gas field to be used for gas storage.

The field includes several different geologic layers, some of which continue to produce natural gas.