Natural steam production is found in reservoir rock overlain by impervious cap rock. These rocks may be volcanic, sedimentary, metamorphic, or plutonic. The age and composition of these rocks are not important but only Pliocene and Recent magmas are known as an underlying heat source. Reservoir rocks usually are highly fractured and rendered porous by solutions, must be of sufficient thickness (minimum 100 meters), and of an areal extent to sustain continuous production. Porosity values should be at least 10 per cent.

Man has utilized natural steam and hot waters throughout historic time. There are many present-day uses for natural heat, such as wood-pulp manufacture, evaporation of salt, space heating, and greenhouse farming. The first electricity made from natural steam was in 1905 at Larderello, Italy. Iceland, New Zealand, and Japan also were early users of geothermal energy. The Geysers in California is the first and only geothermal field generating electricity in the United States. Pacific Gas and Electric installed the first 12,000-kwh. turbine-generator in 1960, a 14,000-kwh. unit in 1963, and in 1966 installed a 27,500-kwh. unit for a total of 53,500-kwh. capacity. A fourth unit is planned in 1968 bringing the total to 81,000 kwh. The rapid industrialization and population growth of the we tern states, as well as the world, has led to increased demands for energy. The U.S. Bureau of Mines Circular 8230 states that, if the growth in electric energy consumption occurs as projected, construction of considerably more thermal-generating plants will be required. Natural steam probably is the cheapest source of energy.

Potential geothermal resources of steam are evacuated by five fundamental criteria: (1) source of heat; (2) regional and local structural features; (3) source of meteoric waters (to infiltrate and circulate to depth); (4) reservoir rock sufficient to retain geothermal fluids in volume; and (5) impervious cap rock overlying reservoir for trapping steam.

Facca and ten Dam recommend exploration in four stages:

1. Preliminary survey and selection of area (includes geological, geophysical, and geothermal information, stratigraphic studies, and porosity and permeability data).

2. Detailed survey of selected area (includes photogeology, volcanological and petrographic studies, hydrogeochemistry, and hydrogeology and gradient surveys).

3. Test drilling.

4. Probing and evaluating (includes output measurements and production tests).

Stratigraphic data of three producing geothermal fields for comparison are given.

Larderello field, Tuscany, Italy, discussed by Facca (1963). He described the sequence and conditions as follows: (1) basement rock, quartzitic and anagenetic; (2) reservoir rock, evaporitic dolomite with high porosity and permeability, Late Triassic; (3) cap rock, flysch-like, shaly, impervious argille scagliose, Late Cretaceous-early Miocene age; (4) no Recent volcanoes; (5) heat flow provided by a deep-lying magma; (6) temperature of producing zone is 200°C. (392°F.); and (7) surface hydrothermal activity occurs as hot springs and steam jets of 100° to 190°C. (212° to 374°F.).

Wairakei field, North Island, New Zealand, was described by W. G. Grindley (1961) as a volcanic region 240 kilometers long, a Recent graben filled exclusively by volcanites. Two volcanoes are active at each end of the graben. The impervious Huka Formation is the controlling factor in heat accumulation. The producing

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aquifer is the Waiora Formation, a porous pumice breccia with buried flows of andesite in the lower part. Underlying the reservoir is the Wairakei Ignimbrite Formation, consisting of impervious dense ignimbrite sheets.

The Geysers, California, was described by McNitt (1963) as having Pleistocene and Pliocene volcanic vents, with rhyolite flows and tuffs, basaltic, dacitic, and andesitic lavas in outcrop in the hydrothermal area. The stratigraphic sequence consists of (1) Cretaceous, massive, yellow-brown sandstone and gray shale 1,700-3,000 meters thick; (2) Knoxville Formation (Upper Jurassic), thin-bedded graywacke and shale, intruded by 150-300 meters of serpentine sills, thickness 1,700 meters; and (3) Franciscan Group (Upper Jurassic), interbedded graywacke and spilitic basalt, and small amounts of shale, conglomerate, and chert; intruded by serpentine sills; thickness, 9,000 meters. The Mesozoic rocks were folded gently, then intensely faulted. Surface thermal activity consists of hot springs, fumaroles, and hydrothermal rock alterations. The producing zone is a sandstone in the Franciscan, shattered severely by faulting, and altered by hydrothermal activity. The sandstone is dense, indurated, with some interbedded shale. Fractures to 0.5 inch wide have been cored. The BHT average is 208°C. (401°F.), pressure is 60 to 150 psi.

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