borhood (200 picocuries per liter or more) were associated with the uraniumrich sheared gneiss. Studies of such radon "hotspots" can permit the USGS to determine where radon "hotspots" may occur elsewhere, thereby providing State and local agencies with much-needed information in dealing with radon hazards. The association of radon with sheared rocks has led the USGS to investigate other areas with sheared rocks in the Eastern and Western United States.

A basic tool for mapping the geologic radon potential of an area is the aeroradiometric data gathered by the Department of Energy in the late 1970's. The data have been used to map the apparent (or equivalent) uranium or radium content of the soils and rocks near the Earth's surface across the United States. Such maps describe the probable strength of radon sources in the ground and will provide a guide to areas

where elevated indoor radon levels are most likely to occur. Maps of Ohio, New Mexico, and Nevada have been published (as USGS Geophysical Investigations Maps GP-968, GP-979, and GP-982, respectively). An equivalent-radium map of the entire United States will be completed in 1989.

The USGS combined several approaches to produce two maps in May 1988 that show the radon potential of rocks and soils in Montgomery County, Md., and Fairfax County, Va. These maps show the correlation between geology and indoor radon and show how existing geologic and soils information could be used alone or with rapidly gathered field data to assess radon potential. The Montgomery County map, based principally on geology and field measurements of radon in soil gas and of surface radioactivity, subdivided the county into three categories of radon potential (low,

Figure 6. Map showing the radon potential of part of Montgomery County, Md., northwest of Washington, D.C. The mapped areas are ranked 1 to 3 with "1" having lowest radon potential and "3" having high radon potential; the letters refer to the rock type underlying the surface. For example, 2m indicates mafic or ultramafic rocks having a moderate potential for elevated radon levels; 2s indicates schist having moderate radon potential. Indoor radon concentrations, in picocuries per liter of air, are shown by triangles: open triangle, less than or equal to 4; half-filled triangle, greater than 4 but less than 20; solid triangle, greater than 20.

moderate, and high). The Fairfax County map, based primarily on geology, aeroradioactivity, and soil permeability mapping, subdivided the county into five radon potential categories. A part of the radon potential map for Montgomery County is shown in figure 6.

Geologic investigations may also contribute to understanding seasonal variations in indoor radon. Long-term study of radon in soils at the Denver Federal Center by USGS scientists has shown that the radon content of soil gas at a depth of 3 feet decreases by an order of magnitude from late winter to late summer, as soil moisture and related factors decrease. The apparent periodic variations in the geologic potential of radon may explain, in part, why there are seasonal variations in indoor radon.

Ongoing investigations by the USGS are focusing on how gas migrates through soils and rocks, the relation between surface radioactivity and radon in soil gas, methods to evaluate the radon potential of undeveloped ground, and the correlation of large indoor radon data sets with the regional geology.

Update on Estimates of Undiscovered Oil and Gas Resources in the United States

By D.L. Gautier, R.F. Mast, and G.L. Dolton

As part of the continuing effort to provide up-to-date estimates of the resources of oil and gas potentially available to the Nation, the U.S. Geological Survey, in cooperation with its sister Interior Department bureau, the Minerals Management Service, developed new methodology and completed a new assessment of undiscovered recoverable petroleum and natural gas resources for the United States as of January 1, 1987. Such estimates are an essential part of the foundation on which the Nation can develop a sound energy policy and can

make critical decisions on public land use and resource management.

In estimating the amounts of oil and gas available for immediate and future development and use, scientists use two broad categories: reserves and resources. Reserves include the amounts of oil and gas that have been measured or closely estimated through engineering and geophysical techniques; resources include the more speculative and long-range estimates of what volumes of oil or gas can be expected to be found based upon estimates of geologic parameters such as the likelihood of oil-trapping structures that have not actually been tested by drilling or other hard, measuring methods.

The commodities included in the assessment were crude oil, natural gas, and natural gas liquids. This assessment was divided into "undiscovered recoverable resources," those resources that exist in conventional geologic reservoirs and that could be extracted using current technology, and "undiscovered economically recoverable resources," those that could be developed and produced under the price-cost relationships, excluding exploration costs, prevailing at the time of the assessment. Specifically not addressed in the assessment were resources from very heavy oil and tar deposits, oil shales, gas from lowpermeability "tight" sandstone reservoirs, gas from fractured shale reservoirs, coalbed methane, gas in geopressured shales and brines, and gas hydrates.

For this most recent assessment, which follows a long history of petroleum resource assessments dating back to the early part of this century, the USGS was responsible for estimating undiscovered resources onshore and in State offshore areas. MMS was responsible for estimating undiscovered resources beneath Federal offshore waters.

This assessment considered new geologic, technologic, and economic information and used more definitive methods of resource appraisal than previous assessments. As part of an effort to ensure the widest possible evaluation of the newly developed methodology, the geologic basis of this current assessment has undergone review by the Association of American State Geologists and the overall methodology and general assumptions

United States oil and gas reserves and resources

are undergoing review by the National Academy of Sciences.

The scientists used an oil and gas "play analysis" approach for the new assessment. For this approach, the United States was divided into 13 regions (9 onshore and 4 offshore). The 13 regions were further divided into 115 provinces. Within each onshore province, some of which extended out into the State offshore areas, the scientists identified "plays" as a group of geologically related known accumulations or undiscovered accumulations, and (or) prospects having similar hydrocarbon sources, reservoirs, and traps. Resource estimates for those undiscovered accumulations greater than 1 million barrels of oil or 6 billion cubic feet of natural gas were made by analysis of the petroleum geology and exploration history of oil and gas plays in each province. This included analysis of large computer data bases containing exploratory drilling data and field-size data for known fields. A statistical extrapolation technique was used to determine recoverable and economically recoverable resource estimates for onshore and State offshore undiscovered accumulations containing less than 1 million barrels of oil or 6 billion cubic feet of gas. Because this new methodology incorporated expected field size and depth ranges, economically recoverable resources could, for the first time, be calculated based on specific economic assumptions that excluded exploration costs.

The undiscovered recoverable conventional resources for the entire United States are estimated to range from 33 to 70 billion barrels of oil and from 307 to 507 trillion cubic feet of natural gas. Corresponding estimates for natural gas liquids in this category range from 6 to 12 billion barrels.

For the category of economically recoverable resources, estimates range from 21 to 54 billion barrels of oil and from 208 to 326 trillion cubic feet of natural gas. Corresponding estimates for natural gas liquids range from 5 to 8 billion barrels.

Looking at the reserve picture, measured reserves at the time of the assessment were 29.5 billion barrels of oil and 206.6 trillion cubic feet of gas. It was further estimated that an additional 21.7 billion barrels of oil and 98.8 trillion cubic feet of natural gas will be added to reserves through expansion of known fields.

The 13 regions of the United States used for the new estimate; estimates of undiscovered recoverable oil and gas resources are shown for each region.

TOTAL UNITED STATES

49 136 63.0-156.9

01-05 <0.1

Undiscovered recoverable conventional oil 332-699 billion barrels Undiscovered recoverable conventional gas 306.8-507.2 trillion cubic feet

0.1-23 6.8-33.7

The results from the current assessment differ from those of previous assessments. Although certain regions, such as the offshore Gulf Coast, show increased undiscovered resources in the current assessment, the overall numbers are reduced from the 1980 estimate. This change is due to such factors as the incorporation of five additional years of drilling information and new sources of field data, as well as the use of the more specific play analysis approach. The play approach allowed for more specific use of existing geologic data and more direct application of computerized data from 1.8 million oil and gas exploration and development wells than was previously

Pacific Coast

Alaska

Colorado Plateau and Basin and Range

Rocky Mountains and No. Great Plains

EXPLANATION

Undiscovered recoverable resource (mean estimate)

Known reserves

Cumulative production

possible. In addition, new sources of data on oil and gas field size were used in this

assessment.

The change in the resource estimates also results from discoveries of oil and gas since 1980, which shifted some of the undiscovered resources from previous assessments into reserve categories. While assumptions about future prices of commodities and technology changes also influenced the results, the overall decline in estimates of undiscovered resources is primarily due to reevaluations that reflect new geological data from exploration drilling in several major regions that were expected to have significant petroleum potential. The new data do not substantiate that expectation. For example, major discoveries that had been expected in frontier exploration of offshore areas off Alaska and the Atlantic and onshore in the overthrust belt of the Western United States did not occur. Moreover, most other onshore regions did not meet the resource occurrence expectations postulated in earlier assessments, although estimates increased in a few areas, such as Alaska. The massive exploration efforts of the industry in the late 1970's and early 1980's simply did not confirm some of the geological expectations for large new accumulations of oil and gas that were included in earlier estimates. Furthermore, most onshore plays showed a decline in sizes of fields discovered over the history of the play.

From the results of this assessment, it is clear that the most prospective areas remaining in the United States for undiscovered oil and gas resources are onshore northern Alaska and the onshore and offshore Gulf Coast (fig. 7). Other important potential resource areas include the Rocky Mountain region, the onshore and offshore Pacific Coast, the mid-continent region, and offshore Alaska.

Because of the critical importance of timely and reliable estimates of oil and gas resources, such assessments will be an ongoing effort of the USGS and other agencies and will be produced on a regular basis, in order to ensure the availability of this information in planning for the Nation's future energy security.

60

Research Advances in the Identification of Disseminated Gold Deposits

By Michael P. Foose

Much of the recent increase in domestic metallic minerals exploration has involved the search for disseminated gold deposits in the Western United States. These deposits consist of extremely finegrained gold dispersed in sedimentary or volcanic rock. Although deposit grades are generally very low (typically between 0.05 and 0.3 ounces of gold per ton of ore), bulk mining and inexpensive oretreatment techniques make them very attractive exploration targets.

Work by the U.S. Geological Survey in the early 1960's was instrumental in identifying and defining these previously unrecognized deposits. Subsequent USGS research has continued to add critical geological, geochemical, and geophysical information needed to understand and explore for these deposits. Important advances in the last year include development of new geologic models, exploration criteria, and geophysical detection techniques.

The development of an ore-deposit model that can be successfully used in mineral exploration requires an understanding of the processes by which the deposit forms. A recently completed cooperative study by the USGS and Freeport Gold Co. on the disseminated gold deposit at Jerritt Canyon, Nev., integrated the results of several different specialized studies to show the following conclusions:

Gold is associated with the introduction of silica into the host sedimentary rocks.

The gold-bearing fluids trapped in the rock indicate that gold was transported in compositionally diverse, moderately saline fluids; gold was deposited when these fluids mixed with oxygenrich, less saline waters, and the temperature of deposition was between 200 and 250 °C (390 and 480 °F).

Isotopes of hydrogen, oxygen, and sulfur can be used as chemical fingerprints to further document the role of fluid mixing in gold precipitation.

• The organic material found in the rocks that host these deposits, which has long been an enigma, was not introduced with the gold but may have acted as an agent to precipitate gold.

The conclusion that gold was deposited when compositionally diverse fluids mixed in fault-controlled aquifers provides an important model for understanding the formation of such gold deposits.

The actual task of finding a deposit, however, requires specific exploration guides. Important guides have recently been developed as a result of USGS research on the Pinson and Preble deposits in Nevada. The waters that transported the gold at these deposits flowed along high-angle faults. Where gold was deposited, silica in these waters both locally replaced parts of the enclosing sediments and also formed crosscutting veins. Geologic maps show that the most silica-rich veins occur close to the main fault and the center of the deposit, whereas carbonate-rich veins are more common at the edges of the deposit (fig. 8). Subtle changes in host-rock mineralogy and composition also accompany the changes in vein composition. The association of vein composition and zoning with gold provides an important new tool to specifically locate disseminated gold deposits.

With few exceptions, the disseminated gold deposits in the Western United States have been found in mountainous areas where bedrock is exposed. Many other deposits must be present in the intervening lower areas but are obscured by a thick cover of younger sediment. The USGS is actively developing techniques to identify areas beneath cover that may be targets for exploration by the mining industry. Because the margins of granitic plutons are commonly associated with gold deposits, one of the recent efforts has integrated aeromagnetic techniques with other geophysical and geological mapping methods to delineate buried granitic bodies.

Further work is now being done to identify geophysical properties to distinguish those granitic bodies that are most