new estimates ranged between 200,000 and 2,000,000 tonnes per year, with a “best estimate” of 600,000 tonnes per year. This revised estimate does not imply that seep rates decreased during the intervening years between 1975 and 1985, but rather reflect a difference in approaches to making the global estimates.

Although only a few new seeps have been identified and estimates of known crude-oil deposits throughout the world have not changed greatly from about 300,000 million tonnes, new technologies, particularly remote sensing techniques, have provided better means of natural seep detection and assessment. Studies in parts of the Gulf of Mexico (MacDonald et al., 1993; MacDonald, 1998; Mitchell et al., 1999), using these new technologies, have resulted in an estimated seepage rate for the entire Gulf of Mexico of 140,000 tonnes per year (range of 80,000 to 200,000 tonnes per year). For offshore southern California, a new estimate of rate of oil seepage is 20,000 tonnes per year (range of 2,000 to 35,000 tonnes per year), based on considerations of work by Fischer (1978), Clester et al. (1996), and Hornafius et al. (1999). The rate of oil seepage for offshore Alaska is also estimated, based mainly on a report by Becker and Manen (1988), to be about 400 tonnes per year (range of 200 to 800 tonnes per year). The North American best estimate is 160,000 tonnes per year, with a minimum of 80,000 tonnes per year (50 percent of best estimate), and a maximum of 240,000 tonnes per year (best estimate + 50 percent1).

The new North American estimate of 160,000 tonnes per year is only 40,000 tonnes less than the 1985 global estimate of 200,000 tonnes per year, suggesting that the 1985 value was grossly underestimated. To accommodate the new information now available, the “best estimate” of the global crude oil seepage rate has been revised to 600,000 tonnes per year, reviving an estimate made originally in 1975. The estimated range of 200,000 tonnes per year to 2,000,000 tonnes per year was developed based on the approach applied in NRC (1985). These limits are set by the amount of crude oil seepage estimated for North American waters and the amount of crude oil ultimately available for natural seepage during geologic time.

Extraction of Petroleum

Global production of petroleum (crude oil and natural gas plant liquids) increased by 1.0 million tonnes per day (7.1 million barrels per day) between 1990 and 1999, an average annual rate of growth of 1.0 percent (International Energy Review, 1999; www.eia.doe.gov). Saudi Arabia, the United States, and Russia were the three largest producers of petroleum in 1999. Together, they produced 31.4 percent of the world’s petroleum. Production from Iran and Mexico accounted for an additional 9.7 percent.

Oil and gas operations include discharges from platforms and other offshore facilities in both federal waters and state waters, produced waters, and atmospheric deposition from the volatile organic compounds (VOC) generated from offshore production platforms. Details concerning data sources, methodology, and computations can be found in Appendix D.

Platforms

Volumes of petroleum hydrocarbons introduced into North American waters from accidental discharge on offshore platforms are relatively well known for the U.S. Outer Continental Shelf and Canada, but data from offshore Mexico and state waters in the United States are generally lacking or scattered and incomplete. There are an estimated 4,900 platforms in North American waters. As of 1993, there were 3,182 offshore oil and gas facilities located in non-North American waters (International Association of Oil and Gas Producers, 2000). Thus in the world’s oceans, slightly in excess of 8,000 platforms and offshore facilities provide the source of petroleum hydrocarbon input into marine waters.

In North American waters, the input of petroleum hydrocarbons from platforms is 146 tonnes per year. Platforms in the OCS account for 39 percent of the discharge, while platforms in state waters account for the remaining discharge (61 percent). As these data are based on relatively good data bases (Appendix D) accumulated over the past ten years, the committee’s best estimate of inputs by offshore facilities is 153 tonnes per year (minimum plus 5 percent2). The committee estimated that the maximum input would probably be on the order of 20 percent3 higher than the calculated amount or 175 tonnes per year. Based on the distribution of accidental spills identified in the databases, it was possible to further pinpoint geographically the sources of inputs. The central and western Gulf of Mexico accounted for roughly 90 percent of the total discharge in North American waters.

In marine waters worldwide (including North American waters), the calculated amount of discharge is 290 tonnes per year, a figure computed on the assumption that the vol

1  

The 50 percent factor applied to develop a minimum and maximum estimate is somewhat subjective and reflects the committee’s confidence in the ability to estimate area of slicks, range in slick thickness, and the likelihood that all seeps are accounted for.

2  

Roughly 5 percent of the spills reported in the available databases did not have adequate geographic information to place them in any region with confidence. The 5 percent factor applied to develop the best estimate reflects the committee’s confidence in the reporting of spills, the completeness of available databases, and a recognition that 97 percent of the total spill volume captured by these databases comes from spills that exceed 100 gallons. The likelihood that a spill much larger than that will go unobserved is, in the committee’s opinion, rather small.

3  

The 20 percent factor applied to develop an maximum estimate is somewhat subjective and reflects the committee’s confidence in the reporting of spills, the completeness of available databases, and a recognition that 97 percent of the total spill volume captured by these databases comes from spills that exceed 100 gallons. The likelihood that a spill much larger than that will go unobserved is, in the committee’s opinion, rather small.



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