Council (NRC, 1975; NRC, 1985) and new information on the natural seepage of crude oil into the marine environment (detailed in Appendix C). Not considered in the new or previous studies is the contribution to the marine environment of hydrothermally-derived petroleum at oceanic spreading axes (see summary by Kvenvolden and Simoneit, 1990). Although it is clear that hydrothermal petroleum occurs in the sea, the rates of seepage are unknown, but are believed to be very small, and therefore are not included in the new global assessment.

In 1975, the estimated worldwide rate of natural seepage of oil into the marine environment ranged widely from 200,000 to 6,000,000 tonnes per year, with a “best estimate” of 600,000 tonnes per year. These rates were based on a comprehensive global survey incorporating extensive geological considerations, but used extrapolations from only a few known seeps. In 1985, little new information had become available, and estimates of individual oil-seep rates had not changed significantly. Thus a revised estimate of the global seepage rate was calculated based on assumptions concerning the amount of crude oil known to be present that could seep over reasonable periods of geologic time. This theoretical approach was first developed by Kvenvolden and Harbaugh (1983) and incorporated into NRC (1985). The