ABSTRACT

A portion of the oil now present in the world's oceans results from the direct discharge of submarine seeps into the water. Proper assessment of the present "hydrocarbon load" carried by the oceans, that is, the proportion contributed by natural sources and by man, requires a reliable estimate of the yearly worldwide seepage rate from marine seeps. Although only a very few marine oil seeps are presently known and located, all evidence suggests that with further investigation, many more will be found. In view of the limited observations, only a few cursory estimates of total worldwide annual marine seepage have been made; there appears to be no substantiation for any of these estimates. This paper, which represents a first documented approach based on geologic considerations, presents an estimate of yearly oil input into the oceans from naturally occurring marine seepage.

A geologic model based on structural styles, recent earthquake activity, and sediment thickness was used to classify the continental margins into areas of high, moderate, and low seepage potential. Flow rate data for each of the three type potential areas were fitted to a log-normal probability distribution, and seepage volumes were determined.

The probable range of seepage into the marine environment is 0.2 to 6.0 million metric tons per year. Within this range the best estimate for present marine seepage worldwide is 0.6 million metric ton per year. On the basis of this estimate, areas of high seepage potential contribute about 45 percent of the worldwide seepage. In the Pacific Ocean, high seep potential areas are by far the major contributors. In the Atlantic and Indian oceans, moderate seep potential areas are most significant. The circum-Pacific area is the area of greatest seepage; it is estimated to contribute about 40 percent of the world's total.