Natural Oil Seeps in South Eastern Alaska - Slides from Field Studies in 1993, 1994 and 1999

David S. Page, Bowdoin College Chemistry Department

These slides are from field surveys we made in 1993, 1994 and 1999 to natural oil seeps in the Gulf Coast region of Alaska east of Prince William Sound (PWS) and at locations on the Alaska Peninsula. The oil seeps and eroding oil source rocks east of Prince William Sound contribute natural petroleum hydrocarbons to areas within the Exxon Valdez oil spill zone.

This work originally came about because of claims by government scientists that petroleum hydrocarbons in seafloor sediments of Prince William Sound (PWS) came from the spill. These petroleum hydrocarbons were shown in 1993 to be related to oil seep areas east of PWS and not to the oil spill and that inputs of the natural background predated the 1989 oil spill. Recently, further claims followed that coal, not oil formed this natural hydrocarbon background in PWS and that our conclusions were wrong. This required additional field and lab work in 1999 to see if these claims were true. The 1993-1999 oil seep field programs collected samples of seep oil and shale to fully understand the sources of hydrocarbons that form the natural baseline in the region affected by the oil spill. Through hydrocarbon fingerprinting of these samples, mathematical least-squares analysis of the results and chemical mass balance constraints, we found that that eroding Tertiary petroleum source rocks and residues of associated seep oils are the dominant sources of the hydrocarbon background rather than area coals or residues from the Exxon Valdez oil spill. Mass balance constraints relate to the fact that coal is mostly carbon, with relatively small amounts of hydrocarbons relative to petroleum. PWS seafloor sediments have low total organic carbon contents and high hydrocarbon contents, consistent with petroleum, not coal. Results of this work indicate that recent claims by government scientists that area coals are the major source of the background are wrong.

alaskan currentsWe found that hydrocarbons from natural oil seep areas east of PWS are the major source of hydrocarbons in subtidal sediments in Prince William Sound. In fact, oil seeps and eroding oil source rocks form the natural hydrocarbon background in many pristine areas in Alaska and have to be taken into account when doing oil spill studies in this region.The Alaskan Peninsula oil seeps shown here are Barbara Creek, Oil Creek and Well Creek. They are all associated with productive salmon streams monitored for numbers of salmon by the State of Alaska. Some of the streams were visited are among the most productive salmon streams in their area. The oil seeps east of PWS shown here are Munday Creek, and Johnston Creek. Seep oil associated with fine sediment and eroded oil source rocks from these and other streams and rivers in this region are transported to PWS by the Alaska Coastal Current. The map to the right shows the general locations of the seep areas shown in these slides.

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