The report includes the results of some chemical tests on a known gas seep from the lower Cretaceous Herendeen formation, on the north side of the peninsula between Port Moller and Herendeen Bay. Because the gas was known to consist of nearly pure methane people have tended to assume that it was biogenic gas, formed by bacterial action in coal beds or organic-laden sediments.

However, the new tests show carbon and deuterium isotope values that indicate that the gas is thermogenic and was formed by the heating of organic material deep underground.

This result points to a gas source in the Mesozoic, rather than in the younger Tertiary strata, Division of Oil and Gas petroleum geologist Paul Decker told Petroleum News. Mesozoic rocks source oil seeps near Puale Bay on the southeastern side of the peninsula and equivalent strata source the oil fields of the Cook Inlet.

There’s long been speculation about how far the Mesozoic source rocks extend under the north side of the peninsula and the Bristol Bay.

The report contains a wealth of new information about the geology of the Alaska Peninsula where the state is holding an areawide lease sale on Oct. 26. It is available at www.dggs.dnr.state.ak.us/pubs/pubs?reqtype=citation&ID=7190.