Migrated hydrocarbons in exposure of Maastrichtian nonmarine strata near Saddle Mountain, lower Cook Inlet, Alaska

Magoon and others (1980) described an 83-meter- (272-foot-) thick succession of Maastrichtian (Upper Cretaceous)
conglomerate, sandstone, mudstone, and coal exposed on the south side of an unnamed drainage, approximately 3 kilometers
(1.8 miles) east of Saddle Mountain in lower Cook Inlet (figs. 1 and 2). The initial significance of this exposure was that
it was the first reported occurrence of nonmarine rocks of this age in outcrop in lower Cook Inlet, which helped constrain
the Late Cretaceous paleogeography of the area and provided important information on the composition of latest Mesozoic
sandstones in the basin. The Saddle Mountain section is thought to be an outcrop analog for Upper Cretaceous nonmarine
strata penetrated in the OCS Y-0097 #1 (Raven) well, located approximately 40 kilometers (25 miles) to the south–southeast
in Federal waters (fig. 1). Atlantic Richfield Company (ARCO) drilled the Raven well in 1980 and encountered oil-stained
rocks and moveable liquid hydrocarbons between the depths of 1,760 and 3,700 feet. Completion reports on file with the
Bureau of Ocean Energy Management (BOEM; formerly Bureau of Ocean Energy Management, Regulation and Enforcement,
and prior to 2010, U.S. Minerals Management Service) either show flow rates of zero or do not mention flow rates. A
fluid analysis report on file with BOEM suggests that a wireline tool sampled some oil beneath a 2,010-foot diesel cushion
during the fl ow test of the 3,145–3,175 foot interval, but the recorded fl ow rate was still zero (Kirk Sherwood, written
commun., January 9, 2012). Further delineation and evaluation of the apparent accumulation was never performed and the
well was plugged and abandoned.

As part of a 5-year comprehensive evaluation of the geology and petroleum systems of the Cook Inlet forearc basin, the
Alaska Division of Geological & Geophysical Surveys obtained a research permit from the National Park Service to access
the relatively poorly understood ‘Saddle Mountain exposure’ that is located in the Lake Clark National Park and Preserve.
This work was done in cooperation with the Alaska Division of Oil & Gas and U.S. Geological Survey (USGS) research
geologists. This report expands on Magoon and others’ (1980) description of the exposure, presents new data on sandstone
composition and reservoir quality, presents new geochemical data on petroleum extracted from the outcropping sandstone,
and describes oil-bearing correlative strata penetrated by the Raven well. Although the exposure is more than a kilometer
(0.6 mile) east of Saddle Mountain (fig. 2), in this report we variously refer to it as the Saddle Mountain succession, Saddle
Mountain section, or the rocks at Saddle Mountain underlain by Upper Jurassic strata of the Naknek Formation.