Preliminary report on the geology of the continental slope adjacent to OCS Lease Sale 55, eastern Gulf of Alaska: Petroleum resource implications

Abstract

Forty samples of Tertiary and Cretaceous outcrop or suspected outcrop were dredged along the 250-km-long northwest-trending continental slope between long 138°00' W. and 142°30' W. These samples, from water depths of 3,150 m to 200 m, combined with seismic reflection data, identify seven rock units in apparently normal stratigraphic position. From oldest to youngest they are: Unit A: Hard graywacke, argillite, and possible intrusive rocks of inferred Late Cretaceous age that crop out in the eastern part of the slope and probably underlie much of the Fairweather Ground. Unit B: At least 900 m of sandstone, conglomerate, shale, and subordinate basaltic volcanic rocks west of Yakutat Seavalley between 142° W. and 143° W. that is not dated but is probably of late Paleocene to early Eocene age. Unit C: 1,300 m of basaltic flow and pyroclastic rocks with associated clastic marine sedimentary rocks that makes up much of the rugged lower slope throughout the area. This sequence is largely, or entirely, of early Eocene age. Unit D: Up to 2,100 m of early and middle Eocene sandstone, conglomerate, siltstone, and tuffaceous organic-rich shale that partly overlies, and partly intertongues with, the volcanic unit. Unit E: 800 m of early to late Eocene and early Oligocene(?) organic-rich shale, tuffaceous shale, siltstone, and sandstone that partly intertongues with, and partly overlies units C and D. Unit F: 300+ m of siltstone, at least in part of late Oligocene age, that crops out locally along the upper slope where it unconformably overlies Unit E. Unit G: Up to 2,000 m of late Cenozoic (middle Miocene? and younger) clastic sedimentary rocks including abundant glaciomarine deposits that unconformably overlie all the older units. The early Tertiary sequence sampled on the continental slope differs strikingly in lithology and structure from coeval rocks exposed onshore or penetrated in unsuccessful exploratory wells on the adjacent mainland. Units B, D, and E include abundant source rocks that have undergone a thermal history resulting in local generation of hydrocarbons. Sandstones from Units D and E locally have secondary porosities up to 31 percent and permeabilities to 36 md. Seismic reflection data indicate that the sequence with the most favorable source and reservoir potential (Units B, D, E) dips northeastward beneath OCS Lease Sale 55 where it could be a favorable exploratory target if involved in suitable traps. Young anticlinal folds are not present, but other important traps are likely to include drape structures, horsts, and faults within the early Tertiary sequence and pinchouts, fault traps, or combination structural/stratigraphic traps along the northeastern margin of the early Tertiary basin.