Structure And Possible Petroleum Traps Of Lower Cook Inlet, Alaska

Abstract

ABSTRACT The regional structure of lower Cook Inlet is a large northeast-trending syncline that extends the width of the inlet. The syncline, which involves Mesozoic and Tertiary rocks, is divided into two oppositely-plunging synclines at the latitude of Augustine Island by the Augustine Seldovia arch. This arch trends east-west, transverse to the axis of the lower Cook Inlet basin, between Augustine Island and the Kenai Peninsula near Seldovia. A number of anticlines have their axes parallel to the basin axis and intersect the arch at oblique angles. Domical structures could be formed where the two structural trends intersect, providing a possible setting for petroleum entrapment. The northeast-trending anticlines also may be structural traps. Seismic data reveal a number of unconformities in the offshore area. In particular, a regional angular unconformity interpreted to separate Mesozoic and Tertiary rocks extends northeastward along the southeast coast of lower Cook Inlet. Truncated strata beneath this unconformity may contain combined structural and stratigraphic traps. INTRODUCTION Lower Cook Inlet is the southern half of an intermontane basin in south-central Alaska (Fig. 1). On the southeast side of the inlet are the towns of Homer and Seldovia, along the northwest coast are Cape Douglas; Augustine Island, and the Iniskin Peninsula. Two seismic surveys that provide public information were conducted in lower Cook Inlet. In July, 1975, the U. S. Geological Survey contracted with Western Geophysical, Inc. to collect 480 km of common-depth-point (COP) seismic data using a 2,700-m, 72-channel streamer, Aquapulse guns, and DDS-888 digital instruments. The grid of seismic lines is about 30 km square. A combined Raydist/RPS navigation system located shot seismic data was processed by Petty-Ray Geophysical, Inc. Copies of the 36-fold final stack sections and shot point maps are available to the public through: National Geophysical and Solar Terrestrial Data Center, Code D-62l, Boulder, CO, 80302. A second seismic survey was conducted in July 1976 aboard the R/V SEA SOUNDER, a vessel operated by the U. S. Geological Survey. A 30-kiloJOUle sparker system obtained data along track lines that form a grid about 15 km square. Some sparker lines are in the same locations as the CDP lines. Although the sparker data has about one-tenth of the sub bottom penetration of the COP data, axes of structures in Tertiary rocks can be clearly seen. The COP lines are too widely spaced to define the extent of structural axes, but show the regional configuration of deep strata beneath lower Cook Inlet. Three seismic horizons were correlated throughout the grid of COP lines; all three are too deep to be seen on the sparker data. In order of increasing depth, the horizons are labeled A, B, and C. The A-horizon is believed to be at the base of Tertiary strata; the B horizon might be at the base of Upper Cretaceous strata; the C-horizon may be near the base of Upper Jurassic strata.