Abstract

The upper Tertiary Sterling Formation of the Kenai Group of Cook Inlet basin, Alaska, contains several trillion cubic feet of gas reserves on the east side of the basin, whereas the underlying Beluga Formation has relatively minor gas reserves in the same area. The difference is due primarily to thick, continuous, high-porosity reservoir sandstones in the Sterling versus thin, discontinuous, low-porosity sandstones in the Beluga.

The Sterling was deposited by moderately large, meandering streams which produced 10- to 15-m-thick point-bar sandstones. These streams flowed south to southeastward parallel to the basin axis in the area of the southern Kenai Peninsula. The Beluga was deposited by smaller, high-gradient, braided streams which produced lenticular, braid-bar sandstones only a few meters thick. These streams built alluvial fans sloping southwestward into the basin.

Beluga sandstones are composed mainly of metasedimentary rock fragments derived from the Kenai-Chugach Mountains east of Cook Inlet basin. These soft rock fragments were squeezed and crushed during burial, destroying nearly all original porosity. In sharp contrast, Sterling sandstones are composed of quartz, plagioclase, and volcanic rock fragments derived from the Aleutian-Alaska Ranges west and north of Cook Inlet. Grains of volcanic glass, trachytic rock fragments, biotite, hornblende, and hypersthene were dissolved during burial to produce secondary porosity on the order of 35 percent. Differences in provenance, depositional environment, and diagenesis all influenced the contrasting reservoir characteristics of Sterling and Beluga sandstones.