Abstract

Seismic data have been collected since the 1950s in Cook Inlet Basin and have been instrumental in the discovery and production of oil and gas in the basin. This chapter provides an overview of how seismic data have been and could be utilized for further exploration. It covers general aspects of data availability, seismic acquisition, processing, and interpretation analysis, with special focus on the extreme or unusual aspects of the Cook Inlet Basin environment, both logistical and geologic, that impact those processes.

Logistical aspects to consider in Cook Inlet seismic acquisition programs include large tidal variations, extensive intertidal zones, long winters, seasonal ice, and numerous land ownership and permitting requirements. Marine and transition zone acquisition is strongly influenced by the tidal range and currents, the weather, and the presence of ice in the Inlet and at the shoreline. The Cook Inlet has the second-highest tidal range in North America, and currents can be quite strong during flood and ebb stages. At low tides, there are large areas of mud flats at shorelines.

Geologic challenges to obtaining high-quality seismic data in the Cook Inlet Basin include areas of steeply dipping beds and faults, a strong azimuthal velocity dependency, the presence of gas, and complicated stratigraphy. Structural and azimuthal components can be addressed in the design and application of modern acquisition and processing procedures and include ensuring that appropriate offsets, azimuths, fold, and trace intervals are collected. Stratigraphic complexities including both high- and low-impedance contrast geologic boundaries and the presence of gas in the section, can be more challenging to deal with, as they lead to weak signal penetration, especially at the higher frequencies.

The Tertiary section, where almost all the oil and gas production occurs, consists of non-marine, fluvial, and alluvial deposits from multiple sediment sources. Formation boundaries are typically gradational in vertical section and time transgressive and often exhibit weak impedance contrasts. Thick and thin, discontinuous, low-impedance coals are common in the section, which also includes localized high-impedance volcanic and/or conglomeratic layers. Preservation of signal is critical in all aspects of acquisition, processing, and interpretive analysis in capturing stratigraphic detail in this kind of environment.

To date, two-dimensional data have been used for most exploration in this prolific, proven hydrocarbon basin. While these data have been sufficient to identify the structurally trapped hydrocarbons discovered to date, no discoveries have been made in purely stratigraphic traps. More high-quality exploration three-dimensional data are needed to identify subtler structural and stratigraphic traps in the Cook Inlet Basin.