The Geological Application of Acoustic Images in Homogeneous Clastic Sediments: Examples from the Tertiary Hemlock Formation, Cook Inlet, Alaska

By

M. Johansson (Schlumberger Oilfield Services) and A. Saltmarsh (Forest Oil Corp.)

The Cook Inlet is a major oil and gas province located south-southwest of Anchorage, Alaska. Cook Inlet basin developed in a forearc tectonic setting, it is approximately 200 mi long, 60 mi wide, occupying 12,000 square miles. Evolution of the forearc basin was controlled by plate convergence, that resulted in the migration and accretion of the Chugach terrane. The overall tectonic regime is predominantly compressional, with the maximum stress orientated northwest-southeast. Thirty thousand feet of Tertiary sediments filled the basin, comprising an extremely thick sedimentary wedge developed in response to the interplay of contemporaneous deposition and active tectonics.

The Hemlock Formation is non-marine in origin and comprises a succession of sandstones and conglomerates of Eocene to Oligocene age. The rocks include braided stream deposits, alluvial fans shed from the basin margins and fluvial deposits developed along the basin axis. Sandstone and conglomerates beds often stack to form thick, relatively homogenous sedimentary units, that can exceed 50 ft in thickness. An acoustic imaging tool commonly used in wells drilled with oil-based mud or other highly resistive drilling fluids, has been used in the evaluation of this formation. The thick units of relatively homogenous rock have acoustic properties similar to the drilling fluids, resulting in limited resolution of stratigraphic details. However, dipping surfaces and textural variations can be observed on the images and these features can contribute to building a sedimentological model. Additionally, borehole-stress analysis based on the log results, can provide key information regarding borehole conditions and break-out orientations.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.