A Study of overpressure in the Navarin Basin, Alaska

Abstract

The Navarin basin is a region to the west of Alaska between the Aleutian Islands and Russia. It has been identified as a potential Petroleum prospect, and exploration wells have been drilled under the ocean up to depths of 17,000 feet. The exploration of the basin was started by Russia and the United States with several exploratory wells drilled in the 1980’s. The geology of the region consists of tertiary sedimentary rock deposited during the Eocene age with mudstone and siltstone from Paleogenic deposition. When dealing with such depths, it is expected that the pressure will increase beyond the hydrostatic gradient. Overpressure, when unexpected, can cause blowouts or oil spills as well as danger to the oil production workforce. Herein, the origin of overpressure in this basin is examined using the well log and geological information, and potential mechanisms responsible for generating abnormal pressure are further discussed. In this study, extensive existing well log data are thoroughly examined and organized to facilitate the characterization of overpressure zones in the basin. As a preliminary step, well logs from eight exploratory wells in the Navarin Basin were digitized and organized as the basis of the analysis. Next, overburden pressure is determined for each applicable well in the target area by examining well log and other geological information. Then, a shale discrimination scheme is applied on the log data to differentiate clay-rich formations (that undergo mechanical compaction) from other rock types. Overpressure horizons are identified and examined through velocity, resistivity and other well logging measurements of clay-rich deposits. As such, sonic velocity vs. density and resistivity vs. density cross plots are constructed to identify signatures of different mechanisms of overpressure. Further characterization of the origin of overpressure involves examination of the tectonics, stratigraphy and source rock in order to characterize the pore pressure regime. Finally, pore pressure is calculated using Eaton (1974) and Bowers (1995) method are utilized to calculate pore pressure within the studied wells and degree of confidence in such calculations are examined.