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Selected Occurrences of Overpressures in Alaska*

David E. Powley¹

Search and Discovery Article #60011 (2008)

Posted April 10, 2008

*Compiled from slides prepared by the author for his numerous presentations about the subject of compartments and pressure regimes, for which he was pioneer, advocate, and mentor.

¹Deceased October 29, 2006. Consultant after retirement from Amoco Production Company.

uLocation map

uCook Inlet

uCopper River

uGulf of Alaska

uNorth Slope

uNavarin

uLocation map

uCook Inlet

uCopper River

uGulf of Alaska

uNorth Slope

uNavarin

uLocation map

uCook Inlet

uCopper River

uGulf of Alaska

uNorth Slope

uNavarin

uLocation map

uCook Inlet

uCopper River

uGulf of Alaska

uNorth Slope

uNavarin

Location Map

Figure 1. Sedimentary provinces of Alaska (from St. John et al., 1984).

Cook Inlet Basin

Figure 2. Cook Inlet Basin, Alaska, with location of cross-section in Figure 3.

	Figure 3. Generalized cross-section of Cook Inlet Basin, showing position of overpressures.
	Figure 4. Pressures in the West Foreland, Middle Ground Shoal, Granite Point and Cook Inlet fields.

	Figure 5. Sonic log responses in shale and drilling mud used in a well (Shell West Foreland No. 1, Section 21, T.8N, R.14W.), drilled in west foreland area, Alaska
	Figure 6. Cross-section, Middle Ground Shoal Field, showing oil accumulation in relation to overpressured section. There is a disproportionately large amount of oil and wet gas in sands in close proximity to the regional seal. Local overpressures due to long gas columns have been encountered shallower than the regional pressure seal in a few fields on the western Kenai Peninsula.

Copper River Sedimentary Province

Figure 7. Map of Copper River sedimentary province, where cross-section in Figure 8 is located.

Figure 8. Generalized cross-section, Copper River Basin, showing position of overpressured compartment and mud volcano; fluidized rock material, mainly shale, and high-pressured water with minor hydrocarbons from the overpressured compartment are being ejected and vented at the surface to form a mud volcano. The rising, high-pressured mixture may pressure-up any shallow, permeable beds encountered, thereby locally complicating recognition of the layered arrangement of hydraulic systems.

Figure 9. Schematic pressure-depth profile, showing seals and pressure in mud column feeding mud volcano.

Gulf of Alaska

Figure 10. Location map of Gulf of Alaska for wells with overpressure. Overpressures have been observed in all wells drilled in the Gulf of Alaska. The top of overpressures is from depths of 6000 to 10,700 feet.
Shell No. 1 OCS-Y-0014 Block 111, 59 o53’05”N, 142o53’03”W, Gulf of Alaska, has overpressures seal approximately 7000-7320 feet. DST 7326-7331 feet, BHSIP 5820 psi. Potentiometic surface +5200 feet. (Assumed gradient 46.5 psi/100 feet.)

North Slope Basin

Figure 11.Well log of Exxon No. 1, East Mikkelson Bay, North Slope Basin, showing seal represented by low-resistivity below oil-bearing unit and shale with normal resistivity..

Navarin Basin

	Figure 12. Pressure-depth profile, Amoco Danielle No. 1, Navarin Basin, showing top of overpressure zone at about 8000 feet.
	Figure 13. Pressure-depth profile, Amoco Nancy No. 1, Navarin Basin, showing top of overpressure zone at 7000-7300 feet.
	Figure 14. Pressure-depth profile, Amoco George No. 1, Navarin Basin, showing seal in interval from approximately 7000 to 7800 feet.
	Figure 15. Pressure-depth profile, Amoco Nicole No. 1, Navarin Basin, showing seal in interval from approximately 6800 to 8600 feet.
	Figure 16. Speculative pressure-depth profile, Arco Cost well No. 1, Navarin Basin, from drilling mud densities from well log header.
	Figure 17. Temperature-depth plot, Amoco Danielle No. 1, Navarin Basin, with interpreted overall geothermal gradient of approximately 2.1oF/100 feet.
	Figure 18. Temperature-depth plot, Amoco Nancy No. 1, Navarin Basin, with interpreted overall geothermal gradient of approximately 1.9oF/100 feet.
	Figure 19. Temperature-depth plot, Amoco Nicole No. 1, Navarin Basin, with interpreted overall geothermal gradient of approximately 2.0oF/100 feet.
	Figure 20. Temperature-depth plot, Arco Cost Well No. 1, Navarin Basin, with interpreted overall geothermal gradient of 1.94oF/100 feet.
	Figure 21. Temperature-depth plot, Exxon OCS-Y-0599 No. 1, Navarin Basin, with interpreted overall geothermal gradient of approximately 2.1oF/100 feet.