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 Palynological and Core Data Suggest Middle-Late Turonian Age for the Avak Impact, Barrow, Alaska*

 

Arthur C. Banet¹,  James P. Fenton², Thomas P. Walsh³, Thomas C. Morahan³ and

Peter Stokes³

 

Search and Discovery Article #30099 (2009)

Posted August 26, 2009

 

*Adapted from oral presentation at AAPG Convention, Denver, Colorado, June 7-10, 2009

 

¹Self, Anchorage, AK   (mailto:[email protected])

²Fugro Robertson Limited, Llandudno, United Kingdom

³Petrotechnical Resources Associates, Anchorage, AK

 

Abstract

 

The Avak impact feature in northernmost Alaska is an integral part of the trapping mechanism for the South, East and Sikulik pools of the Barrow Gas field. The pools are at the periphery of the impact rim. The gas production comes from the Lower Jurassic Barrow sands at the base of the Kingak Formation. Logs and cores show that the upper and lower Barrow sands are coarsening upwards shoreface parasequences and that the sand-rich facies are restricted to the Barrow Arch area.

 

Barrow Field peculiarities include shallow burial depths (<2,250 ft.), approximately 55% water saturation, and reservoir temperatures between about 55 and 65 degrees F. Gas geochemistry (Pixler Plots) show that the gas at Barrow is comparatively dry; drier than other North Slope gas pools. Reservoir studies show the East Barrow Field is within the methane hydrate stability field and that hydrates contribute to the gas charge. The East Barrow Field also initially had anomalously high Helium concentrations which may be related to the impact origin from deep seated faulting.

 

Core data from the Avak well, which is at the center of the Avak impact and between the East and South pools, shows uplifted, out of sequence and disrupted stratigraphy, shatter cones and shocked quartz which is typical of impacts. Basement lithologies, Barrow sand and Pebble Shale clasts have been found and described from several of the Simpson Core test wells which are 50 to 70 km southeast of the Barrow Gas Field. These macroscopic exotic clasts are found in the Seabee mudstones. These clasts appear to be ejecta from the Avak impact. Palynological analyses of the core matrix and clasts suggest the Avak impact occurred in the middle to late Turonian.

 

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