Petroleum Geology of Umiat Oil Field, North Slope, Alaska
Umiat oil field is located on the leading edge of the Brooks Range fold-and-thrust belt of northern Alaska. The Brooks Range originated during a Late Jurassic/Early Cretaceous collision of an island arc with a (then) south-facing passive continental margin. A foreland basin, the Colville basin, formed as a result of crustal loading and filled with sediments derived from the Brooks Range. Both the Umiat source rocks (Jurassic Kingak, Lower Cretaceous gamma ray zone and Lower Torok Formation) and reservoir rocks (Upper Cretaceous Nanushuk Formation) were deposited in this foreland basin. Renewed shortening during Late Cretaceous/Early Tertiary time resulted in thrusting and related folding of proximal parts of the Colville basin as these sediments were incorporated into the leading edge of the fold-and thrust belt. The Umiat structure consists of one of these thrust-related anticlines.
Based on surface expression and oil seeps, the shallow (275-1250 ft/83-381 m) 7500 acre trap was discovered by the U.S. Navy during the late 1940s and delineated with 12 vertical wells. Well log data and hundreds of feet of core were obtained; numerous flow tests were performed (the longest - 400 BOPD for 93 days); different drilling and completion techniques were attempted; and lab tests at reservoir conditions were conducted to arrive at a recovery factor range of 28 to 51%. Despite the desirable nature of Umiat’s light, sweet crude (37°API / 0.84 S.G.) and the good reservoir characteristics of the Cretaceous sands, the field had challenging development issues due to the shallow depth of the reservoir, low reservoir pressures, and the fact that half of the oil accumulation is within permafrost. Uncertainty as to potential application of pressure maintenance led to previous low recoverable reserve estimates.
Reevaluation of Umiat oil field based upon interpretation of an 86 sq mi (223 sq km) 3D seismic survey acquired in 2008 and the original well data has led to a clearer picture of the structural trap. Based on this new data and interpretation, the revised OOIP may be 1 BBO (147 million tons) or greater. With modern-day completion techniques, horizontal drilling, and pressure maintenance mechanisms it is estimated that recoverable reserves could be in the 200 to 300 MMBO (29-44 million tons) range. Application of these proven technologies plus the higher OOIP suggests that this field can finally be converted from an Arctic curiosity to a commercial accumulation.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009