Geology and Natural Gas and Oil Resources of Alaska

Abstract

Abstract Discoveries of natural gas in Alaska have resulted from the search for oil, which has continued intermittently since about 1898. In northern Alaska two gas fields and two prospective gas fields, as well as two oil fields and one prospective oil field, have been discovered in Mesozoic rocks. All other discoveries have been made in Tertiary rocks along the southern coast of Alaska. Except for a small shallow oil field at Katalla, all of these discoveries have been in the Cook Inlet subprovince, which contains a fully developed major oil field, an undeveloped oil field, two major natural gas fields, eight unevaluated or smaller gas fields, and important gas prospects. Alaska is grossly similar in geology, as well as in physiography, to the western part of the conterminous United States and western Canada. During Paleozoic and early Mesozoic time, it was a part of the Cordilleran geosyncline—the eugeosynclinal part over southern Alaska and the miogeosynclinal part over northern Alaska. In mid-Mesozoic time the bedrock surface of Alaska was warped into alternating geanticlines and geosynclines. The sediments that filled the geosynclines were predominantly clastic, and for the most part graywacke assemblages. Alaska has been largely emergent since the end of the Cretaceous Period. During the Tertiary Period, local downwarping formed several elongate basins that were filled with clastic sediments. Most of these were nonmarine, but marine invasions entered a few marginal basins at the southern and northern borders of Alaska. Areas considered for gas and oil possibilities include the Alaska Peninsula-Cook Inlet province, the Bristol Bay Tertiary province, the Pacific-margin Tertiary province, and the Bering Sea shelf in southern Alaska; the Arctic Slope province across northern Alaska; and the Yukon-Koyukuk and Yukon-Porcupine provinces of western and eastern interior Alaska, respectively. The Alaska Peninsula-Cook Inlet province consists of a long, narrow prism (more than 900 mi long and 5 to 60 mi wide) of slightly to moderately deformed late Mesozoic marine clastic rocks, whose composite thickness exceeds 30,000 ft. In the Cook Inlet subprovince, coal-bearing nonmarine, slightly to moderately deformed, Tertiary clastic rocks, whose composite thickness may exceed 25,000 ft, overlie the Mesozoic rocks. The Mesozoic rocks contain several gas and oil seeps and have produced shows of oil and gas in test wells, but no commercial production has been found. The Tertiary rocks contain the major known reserves and the only significant production of oil and gas in Alaska. The area considered to have petroleum possibilities is 5-60 mi wide and about 250 mi long. The Bristol Bay Tertiary province, about 250 mi long and as much as 30 or more miles wide, is underlain by poorly exposed marine and nonmarine Tertiary clastic rocks. No indications of petroleum have been reported, and three test wells were dry. Rocks of the Pacific-margin Tertiary province crop out along the coast from Icy Point in southeastern Alaska to Chirikof Island (southwest of Kodiak Island), and these rocks may extend offshore to fringe the entire Pacific margin and form a belt roughly 40,000 sq mi in area. Many oil seeps, gas seeps, and petroliferous beds occur in the Katalla-Yakataga area. Exploration has been without success, however, probably because of complex structure of asymmetric folds and north-dipping thrust faults, and the failure to find suitable reservoir rocks. Though little is known of the geology of the Bering Sea shelf, it should be considered for petroleum possibilities in view of its large size, possible extension across it of belts of late Mesozoic rocks, and possible Tertiary deposition along its outer edge. Except for the southern part of the Brooks Range, most of the Arctic Slope province, an area of about 125,000 sq mi, may be considered potentially petroliferous. It is underlain by calcareous and clastic miogeosynclinal rocks of Paleozoic and early Mesozoic ages and by clastic rocks of middle and late Mesozoic age. Structure, complex in the Brooks Range, decreases in complexity northward across the foothill belts and the Arctic Coastal Plain. Significant gas and oil reserves have been found in Cretaceous rocks. Gently deformed marine and nonmarine clastic Tertiary rocks crop out in limited areas in the northeastern part of the province. The Yukon-Koyukuk province in west-central Alaska has an area of about 85,000 sq mi. Large parts of the province are underlain by unmetamorphosed mid-Cretaceous rocks that may be locally petroliferous, although the great bulk of these rocks appear to be high-rank graywacke with poor reservoir characteristics. The Yukon-Porcupine province, an area of 15,000 sq mi, is underlain by a thick sequence of moderately deformed, largely unmetamorphosed Paleozoic and Mesozoic strata. Though no real evaluation can be made until test wells are drilled, the province appears to be relatively the most promising area in central Alaska.