Recent production tests near Prudhoe Bay on the Arctic coast show that northern Alaska is a potential major oil province. This potential was first recognized when oil seeps were reported in 1900. By 1930 favorable structures and rocks were known. During exploration of Naval Petroleum Reserve No. 4, gas was discovered in Lower Jurassic rocks at Barrow, oil in Lower Cretaceous rocks at Umiat, and gas in Upper Cretaceous rocks at Gubik. Now oil and gas have been found in Triassic and Mississippian rocks at Prudhoe Bay.

The oil-bearing section consists of Mississippian through Lower Jurassic shelf deposits derived mainly from a northern source, and of up to 20,000 ft of Jurassic and Cretaceous geosynclinal deposits derived mainly from the south.

The Jurassic and Cretaceous Colville geosyncline is bounded on the south by the Triassic and upper Paleozoic rocks of the Brooks Range and on the north by a rise in the pre-Mississippian(?) basement to a depth as shallow as 2,500 ft near Barrow. The stratigraphy of the geosyncline is characterized by northward regression of intertonguing marine and nonmarine detrital sedimentary rocks shed from a southern orogenic source. The structural complexity of the geosyncline

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decreases northward from the mountain front. Twenty anticlines have been tested, and favorable reservoirs in the Cretaceous seem to be limited to narrow nearshore facies.

Pre-geosynclinal rocks are at drillable depths near the edge of the Brooks Range and on the basement rise. Folding and overthrusting make the Mississippian to Triassic rocks along the mountains difficult to evaluate without intensive subsurface exploration. In the north, geophysical data suggest that the basement rise trends southeast from Barrow and may even be an arch with northward regional dips offshore. Recent private exploration resulted in test wells on two presumably separate structures along the trend of the rise. Reservoir rocks between the Upper Triassic and basement were penetrated by the Colville well, and produce oil and gas in the wells at Prudhoe Bay. These Triassic terrigenous clastic and Mississippian carbonate rocks are part of a sequence of deposits that seem to tr nsgress regionally northward across an unconformity and regress southward away from the source. Additional reservoirs on the rise may be present if Devonian(?) carbonate rocks, that discordantly underlie the Mississippian at the front of the northeastern Brooks Range, are preserved below the unconformity.

Transportation facilities necessary for the development of large petroleum resources in northern Alaska will make the development of the extensive coal deposits there more likely and will improve the potential of known phosphate and rich but limited oil-shale deposits.

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