Resource report for proposed OCS lease sale no. 70, St. George Basin, shelf area, Alaska

Abstract

St. George basin is a long (300 km), narrow (30-50 km) graben whose long axis strikes northwestward, parallel to the continental margin of the southern Bering Sea. Located near the Pribilof Islands, and beneath the virtually featureless Bering Sea shelf, the basin is filled with more than 10 km of sedimentary deposits. These sedimentary rocks are ruptured by normal faults associated with the sides of the graben; these ruptures commonly correlate with offsets in the basement surface. Offset along these faults increases with depth implying that they are growth-type structures. Basement rocks, that floor and flank St. George basin are part of an assemblage of Mesozoic eugeosynclinal rocks that extends from southern Alaska to eastern Siberia beneath the Bering Sea margin and outer shelf. A parallel belt of igneous rocks of late Mesozoic and earliest Tertiary age may also extend from western Alaska to northeastern Siberia beneath the inner Bering Sea shelf. The Bering Sea margin and adjacent shelf were apparently uplifted by the end of Mesozoic time, resulting in deep subaerial erosion• Following uplift, the outer Bering Sea shelf has undergone extensional rifting and regional subsidence. Differential subsidence has resulted in the formation of a series of basement ridges and basins whose axes parallel the Bering Sea margin. Some of these basins are very large to gigantic in size, e.g., St. George basin, and involve crustal subsidence exceeding 10 km. Such large scale crustal collapse suggests deep crustal or upper mantle processes, such as thermal metamorphism or stress-induced crustal migration. Nine wells drilled along the northern coast of the Alaska Peninsula, as well as several onshore Soviet wells in northeastern Siberia, relate directly to the submerged basins of the Bering Sea shelf. Although all of the wells on the Alaska Peninsula were abandoned as dry holes, shows of oil and gas were found. In addition, Soviet drilling resulted in the discovery of oil and gas shows in Oligocene and Miocene sandstone. Regional geologic and geophysical mapping suggests that there are suitable source beds, reservoir rocks, and traps within St. George basin. However, it is not known if hydrocarbons are present or if the possible reservoirs are of commercial size. A resource appraisal of St. George basin out to 200 meters water depth indicates that, at 5 percent probability, 6.4 billion barrels of oil and 18.6 trillion cubic feet of gas may be in the basin; at 95 percent probability 0.8 billion barrels of oil and 4.5 trillion cubic feet of gas may be in the basin. The statistical mean of the appraisal is 2.7 billion barrels of oil and 10.3 trillion cubic feet of gas. A large number of faults, evidence for recent movement along some of the faults, and high seismicity all indicate that faulting is a major environmental concern for the outer continental shelf region of the southern Bering Sea, especially in St. George basin. Most of the faults are potentially active and their movement is probably influenced by the local geology, including basement structures and sediment loading. Unstable sediment masses pose potential threats to resource development in the vicinity of the Pribilof Canyon. Volcanic activity along the Aleutian arc south of St. George basin may also pose an environmental hazard to petroleum development in the area. Another environmental hazard is the presence of shallow gas pockets, which could pose such problems during drilling as blowouts and liquefaction of bottom sediment.