Inorganic chemical analyses of black shale from wells in the National Petroleum Reserve in Alaska

Core samples of Mississippian through Upper Cretaceous black shale, siltstone, and limy mudstone from 24 test wells in the National Petroleum Reserve in Alaska (NPRA) have been analyzed for trace elements in order to provide data on regional background concentrations for inorganic geochemical exploration. This study was made because the authors had noticed that several of the rock units cored in the subsurface were associated with surface geochemical anomalies or small mineral deposits in the areas where they crop out. In the southwestern part of the NPRA, the heavy-mineral concentrates from sediments of streams that flow over shale and graywacke of the Lower Cretaceous Fortress Mountain and Torok Formations are unusually rich in lead, arsenic, and silver. Southeast of the NPRA, in the foothills of the Philip Smith Mountains, stream sediments in areas of Permian to Lower Cretaceous shale locally contain anomalously large amounts of zinc and thorium. In addition, the high organic-carbon content of the Shublick Formation, Jurassic (part) Kingak Shale, and lowest Cretaceous pebble shale unit in the subsurface in the Prudhoe Bay area indicate that they may be rich in trace metals. Outcrops of the Shublik in the Brooks Range locally contain much copper, molybdenum, vanadium, andmore » rare-earth elements, and the high gamma-radiation characteristic of the pebble shale unit in the subsurface shows that it is rich in uranium and thorium. The shale section with the most important known metallic deposits is the Mississippian shale and chert now assigned to the Kuna Formation. The distribution of vanadium and nickel may also be of interest in oil exploration. Hughes and others found higher V/Ni ratios in the Prudhoe-Barrow types of oil than in the Umiat-Simpson types and attributed these higher ratios to sources in the Shublik Formation and Jurassic (part) Kingak Shale.« less