Production of Pb-210 from a Slochteren Sandstone gas reservoir

Recently, precipitates of metallic Pb and galena in gas production faciliti es have been reported to contain Pb-210. In the North Sea area, U-bearing K upferschiefer or Carboniferous coal measures have been suggested as a possi ble source of (210)pb. Through coproduction of formation water with natural gas, 210pb may enter production facilities together with non-radioactive P b, and precipitate as metallic Pb or galena. In this study, bituminous sand stones with up to 2.8 wt% organic C from a Pb-210 producing Permian, Slocht eren Formation gas reservoir have been found to contain up to 330 ppm U. Th e sandstones show a complex diagenetic history, during which migrating oil was trapped in secondary pores. Continuing enlargement of pore space and th e wetting characteristics of the reservoir fluids lead to a preferred distr ibution of oil rims around quartz grains, creating fluid-filled voids betwe en the grains. Syngenetic precipitation of finely grained U oxide occurred along the bitumen-fluid boundaries. U-Pb chemical age dating of U oxide gra ins provides an age of 246 Ma for the influx of the oil, and the close asso ciation of abundant anhydrite with the bitumen impregnations points to a Ze chstein source for both bitumen and U. One of the bituminous sandstones sho ws a marked depletion of Pb-210 with respect to U-238 and Ra-226, probably caused by the production of natural gas from the reservoir. Production star ted at the end of the 1960s and induced fluid how throughout the reservoir. This enabled transport of Rn-222 and its decay product Pb-210 away from th e U oxide grains. The observed U concentrations and permeabilities make the bituminous sandstones in this reservoir probably a more important source o f Pb-210 than the overlying Kupferschiefer or underlying Carboniferous coal measures. (C) 2000 Elsevier Science Ltd. All rights reserved.