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Recently, precipitates of metallic Pb and galena in gas production facilities have been reported to contain 210 Pb. In the North Sea area, U-bearing Kupferschiefer or Carboniferous coal measures have been suggested as a possible source of 210 Pb. Through coproduction of formation water with natural gas, 210 Pb may enter production facilities together with non-radioactive Pb, and precipitate as metallic Pb or galena. In this study, bituminous sandstones with up to 2.8 wt% organic C from a 210 Pb producing Permian, Slochteren Formation gas reservoir have been found to contain up to 330 ppm U. The sandstones show a complex diagenetic history, during which migrating oil was trapped in secondary pores. Continuing enlargement of pore space and the wetting characteristics of the reservoir fluids lead to a preferred distribution of oil rims around quartz grains, creating fluid-filled voids between the grains. Syngenetic precipitation of finely grained U oxide occurred along the bitumen–fluid boundaries. U-Pb chemical age dating of U oxide grains provides an age of 246 Ma for the influx of the oil, and the close association of abundant anhydrite with the bitumen impregnations points to a Zechstein source for both bitumen and U. One of the bituminous sandstones shows a marked depletion of 210 Pb with respect to 238 U and 226 Ra, probably caused by the production of natural gas from the reservoir. Production started at the end of the 1960s and induced fluid flow throughout the reservoir. This enabled transport of 222 Rn and its decay product 210 Pb away from the U oxide grains. The observed U concentrations and permeabilities make the bituminous sandstones in this reservoir probably a more important source of 210 Pb than the overlying Kupferschiefer or underlying Carboniferous coal measures. |
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