GEOLOGICAL IMPLICATIONS OF RADIUM AND HELIUM IN OIL-FIELD BRINES - OBSERVATIONS, INFERENCES AND SPECULATIONS

The 1600 yr half-life of radium restricts the time and thus the distan ce over which radium can migrate in sediments. The dominant source of unsupported radium in sandstone reservoir brines must then be close by , likely in shales adjacent to the oil-field reservoirs. Chemical simi larity of calcium and radium can be used to argue for a local shale-so urce contribution to the calcium in reservoir sands-suggesting the pro bability of calcite cementation early in the sedimentary sequence. Hel ium production by radium decay increases with time. Concentrations of helium found in reservoir oil field brines are then used to suggest th at: (a) such reservoirs are dominantly closed systems over geological times; (b) neither methane nor helium in the reservoirs have migrated any significant distance; and (c) the mechanism responsible for the ob served helium in the brine is a continuous on-going process operative today. Diagnetic studies should then deal with both sands and shales i nterdependently, the two are not separable. Shales control the transpo rt mechanisms of migration so that the primary migration of hydrocarbo ns, the result of kerogen catagenesis in shales, should occur sufficie ntly early in the sedimentary sequence in order to avoid exclusion fro m the reservoir by calcite cementation in association with radium tran sport.