Abstract: Measurements of Salinity of Paleoformation Waters in Sedimentary Basins for Better Evaluation of Oil Migration and Reserve Estimation

Peter Eadington, Mark Lisk, Frank Kreiger, Joe Hamilton, Machiketa Das, Mark Person

Irreducible water salinity is important in determining hydrocarbon reserves since relative hydrocarbon saturation is determined indirectly as the difference between reservoir porosity and the water volume determined from its electrical conductivity. Formation water salinity may range from 0-300,000 ppm, corresponding to electrical conductivities of 0-200 mmho/cm. This range in conductivities indicates the potential errors in determining hydrocarbon volume that might arise from using inappropriate salinity estimates.

Current formation waters in Mesozoic basins from Australia and Papua New Guinea generally have low salinities. Further, these Mesozoic sections typically contain siliciclastic sediments deposited when the plate was at high latitudes, and connate waters are expected to have low salinities. Irreducible water is therefore usually assigned low salinity.

Gondwanan Mesozoic basins are often underlain by lower Paleozoic sequences containing carbonates and evaporites. Fluid-inclusion investigations have identified hypersaline paleogroundwaters from some Mesozoic reservoir sandstones in some Australian and Papua New Guinean basins. Cross-formational flow of brines prior to or contemporaneous with oil charge is inferred. The brines probably became immobilized as irreducible water during hydrocarbon accumulation. The electrical resistivity log data would then indicate smaller irreducible water volume than estimated from assumed low salinity, and hydrocarbon volumes would be greater than estimated.

Where hydrocarbons occur in a sedimentary sequence containing evaporites, both hydrocarbons and hypersaline formation waters may use the same migration path to access stratigraphically younger rocks. Determining salinity gradients in current or paleoformation waters can provide a vector to map oil migration.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994