It is possible to transform the sigma value of the Thermal Neutron Decay Time Log into an apparent porosity (porosity transform) which can be compared with the Neutron-log porosity for the purpose of locating hydrocarbons. When the Neutron porosity used is sigmaK, derived from the Ratio curve of the dual-spacing TDT, both sigma and phiN are available from the same tool on the same logging run. These logs are generally run in cased hole. When the formation water is salty, the TDT porosity-transform curve tracks well with the Neutron porosity in water sands but the two curves separate from each other in oil sands. When the zone contains oil, the curve separation can be related to the amount of oil present, irrespective of shaliness; and quantitative interpretations are possible. The separation is also related to the hydrogen index of the hydrocarbon. In a sand containing gas, which has a smaller hydrogen index, a smaller, or even negative, separation is seen. When the formation water is fresh, curve separation between the porosity-transform and the Neutron-porosity curves still identifies oil zones. However, in order to obtain quantitative values it is necessary to account for the fact that the clays and the fresh water do not have the same sigma values; and a knowledge of the amount of clay present becomes necessary. Gulf Coast examples will be shown to illustrate the cases of salty and fresh formation water. The method has been computer implemented.

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