Thinly laminated sands have been a difficult problem for formation evaluation. It is very easy to miss the pay zone on the logs without additional information such as hydrocarbon shows in the mud logs. This is because each sand lamina is so thin that the petrophysical features of the hydrocarbon are often masked by shale lamina. In Japan, particularly in the Niigata area, the mixture of pyroclastic material with sand (Tuffaceous sand) makes this problem more complicated. The calculated Water saturation (Sw) using the conventional equations, such as the Simandoux and Indonesia equations indicate water even if the zone is producing hydrocarbon. For the conventional equations, the problems arise when a particular shale distribution type becomes dominant, because those equations disregard the type of shale distribution. However, their responses on logs are distinguishable. In this paper, a new saturation equation has been proposed which can accommodate three different distribution types of shale (Laminated, Structural and Dispersed shale), in order to obtain more meaningful Sw. Also, the methodology to separate the Dispersed shale portion from the total shale volume (Vsh) by comparing Density-Neutron porosity and Sonic porosity, has been discussed.


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