Distinction Of Oil And Gas In Clean And Shaly Sands As Derived From Well Logs

Well logs assist in the distinction of oil and gas in clean and shaly pay sands, and, under favorable conditions, allow a computed approximation of the hydrocarbon densities. However, estimation of the hydrocarbon densities, especially in very shaly formations, becomes questionable without reliable "shaliness" values and knowledge of the residual hydrocarbon saturation. Properly selected logging programs also assist in the detection of gas/oil or hydrocarbon/water contacts in open and cased holes. Computational techniques and several qualitative methods can be applied. The latter include crossplot techniques and various overlays of comparable scaled porosity or resistivity logs. We particularly recommend these techniques when the interpretation of sidewall samples becomes questionable, such as in shallow pay sands in the Gulf Coast area. Several field examples illustrate the application of the methods.