ABSTRACT

The pulsed neutron log is a valuable tool in evaluating untested or unswept reservoirs even in sandstones with 50% VSh. Five pulsed neutron log analysis techniques have been developed based on case studies and are being used successfully in clean and shaley Gulf Coast and East Texas sandstones. These methods are:Method 1 uses Rule of Thumb Sigma Cutoffs for oil, water, and gas determination. The sigma cutoffs are predetermined and are based on a sand that has fairly constant reservoir properties. It is the easiest to develop and use, but will cause shaley oil or gas productive sands to be interpreted as wet. Method 2 relies on Sigma-Porosity Crossplots with no shaliness corrections. It is similar to Method 1 except that a crossplot is developed to provide quantitative saltwater calculations dependent on sigma log and porosity. It is a quick quantitative chart technique that will still lead to incorrect interpretations for shaley sands that vary from the chart parameters. Method 3 develops a Sigma Matrix corrected for shaliness. This is a quantitative technique that is dependent upon deriving a composite sigma matrix that varies by reservoir. This method eliminates the problems associated with assuming surrounding shales have similar compositions as the clay minerals in the sand of interest. The reliability of a constant Vsh throughout the reservoir plus the requirement of known fluid saturations restricts the use of this technique in many areas. Method 4 uses a Sigma Log Correction based on surrounding shale properties to calculate VSh. This technique assumes that surrounding shales have similar compositions as the shale located in the sands. This technique has a 97% success rate in the Woodbine formation with similar success in other fields, even though the surrounding shales often have different compositions than the shales in the sands. Method 5 has a Sigma Log Correction which utilizes surrounding sand data to calculate VSh. This technique assumes that nearby sands have the same shale compositions, but vary in Vsh percentages. One of the more difficult techniques to use, "Method 5" has a great advantage in needing only the individual well data and does not rely on surrounding shale parameters. It is a reliable technique for all of the fields studied. All of the above methods have been used successfully and should be selected based on the individual field situation. The equations and case histories for these techniques are described for Woodbine, Cockfield, and Frio sands.

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