ABSTRACT

A straightforward procedure for determining water saturation and porosity in laminated sand/shale sequences in the U. S. Gulf Coast is presented and results plotted on an expanded scale. Since deep resistivity and porosity logs exhibit poor bed resolution in this laminae, maximum use of Diplog data is employed owing to its high sample rate and good thin bed resolution. Comparison of results with computations from standard open hole logging measurements are displayed along with whole core analysis. Normalization of Diplog micro-resistivity curves recorded at 64 samples per foot is achieved using other micro-resistivity log measurements. The Thorium-Potassium Index from Clay and Shaly Sand Analysis (CLASS) is used to highlight high shale/sand ratios which coupled with tension curve and/or accelerometer data improves depth matching. Water saturation (Sw) and porosity are calculated using standard micro-resistivity equations, then normalized to whole core analysis and/or thick homogeneous clean zones from open hole log interpretation. The activity coefficient of the Diplog micro-resistivity is employed to isolate discrete sand/shale members. Comparison between these results, the CLASS program, and whole core analysis is presented. Accumulated values of net thickness, hydrocarbon feet, porosity feet and average Sw at different cutoffs are compared between programs to determine minimum and maximum values for economic decisions. Case examples are presented. Limitations of the technique are evaluated, along with detrimental influences such as bed thickness, tool limitations, borehole rugosity, compression of data via tool pulls, variable filtrate invasion radii and mixing, Rmf range of values and discrepancy effects, saturation of gas surrounding the borehole and tight resistive zones.

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