Laboratory measured core grain densities and porosities have been used in a bimodal carbonate reservoir (limestone and dolomite) to establish that at both the low, and the high, porosity endpoints, there is a strong likelihood of the reservoir being predominantly limestone. These two endpoints, in conjunction with an overlying anhydrite bed and a Pef curve, then make up a four-point porosity log quality control procedure. Applying this rock-based QC method to a large number of wells then allows one to make the following general observations:Reliability among the sonic, density and neutron logs is in the order listed. The neutron log can be influenced by relatively light oils by a much larger amount than previously thought. Published environmental corrections to the neutron data may be insufficient, due to parameter variations within the borehole. In wells with three porosity tools and no core, one then commonly uses the sonic log across limestone intervals for benchmarking purposes thereby allowing a valid density-neutron (two-tool) solution in dolomitic rock. In wells with only density-neutron combination, one calibrates the neutron to the density across expected limestone beds. Again a two-tool, dual mineral solution results for rock with dolomitic tendencies. Based upon a review of more than 100 wells, we have found that more than 50% of the neutron logs were in need of calibration.

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