In the early stages of a large integrated study of a cluster of fields in South Oman it became apparent that the selection of proper net reservoir cut-offs was an important topic, with a potential impact on the development planning. As the various fields in the cluster had been studied before by different people and over a period of about two decades, there was unfortunately no coherent regional view on the methodology of establishing net reservoir sand nor on the proposed numerical value of the cut-offs. During the study this problem was addressed through an effort to integrate the data from a variety of sources:

  • Borehole image logs. During the study 19 km of borehole image logs were interpreted, including an extensive analysis of reservoir and non-reservoir facies.

  • Quantitative analysis of UV core photo's.

  • Core data. Analysis of porosity and permeability data on 1350 plugs.

  • Log data. Generation of "Heslop" type of plots in order to steer the selection of cut-offs.

  • Analysis of the EOC (equivalent oil column) vs. the cut-off values.

The above data was gathered for the various stacked reservoirs in the fields, so that it could be established whether cut-off variations were required for the different reservoirs and/or the different fields. Although none of the five steps above provides a final answer to an optimal cut-off value, a reasonably clear picture emerged from the combination of them. This allowed a very robust selection of the net reservoir cut-offs as well as the selection of low and high case values that were tested further in the domain of the static and dynamic reservoir models.

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