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.
