Ranking velocity models for depth conversion, closure confidence and volumetrics

This paper discusses the question of closure confidence, as it applies to the size of a field. The procedure is demonstrated with reference to a particular case study. After introducing the field in question, the size of the field on the time map is examined and compared with a couple of simple depth maps. This leads to the general questions of how to decide which of several possible structure maps is best, and what ‘best’ means in the context of a depth map of an oil field. The approach used in this paper to solve this problem is to create many thousands of depth maps, and use the technique of cross-validation to choose the best depth map(s). The variation of size and extent within these different depth maps is examined, and a statistical attempt made to determine how large the field is, and what the volume uncertainty is. It is concluded that no individual map can be regarded as ‘best’, as the map that predicts the most likely depth at each point on the oil field does not correspond to the most likely volume of oil. A second objective of this paper is to raise awareness of the public domain freeware software http://xval.sf.net that was used to generate these results.