Quantifying the geophysical uncertainties is an important part of field evaluation because it has a direct impact on the extension of the oil pool and on the volumes in place. In low relief structures the uncertainty on Gross Rock Volume can be a major issue because small changes in the top reservoir depth map may drastically impact the closure area.
Our field, offshore Abu Dhabi, is an example where this uncertainty issue on gross rock volume is important. The reservoir levels are located in the Lower Cretaceous Formation. A high standard OBC survey recently acquired drastically improved the vision of the field compared to previous sparse 2D lines of various vintages. This new 3D dataset also emphasized the importance of overburden events effects on the top reservoir time map. Signal phase instability appears to be affecting the dataset and hence the top reservoir time map.
This paper presents the methodology followed to optimize the Gross Rock Volume estimation. Statistics as well as analytical methods are combined to assess carefully the uncertainties. The top reservoir time map is quality controlled with respect to key processing steps, overburden artifacts effects and picking uncertainty. The uncertainty related to time to depth conversion is assessed by scenarios around a selected base case. Multiple top reservoir map realizations are then computed, integrating all uncertainties. The objective of the study is to generate P10, P50 and P90 depth maps for the Top reservoir that fully integrate the whole range of geophysical uncertainties. These maps will then be implemented in the geological and reservoir models for further calculations to get P10, P50 and P90 reserves evaluation.