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 getP10, P50 and P90 reserves evaluation.
Introduction
Our field is a four way dip structure located offshore Abu Dhabi. Since discovery well, six vertical wells have been drilled to delineate the field extension and confirm development decision.
As first phase field development is planned in the near future, a 3D OBC has been recently acquired. The aims of this 3D were to better define structural extension of the field, assess fault network in order to evaluate possible compartmentalization and eventually conduct reservoir characterization models for field development guidance.
In this paper, we will deal with the structural definition of the field and will try to estimate the range of variation of its Gross Rock Volume (GRV).
Compared to other regional structures, our field has a very low relief, estimated depth difference between the structural top and proved oil water contact is slightly over 100ft, which is very low compared to the average reservoir depth.
With these conditions, uncertainty on top reservoir depth map may drastically change field extension and therefore associated volumetric assessments. This justified a study to identify and quantify uncertainties in an attempt to estimate the range of variation of the GRV and rank uncertainties contributors.
Bahrain field is an asymmetrical anticline trending in the North South direction. The sedimentary column extends from Cambrian Saq Sandstone to the Miocene reefal deposits, exposed on the surface.
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