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.
A large 3D OBC seismic survey was conducted offshore Abu Dhabi, United Arab Emirates. This survey covered an undeveloped Lower Cretaceous Thamama field and a producing Middle Cretaceous Mishrif field and was highly-specified to achieve the different development objectives of the individual fields. Data processing was particularly challenging. In addition to the mud-roll and trapped mode noises, inter-bed multiples and acquisition footprint that is associated with OBC data acquired offshore Abu Dhabi, there was a significant difference in raw field data quality between the two fields despite uniform processing that was applied in an attempt to obtain a seamless image across the area. A single set of parameters was applied after extensive testing in both fields using well synthetics and VSP's as to guide parameter selection at key steps in the processing flow. A conservative methodology was adopted for noise attenuation. The objective was to peal noises from the data without touching signal. Multiple targeted noise attenuation processes focused on specific noise types, were applied as opposed to a few harsh filters that addressed multiple noise types in the same step that could produce a uniform but low-resolution data set across both areas. Much of the coherent noise attenuation was performed on the separate P and Z components prior to PZ summation. The processed final data revealed clear features: (1) continuous reflections and discontinuous fault trends; (2) channel-likefeatures in overburden; and (3) reefal reservoir edge; which were not mapped in previous 2D seismic surveys. However, some objectives were not fully resolved and have to be addressed in any future re-processing: (1) difference in data quality between the two fields such as S/N ratio, signal frequency bandwidth and wavelet; (2) remnant acquisition footprint; and (3) limited resolution and offset range. The marked contrast in data quality between the two fields was interpreted as being related to variations in near surface geologies. A separate targeted processing in each individual field could address these problems. The lesson learnt during the processing will help any future processing in a carbonate field of offshore Abu Dhabi, United Arab Emirates.
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