3D OBC Seismic Data Processing for Two Overlapping Carbonate Fields Offshore Abu Dhabi, United Arab Emirates

Abstract 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:continuous reflections and discontinuous fault trends;channel-like-features in overburden; andreefal 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:difference in data quality between the two fields such as S/N ratio, signal frequency bandwidth and wavelet;remnant acquisition footprint; andlimited 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. Introduction A large 3D OBC seismic survey was conducted offshore Abu Dhabi, United Arab Emirates. This survey covered an undeveloped Lower Cretaceous Thamama field (Field A) and a producing Middle Cretaceous Mishrif field (Field B). The outlines of the two fields overlap. The two-way time to the top Thamama reservoir is approximately 1.6 sec and the top Mishrif 1.4 sec. The survey objectives for the two fields were different. Field-A has a subtle low relief structure at Thamama level. In addition, channel-like features filled with high velocity material are present in the overburden at the Upper Cretaceous Fiqa level. These features cause a two-way time pull-up below the high-velocity channel anomalies and have a significant affect on the time structure interpretation. The loose grid of 2D seismic data that was previously available lacked the dense areal coverage to map the field-wide distribution of these features. Field-B is located east of and partially overlaps field-A. Field-B has a structure at the shallower Mishrif level. The reservoir is a reefal facies which changes stratigraphically toward the western flank. No Mishrif reservoir has been confirmed to date in field-A, however the western edge of the reefal facies has not yet been defined. In addition, the structure is highly undulated due to erosional and karstic topography formed during sea-level fall after reefal facies deposition.