High Resolution 3D Seismic Data in Shallow Waters of Arabian Gulf: Acquisition and Processing Challenges

Abstract

This reference is for an abstract only. A full paper was not submitted for this conference. Summary Maersk Oil Qatar acquired a large high resolution 3D seismic data covering the entire Block 5 offshore Qatar during 2006 and 2007. The survey parameters were designed to enable optimum imaging of the shallow target of the Al Shaheen reservoirs. Significant processing challenges are typically encountered in the data originating from this area, with considerable multiples issue and difficult velocity analysis being identified as the main ones in this data set. Despite the challenges, the achieved data quality from the first processing of the Al Shaheen seismic survey has allowed important new geology insight. Data Acquisition During 2006 and 2007, Maersk Oil Qatar acquired a large high resolution seismic survey offshore Qatar covering some 2500sqkm. The acquisition parameters were selected for best coverage of shallow targets in order to enable optimal processing and therefore best resolution in the range of 2000–4000ft. The acquisition challenges were caused by the shallow target depth, shallow water, a complex lithology sequence in the overburden, present-day reefs and small-scale erosional features in the overburden. Due to a complex set of platform and pipeline installations covering multiple fields in the area, twelve smaller OBC surveys were acquired to complement streamer data to ensure continuous data coverage around platforms. Data Processing Two major processing challenges, identified before and confirmed during processing, are inter-related. These are (1) multiple elimination, including both surface-related and inter-bed multiples, and (2) detailed velocity analysis in the presence of multiples and true velocity inversions. Use of forward modelling significantly improved our understanding of the nature of the multiples, and together with well data, corridor stacks and elastic modelling enabled better quality control and optimization of demultiple methods. A large number of wells available in the area enabled a better understanding of the complex velocity distribution. RMS velocities calculated from the wells were used to constrain stacking velocity analysis. Several additional processing challenges were identified during processing. These are cross-line statics, reef statics and multiples, limitations of time-migration algorithms in the presence of laterally changing velocity regimes, merge of towed streamer and OBC recorded data, as well as imaging issues caused by small scale shallow erosional and infil features, small throw vertical faults and apparent collapse features and gas chimneys. Figure 1 shows an example with imaging artefacts causes by dimmed amplitudes, possibly by a gas chimney (left and middle). In contrast, energy scattered by true geological features is properly migrated (Figure 1, right). Results The achieved data quality allows an improved interpretation of both stratigraphic and structural features. Some of the observed stratigraphic features are displayed in the next two figures. Time slice through the variance cube at 400ms shows channel-like features at Umm-3 level in Figure 2. Stratigraphic slice at about 500ms highlights the channels at Halul level as shown in Figure 3. Using these data has significantly improved interpretation and understanding of structural elements with significant impact on both daily operational work and further field development planning. These data have higher resolution than any previously available data in the area, showing that the target-oriented acquisition and processing design is essential. New geological understanding due to the high resolution imaging of overburden will contribute significantly to the next 3D processing sequence.