Online Journal for E&P Geoscientists

Firasat Shah, Syed ¹; Sajjad, Musharraf ¹; Zubay, Erick ^*2; Cunnell, Chris ²; Baioumy Yousef, Mohamed ²
(1) Pakistan Petroleum Limited, Karachi, Pakistan. (2) Schlumberger, WesternGeco, Cairo, Egypt.

We present a case study for advanced pre-stack depth imaging over the Dhok Sultan block, utilizing anisotropic Reverse Time Migration (TTI RTM) to produce a more reliable structural interpretation compared to legacy processing.

The Dhok Sultan block is located in the Upper Indus basin of Pakistan. The data set comprises a single 2D test line of 23km length. A steeply dipping anticline outcrops at the surface with a complex crush zone directly beneath. Basement is overlain by a regional decollment surface (Precambrian salt) which controls the target level structuration.

Legacy processing vintages provided conflicting interpretations of the structural closure at the reservoir level due to the imaging complexity through the overburden. The primary objective of this study was to apply an advanced PSDM workflow and hence minimize these structural ambiguities.

Analysis indicated that velocity anisotropy has a significant effect on the data. The presence of steep dips on the anticline flanks indicated structurally conformant anisotropy was merited. Consequently TTI anisotropy was used throughout. The PSDM velocity model was updated by 3 iterations of gridded tomography. Final migration was performed using both Kirchhoff and RTM algorithms.

The results show a clear improvement compared to the legacy processing. The final Kirchhoff section shows better structural imaging, fault definition and event continuity. The RTM section shows further enhancement in resolution, particularly below the Eocene Kohat formation and under the steep dipping flanks of surface Jhammat anticline. The significant movement of structural features in the 2D plane call for the re-processing of remaining vintage data, using the same processing sequence, followed by a fresh approach to its interpretation. It should be noted that the limitations of 2D acquisition and processing are expected to have a strong influence in this study. Inspection of the final images indicates that unresolved features remain in the model, and illustrates the importance of full 3D acquisition and processing.

This study demonstrates the improvements that can be achieved by a careful TTI tomography approach. It also points the way to realizing further improvements by using more advanced migration algorithms. Of course the uplift from sophisticated migration algorithms rely on the soundness of the underlying velocity model, which reinforces the need for azimuth rich 3D acquisition and well-calibrated processing.