--> --> The Magic of Depth Imaging in a Complex Geological Setting - A Case Study from Upper Indus Basin

2018 AAPG International Conference and Exhibition

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The Magic of Depth Imaging in a Complex Geological Setting - A Case Study from Upper Indus Basin

Abstract

A case study for a HC exploration project from the structurally complex fold and thrust belt of the Upper Indus Basin of Pakistan based on a workflow for pre-stack depth imaging utilizing Anisotropic Reverse Time Migration (TTI RTM) to achieve a more reliable structural interpretation compared to legacy time processed seismic data. A 2D dataset was processed using the workflow to reassess the size and dimensions of a deep seated lead that previously could not attract E&P operators due to its small size and greater depth. Acquiring a 3D dataset would have been the best technical choice to confirm the results of the test performed on a 2D line but due to economic considerations, the 3D idea did not mature. The 2D dataset was treated as a simulated 3D dataset to generate a 3D velocity field to estimate Thomsen’s parameters and perform TTI RTM. The mapping resulted in a completely different picture; a synclinal feature on time imaged data changed to an anticlinal feature at a depth of over 5600 m and lateral movement of the anticlinal feature in the depth image was in excess of four km. Overall size and resource estimates made this feature a drillable prospect. The study area has a steeply dipping anticline at the surface with a complex crush-zone directly beneath it. The Kirchhoff depth imaged section showed better structural imaging, fault definition, and event continuity. The RTM section further enhanced resolution, particularly below the Eocene Kohat Formation and under the steeply dipping flanks of the surface anticline. Mapping and interpretation resulted in a fault-bounded structure that was matured as a candidate for drilling for HC. Drilling of this prospect based on the workflow above resulted in the deepest HC discovery in Pakistan. The study demonstrates improvements that can be achieved by a TTI tomography approach and a RTM migration algorithm. Although, this significant achievement was possible using a 2D dataset with all its imaging limitations, to fully exploit the potential of depth imaging work flow, a 3D data set is recommended. The workflow relies on the soundness of the velocity model, which reinforces the need for azimuth rich 3D acquisition and well-calibrated processing, followed by pre-stack depth imaging. This success calls for a revisit of the many comparable datasets under similar structurally complex geological settings in the area and in its analogs elsewhere in the world.