Depth migration as a valuable tool for prospect derisking in the Colombian Foothills

Abstract

Interpreting land seismic data in the Colombian Foothills poses many challenges. Very often, the data have a low signal-to-noise ratio and the subsurface prospective structures are significantly complex. The level of uncertainty can be so high that even experienced interpreters struggle to reconcile the seismic image with their geologic models. Based on previous knowledge and “fast-track” interpretation of old 2D prestack time migration data (no 3D seismic in the area), we identified two interesting plays for further analysis: a triangular zone (play 1) and subthrust anticlines beneath the frontal fault (play 2). Derisking the different prospects associated with plays 1 and 2 required the application of prestack depth migration (PSDM), which reduce uncertainty regarding the position of the structures, their depth, and even their existence. The seismic image in play 1 structures was improved significantly with better definition of the flanks of the anticlines and the frontal closure of the structures, more coherent events, and sharper definition in fault cut-offs. Some apparent play 2 prospects, that were actually “velocity pull-up” anticlines, were corrected by the depth-migration workflow, whereas other structures experience important modifications in their geometry. In both types of plays, depth migration dramatically changed the initial assessment of prospectivity. Based on the better agreement between seismic and borehole data, significant reduction in residual moveout on final PSDM gathers and more coherent seismic images, we believe that the use of depth migration has allowed us to obtain a more accurate representation of the subsurface, and consequently a more rigorous reserves estimation. The use of PSDM was essential to understand the complexities in the prospects evaluated and the risk associated with their exploration. We consider the lessons learned in this study applicable to similar geologic environments worldwide.