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Using Seismic Forward Modelling to Risk Sub-Thrust Plays

Abstract

Sub-thrust plays and complex compressional geometries present valuable exploration opportunities in a variety of global settings. However, the presence of steeply dipping strata and rapid lateral variation of velocities often cause poor seismic imaging. This results in non-unique and model-driven interpretations, often tested for validity by kinematic restoration. Seismic forward modelling provides an additional source of validation and encourages the identification and use of multiple interpretation cases. Using the output of a traditional interpretation workflow, a three dimensional model of the structure derived from seismic reflection and well data is populated with representative elastic properties. Seismic acquisition is then simulated over this volume using a simplified pre-stack local imaging method, producing a synthetic seismic volume that represents the interpreted structure. This synthetic volume is then interpreted using the same methodology as the initial model. The variance between both interpretations is used to compare variations in geometry between the two volumes. This workflow is demonstrated using a case study from the foothills of the Eastern Cordillera (Llanos Basin, Colombia), generating multiple synthetic volumes representing different scenarios of structural geometry and elastic properties. Comparison of the synthetic and original data challenges the interpreter to justify subtle differences and better understand the constraints of interpretation. Areas of uncertainty may be highlighted when synthetic seismic fails to discriminate between subtle differences in input models, identifying when additional evidence may be required to make a robust interpretation. The consideration of elastic properties during interpretation may enable a more holistic consideration of the impact of seismic processing on interpretation, aiding communication between interpreters and processing geophysicists. This workflow provides an enhanced basis for subsurface-driven risking, encouraging consideration of multiple base models and aiding estimation of their relative likelihood. The output also forms a useful visual reference for discussion between subsurface teams and the broader project team when considering uncertainty and risk in all stages of project appraisal and development.