Abstract: Constructing 3D geological models is a fundamental task in oil/gas exploration and production. A critical stage in the existing 3D geological modeling workflow is moving from a geological interpretation (usually 2D) to a 3D geological model. The construction of 3D geological models can be a cumbersome task mainly because of the models' complexity, and inconsistencies between the interpretation and modeling tasks. To narrow the gap between interpretation and modeling tasks, we propose a sketched based approach. Our main goal is to mimic how domain experts interpret geological structures and allow the creation of models directly from the interpretation task, therefore avoiding the drawbacks of a separate modeling stage. Our sketch-based modeler is based on standard annotations of 2D geological maps and on geologists' interpretation sketches. Specific geological rules and constraints are applied and evaluated during the sketch-based modeling process to guarantee the construction of a valid 3D geologic model.