Abstract: Petroleum reservoir model building is a fundamental but complex task present in all stages of oil/gas exploration and production (E&P). Reservoir models are built incrementally using multi-disciplinary data (e.g. from geophysics, geology, reservoir engineering) and the domain expert interpretation of that data. The first reservoir models are constructed at the appraisal stage, where the available data presents inaccuracies and a high degree of uncertainty. In this paper we present a set of sketch-based interface and modeling operators integrated in a system for the early appraisal stage in oil/gas E&P for the tasks of seismic interpretation and reservoir model building. Our system allows the user to sketch directly over the raw seismic reflection volume and its derived data. These data guide the expert in the key tasks of seismic interpretation and building the structural framework of the reservoir. We propose a novel set of sketch-based modeling operators designed by specific domain requirements from geophysics and geology. A novel architecture using adaptive meshes is also developed to create a more flexible sketch-based system.