EAGE Workshop on Detective Stories Behind Prospect Generation - Challenges and the Way Forward

Exploration and drilling practices in a complex tectonic environment have always been a challenge. Look-ahead VSP has been introduced to help identifying the potential hazards below the bit prior to drilling. The procedure is to invert the VSP data for acoustic impedance while accounting for the structural effects. The technique used is based on Monte Carlo sampling of reflection data while incorporating known impedance and structural dip constraints (model) at different times through Bayes’ rule. The technique has been successfully employed for look-ahead purpose in complex geological structures of North Pakistan. The borehole deviation, however, remains a critical challenge for the quantitative applicability of the technique since the direct rays traveling down to the bottom-most receivers follow highly slant trajectories in this case. In such cases, the inverted acoustic impedance gives a reasonable quantitative guess and a good qualitative look-ahead prediction.

The joint interpretation of digital elevation models (DEM) and multispectral remote sensing data in connection with stratigraphic and geologic information reveals the geologic structure of the earth surface, particularly in desert terrain. The characterization and classification of the DEM using spatial statistics provides hints regarding formation tops, which are validated by mineral spectroscopy of multispectral remote sensing data. The idea behind this approach is that the earth surface topography is the result of geological processes such as deposition, erosion, and tectonics.

We acquired a VSP survey for look-ahead purpose in a deviated well in the tectonically complex area of Kohat Plateau, North-West Pakistan. The dips from VSP image were found contradicting with the surface seismic image. Because of the very small lateral extent of VSP image at the reservoir level, the VSP image alone could not provide confidence in the structural interpretation. For that reason, we analyzed various VSP migration techniques to analyze the reflections from top of the reservoir. The uncertainty due to non-uniqueness of the tomographic inversion of VSP travel times and the reflector geometry was reduced by iterative numerical modeling of VSP wavefield. The VSP image showed that the reservoir had been missed because the well had entered into the fault block. After validating the interpretation by drilling further 100 m, a side track was drilled based on the VSP image. The side-track hit the reservoir as predicted.