Core Analysis and Well Data Interpretation for Facies Extrapolation with a focus on coals and coarse sand in the Carboniferous of the Southern North Sea

Abstract

In 1990, the 44/21a-6 well was drilled in the Southern North Sea. Core material was taken from the Westphalian A/B part of the core. Some of this core material was recently obtained by the TU Delft via the North Sea Core Initiative. Since the Westphalian A/B in the Pennine Basin is fluvio-deltaic in origin, it is highly variable both vertically and laterally. In this thesis we set out to develop a method to extrapolate facies found in the core material to recognize if these are
present in other locations in the well. This means that core material is correlated to the well log. This correlation is then extended over the rest of the well. Two investigation targets were chosen, the first facies is channel base facies, consisting of very coarse sandstone, the second a swamp facies. This firstly requires a detailed core log, and a short study of the well log data. The core log is interpreted in order to try to establish some form of elektrofacies. The well data is then formatted and different principal component analyses (PCA’s) are performed in order to differentiate the facies from their surroundings. For both analyses more than 90% of the variance was explained by the first two component axes, making them reliable. Both target facies require different data sets in order to distinguish them from their surroundings. The swamp facies
becomes easily visible from the third principal component axis, while the channel base sand is harder to differentiate, being recognizable from a point cloud in a principal component plot. In total 40 coals were found to be present over the researched well interval, starting underneath the Zechstein formation, from 12800 to 13800 ft. No other similar channel base coarse sands
were found over the interval apart from the one in the cored material. Overall, it seems that the basics for this method of detection and extrapolation is quite reliable and better than using the original log data, although it does need some further development and improvement.