The role of unconformities in the distribution of clay and porosity in North Sea clastic reservoirs

Abstract

The effect of an unconformity surface on the quality of underlying clastic reservoirs has long been a matter of considerable debate. Two opposing hypotheses have been proposed. One supports a relationship between the presence of an unconformity surface and the quality of the underlying reservoir, either by enhancement or reduction of porosity (e.g. Ketzer et al., 2009 and Shanmugam, 1990). By contrast, the opposing hypothesis argues that there is no evidence for the existence of such a relationship (e.g. Ehrenberg and Jakobsen, 2001 and Bjorkum et al., 1990). In this study, both hypotheses were examined by evaluating the role of the unconformity surfaces on the quality of selected North Sea clastic reservoirs. In particular, the study focussed on the Late Triassic Skagerrak Formation in Kittiwake Field, Central North Sea, which underlies the Mid-Cimmerian unconformity, and the Middle Jurassic Brent Group sandstones in the Tampen Spur area, Northern North Sea, which underlie the BaseCretaceous Unconformity. This thesis presents the results of detailed sedimentological and petrographic analysis of over 200 core samples, including their lithology, texture, mineralogy and diagenetic characteristics, in order to investigate the existence of any diagenetic changes that may have affected the reservoir quality, and to consider any possible relationship with the overlying unconformity surfaces. Thin-section petrography and SEM analysis shows that there are several lines of evidence for the existence of post-depositional diagenetic changes within the studied sandstones, which included the generation of secondary porosity and the formation of authigenic kaolinite. These diagenetic changes are mainly due to the dissolution of detrital framework grains (particularly feldspar) and intergranular cement (particularly calcite), and the local precipitation of authigenic clay. Both of these diagenetic processes have had a significant effect on the reservoir properties. The distribution of the secondary porosity and authigenic kaolinite clearly indicate late stage diagenetic processes, which clearly post-date the majority of mechanical compaction, and therefore also post-date the origin of the unconformity surfaces. The study demonstrates that there is no relationship between the studied unconformity surfaces and the observed diagenetic changes (i.e., secondary porosity and authigenic kaolinite) within the selected reservoirs. Therefore, any such relationship reported elsewhere is most likely to be a localised phenomenon, and should not be extended as a general rule to all unconformities.