Can thin coals be resolved in seismic data? An example of the challenges for Carboniferous (Visean to Westphalian) source-rock definition across the Elbow Spit Platform (Netherlands) and the Mid North Sea High (UK) through well synthetic modelling

Abstract

The ability to predict coal units from seismic data has become important for hydrocarbon exploration, especially in areas away from traditional play sweetspots. An understanding of coal distribution/thickness from seismic data can influence the risking of basin source-rock presence/charge efficiency. A selection of wells is presented from the Carboniferous of the Elbow Spit Platform (Netherlands), the Mid North Sea High (UK) and adjacent areas. These wells penetrated various coal thicknesses, as well as sandstones, mudstones and thin limestones in different stratigraphic configurations. For each well, full-stack synthetic seismograms (acoustic impedance domain) using a conventional seismic wavelet are generated along with supporting rock physics. This analysis suggests, as a guideline, that coal thickness needs to meet a threshold of 25–35% of loop thickness to be uniquely distinguishable from the surrounding lithologies (discernible soft kick). If the coals are thinner than this threshold, they will be comparable to soft sandstone-related kicks, although some exceptions exist where thin coals directly overlie soft sandstone intervals. The exact percentage value will depend on seismic-data quality and rock properties. This study highlights the need for careful consideration of thin-coal seismic expression when devising interpretation strategies and risking source-rock presence.