Raiders of the Lost Mud: the geology behind drilling incidents within the Balder Formation around the Corona Ridge, West of Shetland

The Faroe-Shetland Basin, NE Atlantic continental margin, hosts a number of important hydrocarbon fields, though deep water and narrow weather windows mean drilling costs are considerably higher than other parts of the UK Continental Shelf. Any additional drilling complications are therefore important to predict and negate as such issues can result in avoidable multi-million pound cost implications. This study focuses on the Corona Ridge, an intra-basinal high which contains the Rosebank Field, where a plethora of drilling issues, of enigmatic origin, are common within a key stratigraphic marker known as the Balder Formation. Drilling fluid loss, bit balling, wellbore breakouts, and wellbore 'ballooning', where lost drilling fluid returns to the wellbore, are all recognised within the Balder Formation along the Corona Ridge. We find that many of the drilling incidents can be traced back to both the lithological character of the Balder Formation, and the mid-Miocene tectonic inversion of the Corona Ridge. Moreover, we find that this geological explanation has wider implications for exploration in the region, including mitigation of drilling incidents in future wells through drill bit selection.