The Cleaver Bank High in Quad 49 in the Southern North Sea is Shell Expro's and joint venture partner ExxonMobil's Rotliegend transition zone heartland. It contains several discoveries and fields that are being appraised and developed. The Carrack field is the JV’s most significant discovery and will be brought on stream in Q4 2003. This new hub increases the attractiveness of the area for further exploration.
Since Tertiary charge seems to be abundant and the Rotliegend reservoir is capped by a thick sequence of Zechstein evaporites, the major risks and uncertainties are "structure" and "producable reservoir". Unlike in the Dutch sector Top Rotliegend in Quad 49 is a fairly low relief surface. The Upper Rotliegend sediments in this area are an inter-play of fine grained sabkha deposits and coarser grained sands of predominantly fluvial origin with minor aeolian input. Further to the north the formation shales out, becomes interbedded with halite and forms a seal to the underlying Basal Leman Sandstone, at e.g. Cutter (49/09-3). The presence of this sandstone seems to be controlled by paleo-topography and/or syn-sedimentary faulting and is therefore hard to predict.
The combination of these risks makes it critically important to understand the geological model and to have a proper control on time-to-depth conversion. Due to the low relief nature of the structures a waste zone in the upper section of the Rotliegend can significantly reduce the attractiveness of a prospect. Based on well data a regional geological model can be established which however is not detailed enough for prospect evaluation. Depth migration based on anisotropic velocity models and seismic inversion can mitigate these risks and lead to a geological model suitable for prospect maturation.