f Channel deposits of the Palaeocene Forties Fan, Nelson Field, Central North Sea

The Nelson Field, situated on the Forties-Montrose High in the UK Central North Sea, is a simple, low relief anticlinal structure with four-way dip closure. First discovered in 1988, the Nelson reservoir is found in the Palaeocene Forties Sandstone Member of the Sele Formation and comprises predominately of three main submarine channel systems that run in a NW-SE direction across the structure. Reservoir quality sandstones are found in the axis of the channels as well as in the channel margins and interchannel areas. Core intervals from three producer wells show the sedimentological characteristics of these three depositional environments. Core from 22/1-N9 shows an example of the deposits from a channel axis. The core is characterized by a high net to gross section (up to 95%) dominated by massive, structureless, amalgamated, predominately medium grained sandstone deposited from higher-energy flows. Muddy sediments are only preserved at the very top of a limited number of flows. A channel margin setting, as seen in core from 22/11-N16y, shows a lower net to gross range (50-60%). However, a large portion of the sands in the core, while thinner than the channel margin, posses very similar characteristics of the sandstones seen in the channel axis. The remaining sand in the cores preserves sedimentary structures (laminations, ripples, alternations of silts and sands) from deposition by lower-energy flows. Deposits in the area between the three major channel systems, as shown in the 22/11-N1 core, are dominated by muddy/silty sediments interspersed with sands. Again, even though the overall net to gross is in the range of 30-40%, the sands seen in the core are similar to the sand seen in both the channel axis and channel margin deposits. As the Nelson field is maturing, understanding the connectivity of the sands from the channel axis through the channel margin and into the interchannel areas is of paramount importance to determine what part of the reservoir has already been swept and where to find the remaining oil in the field.