Stratigraphic Traps in Paleocene Sands in the Balder Area, North Sea
The Balder oil field in Blocks 25/10 and 25/11 in the Norwegian sector of the North Sea is a submarine-fan complex of middle to late Paleocene age deposited in a series of prograded and channelized suprafan lobes. The reservoir sequence at Balder (upper Thanetian) is 20 to 60 m thick and consists of a highly quartzose sand-prone suprafan complex. Depositional topography and subsequent submarine erosion produced a mounded upper surface on the fan, and oil is stratigraphically trapped by this closure. Shale occurs as a hemipelagic drape over the entire suprafan complex and serves as the seal.
A sand-rich suprafan typically consists of a lower fan fringe facies characterized by prograded, coarsening-upward siltstone/sandstone sequences, and a channelized inner fan facies. The channelized facies migrates over the fan fringe and consists of massive fine to medium-grained sandstone in a series of coalesced, fining-upward channel-fill deposits. Eight individual suprafan lobes were identified and mapped within the reservoir sequence. Major downlapping cycles and topographic crests define lobes. The lobes have an elongated north-south orientation, and dominant progradation was from north to south. The shale within the reservoir sequence generally occurs as a thin (< 15 m) drape over individual suprafan lobes. A distal turbidite unit of shale, siltstone, and fine sandstone blanketed the area and is preserved in the crests of the highest mounds and at the southern and western margins of the field.