Channel-Rich Versus Channel-Poor Deep-Marine Megasequences in the Post-Rift Cenozoic Succession of the Central North Sea: from Integrated 3-D Seismic and Well Log Study
A combined study of 3D seismic and well-log interpretation in an area of 314 sq. km reveals several megasequences within the post-rift Cenozoic succession of the North Sea. Based on strong seismic-reflection character and reflector termination pattern, four seismic horizons (hs_1, hs_2, hs_3 and hs_4) were traced in the 3D seismic dataset. These horizons are interpreted to represent megasequence boundaries that encase three megasequences, namely A (~300 m thick), B (~200 m thick) and C (~700 m thick), from older to younger. Published biostratigraphic results show that megasequences A and B represent Paleocene-Eocene successions, whereas megasequence C represents Oligocene deposits.
Gamma log signatures correlated to previous core studies indicate that these megasequences were deposited in deep-marine systems, where megasequence A contains numerous channelized sandstones encased within mudstones, megasequence B is dominated by mudstones with fewer channels, and megasequence C consists of mostly interbedded sandstones and mudstones. Based on reflector termination pattern and overall depositional style, sea-level history of these megasequences was interpreted. In megasequence A, deposition was thought to take place during an overall low sea-level, depositing abundant channelized sandstones. Megasequence B was likely deposited during a major phase of marine transgression, generating channel-poor mudstones that show an overall fining-upward facies trend.
Techniques of seismic-geomorphology using attribute (amplitude and instantaneous frequency) analysis also reveal that megasequence A contains extensive cross-cutting channels, whereas megasequence B shows only a few, isolated channels. In seismic sections, megasequence C shows offlap to downlap reflectors, and is interpreted to represent a depositional phase during an overall high sea-level. From hydrocarbon exploration point of view, the channelized sandstones of megasequence A represent potential stratigraphic traps, where inter-channel mudstones can act as seals.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009