Fluviomarine Sequences in the Lower Cretaceous of the West Netherlands Basin

The Lower Cretaceous of the West Netherlands Basin contains fluvial deposits of the Delfland Group, overlain by shallow marine sediments of the Rijnland Group. Sequence stratigraphy was applied to both succession, to study the nature of the transgression and to identify possible reservoir-seal pairs. A total of six depositional sequences has been identified using extensive new biostratigraphy and sedimentology. Three of the sequences contain fluvial sediments only, two are mixed fluvial and marine, and one is fully marine. The typical thickness per sequence is 200-400 m. Characteristic for the fluvial sequences is an overall fining-upward pattern. The sequence boundaries have been correlated on logs and on regional seismic. Each of the main depositional settings displays a characteristic seismic facies, which has been used to reconstruct the sedimentological facies distribution. The results demonstrate that the source direction of the Delfland Group was predominantly southeast, controlled by the tectonic grain. Strong thickness variations within the lowermost sequence indicate syndepositional rifting, which confined the main channel systems. The major intervening flood-plain shales correspond to base-level highstands, which implies they can be correlated regionally and may act as intraformational seals. The marine transgression entered the basin from the north during the late Valanginian. Bymore » the early Barremian, it covered the entire basin. Retrograding barrier sands from the transgressive systems tract are the best reservoirs within the marine succession. In both fluvial and marine settings, the use of sequence stratigraphy has enabled recognition of several types stratigraphic traps.« less