Fluviomarine sequences in the Lower Cretaceous of the West Netherlands Basin: correlation and seismic expression

Abstract

The Lower Cretaceous of the West Netherlands Basin is characterised by fluvial deposits of the Delfland Subgroup, overlain by shallow marine sediments of the Vlieland Subgroup. Sequence stratigraphy was applied to study the distribution of reservoirs and seals within both successions. A total of six depositional sequences has been identified, using extensive newly acquired 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 to 400 m. Characteristic for the fluvial sequences is an overall fining-upward pattern. The marine sequence starts with a fining-upward interval (Transgressive Systems Tract), followed by a prolonged coarsening-upward interval (Highstand Systems Tract). The sequence boundaries have been correlated on logs and on regional seismic. Each of the main depositional settings displays a characteristic seismic faciès, which has been used to reconstruct the sedimentological facies distribution. The results demonstrate that the main transport direction for sediments of the Delfland Subgroup was SE — NW, controlled by the tectonic grain. Strong thickness variations within the lowermost sequence indicate syn-depositional rifting, which confined the main channel systems. The major intervening floodplain shales correspond to base-level highstands, and it appears that these can be correlated on a regional scale. The marine transgression entered the basin from the north during the Late Valanginian. By the Early Barremian, it covered the entire basin. The overall transgressive regime was punctuated by periods of coastline progradation, which determine the sequence boundaries. Good reservoirs for oil are found in marine transgressive sands (Rijswijk Member), and in prograding shoreface sands (Berkel Sandstone, IJsselmonde Sandstone and De Lier Members), as well as in the fluvial sands of the Delfland Subgroup.