Tectonostratigraphy of the Nieuwerkerk Formation (Delfland subgroup), West Netherlands Basin

Abstract

The Nieuwerkerk Formation is a major Lower Cretaceous synrift and postrift fluvial unit in the West Netherlands Basin (southwest Netherlands) that attains thicknesses in excess of 1 km in places. A strong tectonic overprint on its deposition and a large degree of facies heterogeneity have complicated correlation and greatly hampered understanding reservoir and seal distribution within the unit. The integrated application of fluvial sequence stratigraphic concepts with biostratigraphic dating and the correlation of cycles of changing accommodation to sediment supply ratio (A/S cycles) on three-dimensional (3-D) seismic, well-log, and core data have allowed a much improved understanding of reservoir facies distribution within the Nieuwerkerk Formation. A major intraformational unconformity divides the Nieuwerkerk Formation into two members. The lower of these, the Alblasserdam Member, is predominantly nonmarine and has a significant tectonic depositional overprint. Correlation within this member is dependent on the identification of base-level transit cycles probably induced by pulses of tectonism. The inclusion of 3-D seismic isopach data facilitates mapping thicknesses and reservoir properties of the Alblasserdam Member in areas with no well control. The upper member, the Rodenrijs Claystone Member, was deposited during the postrift stage and is predominantly a coastal-plain succession. Biostratigraphic correlation proved useful in subdividing this unit and correlating key seismostratigraphic markers. Use of biostratigraphic and cyclostratigraphic correlation techniques allowed chronostratigraphically consistent reservoir maps to be made of the constituent members of the Nieuwerkerk Formation. These maps exhibit localized nonmarine syndepositional basins (Alblasserdam Member sand depocenters), followed by gradual southward, landward stepping of facies tracts of the Rodenrijs Claystone Member above the intraformational unconformity. The Rodenrijs Claystone Member is capped by a marine transgression that terminated fluvial deposition in this part of the basin. Interestingly, no major lacustrine facies have been identified in the fluvial units in the West Netherlands Basin, rendering it somewhat anomalous among rift basins.