Relating Main Buntsandstein paleo-environments to present-day porosities in the West Netherlands Basin

Abstract

One of the most promising geothermal plays of the Dutch subsurface is the Lower Triassic Main Buntsandstein Subgroup of the onshore West Netherlands Basin, SW Netherlands. The relation between present-day porosity of Main Buntsandstein strata and burial and diagenetic history of the subgroup is extensively studied, but the relation of present-day porosity with Main Buntsandstein depositional environments and synchronous topography has not received as much attention.

I studied the Main Buntsandstein Subgroup of the onshore West Netherlands Basin along 5 seismic cross sections and in 35 wells. I researched gamma ray, sonic, bulk density, and neutron porosity logs and a total of 361 m of cores from 15 of these wells. The study of petrophysical logs resulted in the description of wireline shapes and ranges, whereas the core study enabled the interpretation of 12 different depositional environments and description of corresponding lithofacies. A broad range of continental depositional environments—i.e. fluvio-lacustrine to eolian—is recognised. These depositional environments are grouped into 4 petrophysical facies groups by combing core and wireline descriptions. These facies groups and subordinate depositional environments are included in a classification key that provides a comprehensive way of describing Main Buntsandstein wireline and lithofacies. This approach solves the lack of unambiguous nomenclature and description in wireline and lithofacies in previous research. Another utilization of the key is the prediction of petrophysical facies groups from wireline data. This enables extrapolation of petrophysical facies groups throughout the study area. This extrapolation shows that the basin centre is characterised by two zones of confined flow environments—such as braided river channels—which are parted by unconfined flow environments, like floodplains and lakes. Synsedimentary faulting and resulting paleo-topography influenced the distribution of Main Buntsandstein depositional environments.

The interpreted seismic cross sections shows the existence of two non-synchronous fault-bounded zones which were generally active during the deposition of different Main Buntsandstein members. Paleo-topography influenced the distribution of depositional environments by 1) migrating confined and unconfined flow zones as a consequence of emerging fault-bounded paleo-swells and 2) alternating the presence of dry or damp eolian sandflats in the Hardegsen fringe area. The fringe area holds the highest porosities throughout the entire Main Buntsandstein Subgroup, regardless of the locally dominating petrophysical facies group.

Differential burial and diagenetic history are most important in determining porosity trends in the Main Buntsandstein West Netherlands Basin. Porosity generally decreases as a result of cementation by diagenetic processes like dolomitisation, anhydritisation, and pyritisation but locally increases due to the creation of secondary porosity. The major influence of burial history is emphasised by the fact that porosity trends tend to follow Jurassic fault patterns that are representative for times of both Jurassic–Early Cretaceous deep burial and Late Cretaceous–Paleogene inversion. However, diagenetic processes are occasionally facies-related. It is therefore required to clarify the relation between porosity and distribution of paleo-environments to fully understand the present-day porosity in the West Netherlands Basin’s Main Buntsandstein Subgroup.