The Uppermost Ablasserdam Member and Delft Sandstone Member are of high interest for geothermal energy extraction. The stratigraphy, sedimentology, and depositional environment of these members have been studied by different researchers over time. However, due to the lack of physical and extensive logging data within the West Netherlands Basin, there is still uncertainty about their depositional environment and how these sandstone bodies are vertically and laterally connected. Without this data, determining the depositional environment is challenging, and so is developing an optimized reservoir model.

This study performs a sedimentological analysis on the core, sidewall cores, and plugs, together with an investigation of the wireline logs of the DEL-GT wells. The main goal behind this is to determine the different lithofacies types and facies associations and interpret this while considering the limitations of the available data.

The Uppermost Ablasserdam Member and Delft Sandstone Member show characteristics of a fluvio-deltaic system and is located in an upper to lower delta plain environment. With observed facies associations of multi-stacked fluvial to distributary channels, single fluvial channels, crevasse splays, levees, overbank fines, and paleosols. The wireline logs help determine the vertical distribution of the facies, which showed a trend and lateral heterogeneous distribution between the wells. Thus, I could not correlate the wells to each other due to the lack of clear markers and indications that certain sandbodies are connected. Hence, more research on this is needed.

The wireline logs did show a trend from more sand-prone deposits to more silt-prone deposits, which can be related to the the different allogenic and autogenic controls. The controls likely influencing the depositional patterns are the tectonic activity and eustatic sea level change. In combination with climate change affecting the sediment supply and discharge rate, which influences the autocyclic controls of the channels.

This study is based on physical data from the first 80m interval of the one dimension DEL-GT-01 and side wall cores from DEL-GT-02-S2 wells. This brings an uncertainty in determining the lithofacies types and facies associations. Broader research must be done not only on the lithofacies types and facies associations but also on the lateral and vertical distribution. This will help determine the determine the limits of the reservoir.

Moreover, this study brings attention to the importance of extensive core and wireline log data collection.This data will reduce uncertainty for the reservoir model and increase the efficiency of the project and other future projects. It is also highly advised that all the findings and data should be open and free to use for future projects to reduce the uncertainties within the reservoir model.