Gas-Water Contacts, Free Water Levels Analysis in Support of Petroleum Exploration in Offshore Netherlands (2017)

This MSc thesis has been written by former intern Malik Belabed.

Hydrocarbon (HC) contact depths, e.g. gas-water contacts (GWCs) in a HC reservoir are crucial for volumetric and petrophysical calculations. It is therefore of great importance to determine the GWC and free water level (FWL) correctly and to understand the uncertainties and (often financial) limitations these determinations inherently possess.

The aim of this study was to enhance the subsurface knowledge of the K- and L- blocks of offshore Netherlands through an analysis of HC contact depths using operator data, log evaluation, pressure data and saturation modelling. The goal was also to ultimately assess the significance of HC contact depth and uncertainty in support of petroleum exploration, and to provide innovative new ways to asses reservoir characteristics using HC contact depths.

Using data from approximately 122 wells, the K- & L-blocks of offshore Netherlands have been mapped using operator-sourced data, and verified using pressure data, quick-look petrophysical analysis and saturation modelling. The Permian basins form the overwhelming majority of data with 81% being Upper or Lower Slochteren Member. An uncertainty analysis of the Slochteren Formation shows that the pore throat radius is the most important variable for the capillary height. The capillary height is defined by the difference in depth between the GWC and FWL. The operator standard deviation in FWL/GWC height is approximately 55 m for the Slochteren Formation.

Using data visualisations, it can be concluded that the age of the formation containing the HC contact increases going westwards. Triassic gas-filled reservoirs are solely present in the easternmost region of the L-block, whereas the oldest gas-filled Carboniferous reservoirs are present in the northwestern part of the K-block. The Slochteren Formation also displays a clear separation, with the Upper Slochteren gas-fill being situated more in the southeastern part and the Lower Slochteren towards the northwestern part of the region.

The HC contact database can be used to validate the compaction of reservoir materials judging by the change in the capillary height, and the facies distribution of the Upper Slochteren can be verified using the HC contact database. Assuming that the other variables in the capillary equation remain constant, we can see that the pore throat radius reduces with approximately 28.8% in the fluvially dominated part of the Upper Slochteren sandstone.