Basin Analysis of Tertiary Deposits in the Gorredijk Concession Using 3D Seismics and Well Data (2011)

Report by former intern Anja Dijkhuizen.

The 1000m thick Tertiary sequence present in the Dutch onshore subsurface can hold prospective reservoirs of shallow gas. In this report, an inventory of Tertiary reservoirs is made for the Gorredijk concession (Friesland, northern Netherlands). By using well, log and seismic data, the structural elements of the studied area are investigated. Thickness maps of the three Tertiary groups show a thickening trend towards the west. However, individual differences exist within the groups, implying basin shifts through the Tertiary Period. Gas migration would therefore be to the east, but timing is important due to the differences within each group. Indications of an erosive surface in the lower Tertiary Brussels Sand Member give more insight in the different Alpine phase pulses during the Tertiary. A large erosive event during the Miocene has eroded parts of the Upper Tertiary deposits. The associated angular unconformity, the ‘Mid Miocene Unconformity’, is shown not to be of tectonic but of sedimentary origin. Large Neogene delta foresets from the Eridanos deltaic system onlap on the Mid Miocene Unconformity. These delta deposits are so far only described to be found offshore. No closures from structural maps of key well tops and related seismic reflectors are found within the Tertiary deposits. The only prospective formation is the Lower Tertiary ‘Basal Dongen Tuffite Member’, which is known as a producing formation in areas around the studied area.