Triassic Reservoir Development in the Northern Dutch Offshore (2016)

This report has been written by former intern Cas van der Kooij.

The Triassic Main Buntsandstein Sugbroup (RBM) play is established in the Southern North Sea. After the Rotliegend play, the RBM play is the most prolific hydrocarbon play in Dutch exploration. In the DEFAB area a P50 GIIP of 80 BCM (unrisked) of gas is still to be found (EBN, 2016). The Lower Volpriehausen Sandstone Member (RBMVL) deposits form the main reservoir rock in the RBM play. It is generally perceived that the reservoir quality decreases towards the north and that Triassic prospectivity is limited in the northern Dutch offshore. However, the northern Dutch offshore is relatively under-explored and only a few wells have actually tested Triassic reservoir rocks in this region. A recent study by EBN (2015) suggests the possibility for an alternative reservoir provenance in the marginal Step Graben. To better understand the Triassic depositional environment a regional study on wells and seismic data in the northern Dutch offshore and the surrounding territories (Five Countries Area) has been performed in this study. Ongoing Triassic sedimentation on top of Zechstein salt deposits often forms sediment pods (e.g. Smith et al., 1993). The formation of these syn-tectonic deposits in local depocentres may have formed due to early halokinesis in the Triassic. In this study accommodation space generation in the Step Graben is assessed by creating timeisochore maps for three Triassic intervals in Dutch blocks A15 and A18. The time-isochore maps show thick time anomalies likely related to early Triassic salt movement and faulting. This study proposes that oblique pull-apart faults are more likely to form at the north and south flanks of the Elbow Spit High, causing enhanced roof top weakening and development of local depocentres. In the observed areas with enhanced accommodation space sand might have accumulated from either a local source or a more distal Southern Variscan source. Two BSc projects have been set-up to provide data on heavy mineral assemblages (sediment provenance) and grain size distribution analysis (transport mechanism) on wells in the  Northern Dutch offshore. Heavy mineral assemblages suggest the contribution from the local Mid North Sea High – Ringkøbing Fyn High. In the local depocentres an interplay between accommodation space generation, erosional processes, and sand supply is established. This interplay plays an important role in the development of potential reservoir rock. A thick Early Triassic interval might indicate axial fluvial sedimentation pathways. Heavy mineral analyses (Vonk, 2016) suggests that both a northern and a southern provenance of sediment is established in the northern Dutch offshore (Step Graben). Triassic reservoir development in the northern Dutch offshore is also seen in the offshore of the United Kingdom and Denmark. Uncertainty of the paleogeography is large, therefore making it difficult to assign local highs as the main sediment source for reservoir rock. Besides the early Triassic RBM sandstones, the Bertel-1 (DK) and A05-01 wells possibly suggest late Triassic Schilfsandstein deposits.