A review of the Triassic play in the Roer Valley Graben, SE. onshore Netherlands

  • Winstanley A
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Abstract

The Roer Valley Graben is a NW-trending fault-bounded graben marking the eastern limit of the Sole Pit/West Netherlands basins. Sedimentation is composed of four megasequences. The first is a broadly regressive Carboniferous (Visean to Stephanian) sequence which is terminated by the Saalian Unconformity. This is followed by a Zechstein to Middle Jurassic megasequence comprising marine fringe facies and alluvial clastics which grade upwards into open marine sediments of Middle Jurassic age. Megasequence three is composed of syn-rift Upper Jurassic deltaics and is very sparsely preserved due to inversion-related erosion. The final megasequence is composed of post-rift Upper Cretaceous chalk and Tertiary clastics. A major inversion occurred during the Maastrichtian and subsequent erosion removed all the chalk (up to 1000 m) from the graben area but left the sequence on the flanks largely unaffected. Tectonic development of the basin is dominated by the Late Jurassic rifting phase. This was largely extensional; however, a component of oblique-slip motion is evident and is responsible for the intersecting pattern of N–S- and more NW–SE-trending faults. This structural fabric was subsequently overprinted by NW-trending Cretaceous inversion structures. Alluvial fan sandstones within the Main Buntsandstein and Röt formations are the primary reservoir target. Repeated rejuvenation of early faults caused renewed influxes of sediments and stacking of the fan sequences. Subsequent diagenesis, primarily associated with the rifting phase, has reduced porosities and permeabilities. The source rocks for this play are the Westphalian and Namurian coals and shales. These are gently folded and subcrop to the Zechstein-Upper Jurassic sequence across the graben. Peak generation is estimated to have occurred in Early to Late Cretaceous times, prior to the main period of inversion which effectively shut off hydrocarbon generation over much of the basin. Trap development was initiated during the Early Jurassic and developed further throughout the Late Jurassic rifting phase. However, substantial modifications occurred as a result of the Cretaceous inversion and, more locally, relaxation of the graben during the Tertiary. The two discoveries at Waalwijk are early rift (Early Jurassic) tilted fault block structures, which were modified during the Cretaceous inversions. Oil and gas charges followed each other into the reservoirs, with gas probably displacing the oil. Other tested trap types include both pure inversion and inverted rift structures. The latter trap type has met with some success, whereas all wells on the former have been dry to date. The overall success rate for the area is good but not as high as that achieved in the offshore extension of the play fairway. This is primarily a function of the Cretaceous inversion of the Roer Valley Graben which not only downgraded trap integrity and reservoir quality but also shut off hydrocarbon generation.

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Authors

  • A M Winstanley

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