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Dolomite Cementation as Prime Control of Reservoir Quality in the Permian Rotliegend Sandstones, Offshore The Netherlands (Block K6) in Relation With Mudstone/Sandstone Ratio

Abstract

The Upper Permian eolian-fluvial sandstones of the Rotliegend formation deposited in the Southern Permian Basin (SPB) constitute important gas reservoirs in The Netherlands and the southern North Sea. It has been documented since the 70’s that the reservoir properties of these deeply buried (~3-4 km) sandstones are strongly affected by diagenesis. However, the prime diagenetic factors impairing reservoir quality and their cause(s) remain somewhat variably interpreted in the literature. In this study, we investigated the Lower Slochteren formation offshore The Netherlands, with particular focus in the K6 block and around, in order to precisely document and quantify diagenetic processes controlling reservoir quality and provide guidance for future exploration. Over 200 samples from 21 wells, covering all main depositional facies (dry eolian, damp eolian, proximal fluvial, distal sheet-flood shaly sands and playa-lake shales), were studied by optical microscopy and point-counting analysis to characterize diagenetic transformations. Diagenetic cements include (chronological order): Fe-oxide coats, dolomite, kaolinite, quartz, siderite, anhydrite, barite and illite. Integration of petrographical observations with log data and core description reveals that the distribution of cements is not driven by depositional facies, and nor are the reservoir properties. Early pore-filling dolomite cement can be as high as 40% in volume and represents the prime control on reservoir properties. Detailed analysis of the spatial distribution of dolomite cement shows that it is distinctly related to mudstone proximity and mudstone/sandstone ratio. Sandstones occurring as thin beds in mudstone-rich depositional sequences (high mudstone/sandstone ratio) typically exhibit strong pervasive carbonate cement regardless of sedimentary facies. In contrast, sandstones forming thick beds in mudstone-poor sequences (low mudstone/sandstone ratio) are commonly free/low in dolomite cement. Our results demonstrate that reservoir quality in the Rotliegend sandstones offshore The Netherlands is primarily controlled by early dolomite cement. The latter is most developed in areas with high (>75%) vertical mudstone/sandstone ratio, where it can constitute a killing factor for the reservoir quality. As a consequence, best reservoir sandstones should be expected where depositional stacking pattern is poor in shaly deposits (playa-lake and distal sheet-flood facies), such as in the southern and southern-eastern part of block K6.