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ABSTRACT

The Permian Rotliegend clastic reservoirs form the main gas-bearing intervals in the Netherlands, Northwest Germany, and the Southern North Sea. We review and summarise the results of more than thirty years of diagenesis and reservoir-quality studies in Rotliegend sandstones of the Netherlands and adjacent areas.

The Rotliegend sediments were deposited in an alluvial–wadi–aeolian dune–sandflat–playa lake depositional setting, marked by an arid to semiarid climate, in which drier and wetter climatic cyclicity drove sedimentary processes. Present depths of Rotliegend reservoirs in the range of 2 to ca. 4.5 km, are often referred to as maximum burial depths. Rotliegend clastics were exposed to temperatures between 60°C and180°C. Structural uplift during Late Jurassic and Cretaceous times influenced the Rotliegend pressure and fluid-flow regime.

The paragenetic sequence is spatially variable and comprises a wide variety of authigenic minerals, with several early cements typical of continental red-bed sequences. Characteristic of semiarid (to arid) environments is reddening and the presence of grain-coating metal (Fe, Al, Mn, and Ti) oxides as well as smectitic, illitic, and chloritic grain-coating clays. Early blocky and often pore-filling cements include dolomite, gypsum, anhydrite, and halite, but also quartz and K- and Na-feldspar overgrowths. Burial-related authigenic precipitates are Fe-dolomite, calcite, siderite, and quartz cements, kaolinite, dickite, chlorite, and mainly fibrous illitic clay.

Dissolution of feldspar and volcanic rock fragments, of soluble pore-filling carbonate, sulphate, and halite cements, and the formation and destruction of secondary porosity are important factors in determining current reservoir properties. Most of the variance in porosity and permeability can be explained by a small number of significant variables: carbonate (and anhydrite) cementation, initial mineralogy, grain size, clay matrix content, diagenetic clay association, diagenetic quartz, and feldspar dissolution. Pore-blocking anhydrite and carbonate cements are the most pronounced phases that impacted on porosity. Impairment of permeability is due mainly to authigenic clays (illite, kaolinite, chlorite).

Even after pronounced diagenetic alteration the depositional setting remains as an important control on overall reservoir quality. Aeolian dunes and dry aeolian sandflat deposits remain the best potential reservoirs even under deep burial. However, pore-occluding blocky cements, mechanical compaction, or clay growth can heavily impair reservoir quality in optimum depositional facies, particularly under extended exposure times to high temperatures. Long-term or continuous gas fills preserve favourable reservoir properties. The spatial proximity of Carboniferous source rocks to Rotliegend reservoirs in the Netherlands is considered to be a smaller risk for reservoir quality compared to northern German subsurface analogues.

Reservoir characterisation studies spanning more than three decades clarified the mechanisms, controlling factors, and relative timing of many diagenetic processes, but uncertainties about the quantities of resulting products remain. The multitude of interfering factors that control Rotliegend reservoir properties and the geological heterogeneity in the area does not favour conceptual models of regional applicability. Evaluation of the existing concepts on Rotliegend reservoir quality indicates the necessity of combining all available data to constrain the complexities of depositional facies, diagenesis, structuration, and charge history in the specific cases.

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