The Unusual Carbonate Reservoir of the Dutch Lower Muschelkalk (Middle Triassic)—Learning Points for Similar Epeiric Dolomites

Borkhataria, Ravi¹, Thomas Aigner¹, Koos Pipping² (1) University of Tübingen, Tübingen, Germany (2) Nederlandse Aardolie Maatschappij B.V, Assen, Netherlands

The mud-dominated Lower Muschelkalk carbonates (Middle Triassic) are a unique reser­voir currently producing gas in the Dutch De Wijk field. Scattered gas shows suggest fur­ther, currently unrecognised potential. This study complements an extensive subsurface data set with selected outcrop information to localise missed opportunities and evaluates the factors controlling the development of this unusual reservoir type.

The “muddy” carbonates are deposits of a storm-dominated epeiric carbonate ramp with negligible depositional gradient. Different mudstone facies types reflect the low energy coastal plain/inner ramp and higher energy mid ramp environments. The best reservoir facies is recognised in distal inner ramp laminated dolo-mudstones (permeability up to 32 mD). Due to the variable intensity of early, facies-related diagenesis the reservoir quality of dolo-mudstones decreases markedly in landward and seaward direction from the inner ramp. The stacking of few decimetre- to metre-thick reservoirs is reflected by a four-fold hierarchy of depositional cycles that can largely be recognised with conventional wireline logs. Stacks of thin-bedded reservoir units most probably pinch out within a few kilometres and their lateral continuity should not be overestimated despite the epeiric “layer-cake” set­ting. Patches of reservoir facies are frequently located above palaeohighs tens to hundreds of kilometres away from the palaeoshoreline. Surprisingly, not only the reduced thickness of the succession above these palaeohighs but also the presence and lateral extent of reservoir patches may be detected with seismic data.

The general patterns in this case may also improve reservoir prediction in epeiric set­tings elsewhere, e.g., the Middle East.