An unusual, muddy, epeiric carbonate reservoir: The Lower Muschelkalk (Middle Triassic) of the Netherlands

Abstract

The mud-dominated Lower Muschelkalk carbonates (Middle Triassic) are a unique reservoir currently producing gas in the Dutch De Wijk field. Numerous gas shows scattered over the Netherlands onshore and offshore suggest further, currently unrecognized potential. This study complements an extensive subsurface data set with selected outcrop information to localize 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 marlstone, dolomudstone, and lime mudstone facies types reflect the low-energy coastal-plain to inner ramp and higher energy midramp to proximal outer ramp depositional environments. The best reservoir facies is recognized in distal inner ramp algal dolomudstones (porosity up to 24% and permeability up to 32 md). Because of the variable intensity of early, facies-related diagenesis, the reservoir quality of dolomudstones decreases markedly in landward and seaward direction from the distal inner ramp. The stacking of decimeter- to meter-thick reservoirs is reflected by a fourfold hierarchy of depositional cycles that can largely be recognized with conventional wire-line logs. High-resolution outcrop and subsurface correlations reveal that stacks of thin-bedded reservoir units most probably pinch out within a few kilometers, and that their lateral continuity should not be overestimated despite the epeiric layer-cake setting. Local reservoir occurrences are commonly situated above paleohighs of tectonic or halokinetic origin. Not only the reduced thickness of the Lower Muschelkalk above these paleohighs, but also the presence and lateral extent of local reservoirs, might be detected with seismic data. The general facies and reservoir patterns exemplified by the Lower Muschelkalk may also be of use for reservoir prediction in similar epeiric settings elsewhere, for instance, in the Middle East.