Cycle stacking pattern, diagenesis and reservoir geology of peritidal dolostones, Trigonodus-Dolomite, Upper Muschelkalk (Middle Triassic, SW-Germany)

Summary Peritidal dolostones (Trigonodus-Dolomite) characterize the back-bank environment of the Upper Muschelkalk (Middle Triassic) carbonate ramp of SW-Germany. These deposits represent the Late Highstand Systems Tract (HST) of the ‘Third-Order’ Middle to Upper Muschelkalk depositional sequence. The HST forms an overall shallowing-upward trend and is build by a progradational stack of 1–2 m thick shallowing-upward cycles. The latter vary from subtidal-to-intertidal cycles at the base of the investigated section to intertidal-to-supratidal cycles at the top of the section. Six major facies types can be recognized: subtidal associations are characterized by oolithic grainstones, lagoonal oncolithic wackestones and peloidal mudstones. Intertidal associations are characterized by ostracod wackestones and laminated mudstones, supratidal facies consist of laminated mudstones with tepee horizons and flat pebble conglomerates as well as paleosol horizons. Thin section petrography, cathodoluminescence-microscopy and stable isotope geochemistry reveal a complex dolomitization history (evaporative dolomitization; burial dolomitization). The strong negative oxygen isotope signatures(−3.28 to−5.85‰) point out burial dolomitization as the dominant stage. The Trigonodus-Dolomite shows intercrystalline porosity and some vuggy porosity. Subtidal dolo-grainstones with idiotopic texture at the base of the investigated section have fair permeabilities (5–30 mD) and high porosities (14–32%). Inter-to supratidal dolo-wackestones and dolo-mudstones with xenotopic texture at the top of the section have very low permeabilities (0.3–1.0 mD) and lower porosities (11–16%). The reservoir characteristics with lateral continuity of porous and permeable zones at the base of the section and less porous and impermeable zones at the top again reflect the stacking pattern of shallowing-upward cycles within the overall shallowing-upward trend of the HST. Primary facies and dolomitization processes thus control the distribution of porosity and permeability.