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The Upper Jurassic carbonates represent important potential targeted reservoirs for geothermal energy in the Geneva Basin (Switzerland and France). Horizons affected by dolomitization, the focus of the present study, are of particular interest because they proved to be productive in time-equivalent deposits currently exploited in Southern Germany. The study is based on sub-surface samples and outcrops in the Geneva Basin. Petrographic analyses allowed to constrain the paragenesis of the Upper Jurassic units prior to discussing the cause(s) and effect(s) of dolomitization. Data reveal that the facies are affected by early and late diagenesis. All samples show at least two stages of burial blocky calcite cementation with the exception of those from the sub-surface, which display an incomplete burial blocky cementation preserving primary intercrystalline porosity. Dolomitization affected all units. The results point to an early dolomitization event, under the form of replacement dolomite. Dedolomitization, through calcitization and/or dissolution, is an important process, creating secondary pore space. Results of the present study favor a reflux model for dolomitization rather than the mixing-zone model suggested in earlier work. However, considering the geodynamic context, other dolomitization models cannot be excluded for the subsurface. The presence of secondary pore space might contribute to the connectivity of the porous network providing enhanced reservoir properties. These results are a first step towards a better understanding of the diagenetic history of the Upper Jurassic in the Geneva Basin. Moreover, it provides a reasonable framework for further geochemical analyses to constrain the nature and timing of fluid migration. The paragenesis and the dolomitization model hold the potential to help in ongoing exploration for geothermal energy beyond the Geneva Basin. [ABSTRACT FROM AUTHOR]

This paper presents a review of the criteria for and limitations to the definition of a usable long distance correlation scheme for the Oxfordian/Kimmeridgian stage boundary, based on the example of the Late Jurassic of the Swiss Jura range. Two main issues and their stratigraphic consequences are more specifically addressed, namely the interpretation of calcareous oolites in terms of depth of deposition and the dependence of ammonite distribution on the paleoenvironment. Calcareous oolites are not diagnostic for depth of deposition, as demonstrated by the conflict with other detailed sedimentary observations. When the sedimentary record is correctly interpreted in terms of water depth, the distribution of ammonites proves to be dependent on the paleoenvironment. Therefore ammonites cannot be directly used as a time correlation tool in a laterally heterogeneous regional context such as the Swiss Jura range. As a consequence, additional proxies are needed to circumvent the limitations of ammonite biostratigraphy in shallow carbonate platform environments. These proxies can ultimately be calibrated with high-resolution ammonite biostratigraphy in basin setting, which remains the most accurate method for long distance time correlation (provided it is not based on endemic or low diversity assemblages). The analysis of the biostratigraphic and lithostratigraphic schemes for the Swiss Jura range with respect to the position of the Oxfordian/Kimmeridgian boundary leads to the conclusion that the former suggestion of a suitable type section on the slope of Summerhalde (3 km NNW of the city of Schaffhausen) has to be replaced by another section outside Switzerland. Indeed, the Oxfordian/Kimmeridgian boundary in northern Switzerland lies within the Knollen Bed (uppermost part of the Gredingensis Zone) rather than at the base of the Reuchenette, Burghorn and Schwarzbach Formations (boundary between Galar and Platynota Zones with low long distance correlation potential). [ABSTRACT FROM AUTHOR]

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