Diagenetic Evolution and Inorganic-Organic-Interaction in Triassic High-Pressure/High-Temperature (HP/HT)-Reservoirs in the Central Graben, North Sea

Ondrak, Robert¹, Robert Lippmann², Reinhard Gaupp², Rolando Di Primio¹, Brian Horsfield¹ (1) Geoforschungszentrum, Potsdam, Germany (2) University Jena, Jena, Germany

High pressure- high temperature clastic reservoirs at 3-5 km depth in the Jade and Judy fields (UK) are typical features of the North Sea Central Graben. We focused on detailed descriptions of petrography and quantitative analysis of mineral compositions to decipher the diagenetic history of clastic sediments from the Triassic Skagerrak Formation within the framework of modelled burial and temperature histories of the study area. We recognized that distribution and evolution of eodiagenetic phases were predominantly determined by depositional facies. During mesodiagenesis sandstones were strongly affected by interac­tions with interbedded mudstones and aggressive fluids enriched with organic maturation products, causing the dissolution of carbonates and feldspar. Best reservoir properties appear to be confined to high porosity zones in the central sections of sand-intervals where additional porosity probably was created by mineral dissolution. We found evidence that the reaction products caused the precipitation of ankerite or were consumed by mineral reac­tions in the surrounding mudrocks. Geochemical modeling was used for mass balance cal­culations and to derive initial fluid compositions for reactive-transport modeling which was applied to study the interaction between sandstones and mudstones. First results of 1D-reactive-transport modeling indicate that K-feldspar dissolution and albite formation is affected by organic maturation products formed during thermal maturation. Modeling results also support the assumption that diffusive transport of silica derived from clay min­eral alteration in the mudstones caused quartz cementation of the sandstones. No evidence was found for an influence of overpressure on diagenetic evolution of the reservoir rocks.