Bioclast-controlled patchy barite cementation – Origin and impact on reservoir properties in deeply buried Upper Jurassic sandstones, North Sea

Reservoir non-destructive patchy barite cement occurs in volumes up to 16 vol% in deeply (>5150 m) buried shoreface sandstones in the Hejre field, Danish Central Graben, North Sea. Barite typically occurs as an accessory cement in the sandstones of the Upper Jurassic Heno Formation; however in the Hejre field, barite appears more abundant in the crestal-most well than in other wells. The purpose of this study is to describe the characteristics and the origin of patchy barite cement besides implications for reservoir properties. Petrographic investigations by optical and scanning electron microscopy supplemented by isotopic analysis by SIMS (secondary ion mass spectroscopy) were applied to reveal the barite cementation process. Porosity and permeability of the sandstones were measured by conventional core analysis. Barite cement occurs in the Hejre field with three morphologies: crystals arranged in rosettes, crystal fillings of bioclasts, and multiple-oriented intergrown small crystals in veins. SIMS showed similar sulphur and oxygen isotopic compositions for these barite morphologies, and this composition resembles evaporite-derived barite cement. A hydrothermal origin of the fluid is supported by the associated precipitation of pyrite, chalcopyrite, sphalerite and galena, and a high Sr/Ba ratio in barite. Conventional core analysis revealed major reductions in porosity and permeability when barite cement occurs as fracture filling or in combination with calcite cement in shell lags. However, these occurrences are rare and have a minor influence on the overall reservoir properties. Minor reduction of porosity and permeability characterizes sandstones with barite cement in rosettes of radiating crystals. This growth pattern was probably promoted by slow seep of sulphate-rich fluids which, by mixing with barium-bearing formation water became super-saturated with respect to barite as they moved towards lower temperature and pressure zones. Faults acted as pathways for transport of sulphate-rich fluids possibly from evaporite deposits, whereas barium may have originated from compactional fluids leached out of underlying volcaniclastic rocks. Thermochemical sulphate reduction led to precipitation of sulphide minerals, minor sulphur fractionation and fluid acidification, whereby calcareous bioclasts dissolved. Barite precipitated when the fluids entered these large- or over-sized pores with small obstacles causing prolonged fluid-rock interaction. The bioclast-replacing and patchy barite cement caused only limited reduction of reservoir porosity and permeability compared with barite-free sandstones from the Heno Formation found at a similar burial depth.