Hydrodynamic model of the Troll Field reservoir.

The main goal of this work is to provide a description of the Troll Field reservoir hydrodynamic model, integrating the depositional and structural model with formation fluid pressure data. First of all, an overall overview of the North Sea Basin inception, with structural and stratigraphy evolution, is given, being followed by a description of facies and paleo-environments of the Sognefjord Formation. The Sognefjord Formation is an important reservoir for oil and its paleo-environment is given by cyclic progradation of spit system, with intervening flooding events and tide-dominate coastal plains eastwards. A typical vertical section of the Sognefjord formation is characterized by offshore fine- to medium-graded bioturbated transition sandstones, passing upward to lower shoreface sandstones, up to shoreface and foreshore coarse-graded sandstone. Intervening thin siltstone intervals mark flooding events. The offlapping geometry of this succession reflects the progradation of spit system, which is flanked eastwards by muddy- tide-dominated coastal plain deposits with ondulatory geometric in seismic section. In tidal facies heterolithic deposits were developed in tidal flats or tidal channel, whereas seawards mouth bars, sandy ridges, muddy shelf and prodelta deposits can be found in some area. A faults swarm within the Troll Field developed during the Kimmeridgian and affected the reservoir units. After describing Troll Field paleoenvironment, a batch of formation fluid pressure data have been collected from seven wellbores and then plotted in a pressure-depth diagram. Fluid pressure gradients have been calculated and then compared with theoretical hydrostatic ones. The quantification of the shift between the formation fluid pressure data and theoretical hydrostatic gradient has allowed to calculate overpressure values. Both gas-oil- and oil-water-contact have also been measured, whose related depths have provided a first clue of poor laterally communication. The presence of this overpressure system is justified by existence of the lateral and overlying, poorly porous-permeable formations. These units do not enable fluids to drain. Once differences of overpressure among wellbores have been defined, a depositional and structural model has been developed. This model takes into account the alternation of sandstone and siltstone that point to a “shale smearing” model, considered to be the main control factor of the seal efficiency and responsible of overpressure differences.