Integrating Basin and PVT Modeling to Improve Prediction of Fluid Compositional Evolution in High-Pressure/High-Temperature (HP/HT) Settings of the Central Graben, North Sea

Neumann, Volkmar¹, Rolando Di Primio¹, Robert Ondrak¹, Robert Lippmann², Reinhard Gaupp², Brian Horsfield¹ (1) Geoforschungszentrum, Potsdam, Germany (2) University Jena, Jena, Germany

We analyzed the compositional evolution of hydrocarbon fluids on a basin-wide scale using fluid samples and core extracts from high pressure-high temperature (HPHT) reser­voirs in the British and Norwegian Sector of the Central Graben. The results were integrat­ed in 3D-basin modeling in order to reconstruct burial, temperature and HC-fluid flow his­tory of the study area. The Mesozoic reservoirs are situated at 4-5 km depth reaching tem­peratures of up to 192°C and pressures of up to 80 MPa. Reservoir fluids are gas conden­sates and black oils. A compositional kinetic model calibrated with PVT-data from reservoir fluids was been used in the 3D-models to determine timing of first hydrocarbon fluid emplacement, and fluid phase evolution in the reservoir. Results indicate a significant differ­ence in the charge histories of both areas. The reservoir in the UK study area was charged early before significant burial and contained at first a saturated gas phase. With increasing burial, additional kitchen areas became mature and contributed to the HC-fluid in the reser­voir, increasing the liquid content as indicated by decreasing GORs. The fluid phase became undersaturated with onset of overpressure during Late Cretaceous. Present composition and phase behavior are predicted correctly by the model. Fluid emplacement in the Norwegian study area occurred during Late Cretaceous to Early Paleocene as saturated black oil. These fluids increased in GOR during further source rock maturation and by in­reservoir oil to gas cracking driven by reservoir burial. Present undersaturated gas-conden-sate compositions encountered are also correctly reproduced by the phase modeling results.