Pressure–temperature–time–composition ( P– T– t– X) constraints of multiple petroleum charges in the Hild field, Norwegian North Sea

Abstract

The Brent reservoirs of the Hild field, situated at 3.8–4.3 km depth in the northern North Sea, show evidence of multiple episodes of diagenetic events, fracturing and petroleum charging. Data from fluid inclusions and drill stem test samples characterise the petroleum filling history of the Hild field. Two main types of petroleum inclusions occur. Type 1 inclusions, which occur in two of the fault compartments of the field, show a ‘heavier’ composition than the present reservoir fluids. From a comparison of fluid inclusion analyses and burial history, onset of reservoir filling is most likely represented by type 1 inclusions and took place at 125–130 °C in well 30/7-8R. This temperature corresponds to a burial depth of 3.5–3.7 km, which occurred 35–32 Ma ago. Type 2 inclusions occur in all wells and are compositionally similar to the reservoir fluids, suggesting recent trapping. Considerable differences in homogenisation temperature and phase behaviour of type 2 inclusions between the three wells of the Hild field indicate the compartmentalised nature of the reservoir. PVT modelling of fluid properties and trapping conditions was based on microthermometry, GC data for type 1 inclusions and present-day fluid compositional data for type 2 inclusions. The results show generally good agreement between estimated pressure for type 2 inclusions and present-day reservoir conditions, and an increase in overpressure between trapping of types 1 and 2 inclusions. We suggest that quartz cementation may continue after onset of reservoir filling, and may exert major control on the pressure evolution in deep reservoirs.