The Progress of Geophysical Reservoir Description at Hibernia

Hibernia is located at latitude N. 46°44', longitude W. 48° 46' on the continental margin of the North Atlantic Ocean. Water depth is 78 metres, deepening east of the field as the Grand Banks of Newfoundland descend to the abyssal plain. The local marine environment is severe with storms, 20 metre waves and frequent icebergs demanding attention in both the design and operational phases of the project.

Ten Hibernia delineation wells were drilled during the period from its discovery in 1978 to 1985. This was a period in, which borehole compensated sonic and density logs were technologically mature. Hole conditions were generally adequate to obtain good sonic information but density logs were often affected by washed-out intervals. Vertical seismic profiling was not yet being done in the high cost regime of the Newfoundland Offshore. Checkshot surveys were obtained at all but one of the delineation sites.

Two 3D seismic surveys have been acquired across the field, one in 1980/81 and another in 1991.

Data quality in the early survey was good but positioning accuracy was lower than required for application as well piloting data in high angle drilling. Temporal resolution was improvable through reduced streamer depths and digital cable technology. Likewise, spatial resolution was improvable through the reduction of line and trace spacing available via multistreamer and multi-source equipment. The new seismic survey covers an area of approximately 450 square kilometres.

The dominant lithologies in and above the field are thick sequences of Cretaceous and Tertiary siliciclastic rocks. However, significant seismic reflections in the field originate from four thin limestone strata, three between the two principal reservoir zones and one above them. These limestone horizons are critical in tying seismic character to well log data and in correlating from one fault block to the next. Amplitudes within the reservoir zones themselves depend on weak impedance contrasts between oil or gas bearing sandstones and the intervening shales.

Through the main reservoir zone infield wells typically encounter 75 m of net sand with porosities of about 15%. These wells are expected to produce approximately 15,000 bbls/d.

Faults are abundant throughout the Hibernia field by virtue of its position at the junction of two basin scale faults. The Murre fault forms the western boundary of the host basin and the Nautilus fault forms the northern boundary of the field. At the level of the primary reservoir (approximately 3500 metres) there are three fault systems two of which extend upwards to the secondary reservoir (approximately 2500 metres).

Mapping individual fault planes contained within these systems was necessary for three reasons. First they constitute the migration path from Jurassic source rocks to Cretaceous reservoirs. Second, comprehensive fault identification was required for mapping unconformities through faulted areas. Third the assessment of reservoir connectivity through the field depends on fault seal analysis. The mapping process was simplified by adherence to specific procedures which will be described.

Seismic fault plane mapping described at the luncheon, together with data from reservoir mapping, clay smear analysis, geochemistry, and RFT pressures allowed the creation of a self-consistent distinction between sealing and nonsealing faults. The resulting reservoir description was simplified through the identification of megablocks which are expected to be unique with respect to fluid contacts and compositions.