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The Njord Field: a dynamic hydrocarbon trap

1997; Elsevier BV; Linguagem: Inglês

10.1016/s0928-8937(97)80018-5

2212-1390

Trond Lilleng, R. Gundesø,

Geological and Geophysical Studies

The Njord Field is located at Haltenbanken 30 km west of the Draugen Field at a water depth of 325 m. The field was discovered in 1985. Plans for the development and operation of the field were submitted to the authorities in Spring 1995 and planned production start-up is Autumn 1997. The expected recoverable oil reserves for the main production phase are estimated to be 32 MS m3. The field is covered by high quality 3-D seismic and has been delineated by seven wells. Four wells proved to have producible hydrocarbons in marginal marine, heterogenous sandstone reservoirs of Early-Middle Jurassic age (Tilje and Ile Formations). The Njord structure developed during the Late Jurassic by downfaulting and rotation of a large hanging-wall fault block along a major listric shaped fault plane belonging to the Vingleia Fault Complex which separates the Frøya High from the Halten Terrace. The structure is compartmentalized by a complex set of faults. Although no firm fluid contacts have been proven by the wells, a total hydrocarbon column of approximately 400 m is inferred from formation pressure data. The Late Jurassic Spekk Formation represents the major source rock, charging the structure, during Eocene to Recent. Within single reservoir units, formation pressure data indicate lateral stepwise increasing overpressures from approximately 70 bar above hydrostatic in the south-east to approximately 120 bar in the north-west, controlled by major northeast trending sealing faults, which subdivide the Njord structure into a series of hydraulic compartments. There is also a stepwise formation pressure increase with depth, corresponding to Triassic and Jurassic stratigraphic boundaries. The main reason for the observed overpressures is believed to be related to the dramatic increase in subsidence rate of the Njord area during the Pliocene to Recent causing a rapid increase in overburden loading and a renewed pulse of intense fluid charge to the reservoir units. Sealing along the major northeast trending faults has prevented lateral pressure dissipation, and allowed formation pressures to reach the level controlled by the vertical top seal strength of the structure. Residual hydrocarbons within the Triassic and hydrocarbon shows within the Cretaceous overburden support the concept of a dynamic model with an element of active vertical flux through the Jurassic sequences implying breaching of the reservoir top seal and vertical leakage. The relationships between the pore-pressures of the Jurassic reservoirs, the estimated overburden pore pressures and the formation integrity trends of the structure are taken to suggest that capillary entry pressures (membrane seal failure), possibly in combination with cap rock microfracturing, are the main controlling mechanisms for vertical leakage. A proper understanding of the above items, including maturation and filling history, formation pressure distributions, intra-reservoir communications, fault and top seal potentials, and leakage mechanisms, is considered essential for resource assessment, safe drilling of further exploration/delineation and production wells, and for reservoir management and production planning of the Njord Field.