--> Abstract: Central North Sea Hydrocarbon Systems: Generation, Migration, Entrapment, and Thermal Degradation of Oil and Gas, by Gary H. Isaksen; #90914(2000)

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Gary H. Isaksen1
(1) Exxon Production Research Company, Houston, TX

Abstract: Central North Sea hydrocarbon systems: generation, migration, entrapment, and thermal degradation of oil and gas

The high-pressure and high-temperature areas of the Central North Sea constitute an important hydrocarbon province. This includes the deep, Mesozoic reservoirs within UK quadrants 22, 23, 29, and 30.

The Late Jurassic Kimmeridge Clay has been the source for both oil and gas over the entire area, with additional gas charge from the humic coals of the Pentland Formation in the Western Graben areas. The southern Forties Montrose High, with its southward plunging Mesozoic terraces, is host to numerous oil and gas fields with temperatures ranging from 90 °C to 180 °C, and pressures exceeding 0.8 psi/ft. Several of these oil accumulations have undergone in-reservoir thermal cracking resulting in a volatile oil and gas phase together with a pyrobitumen residue in the pore volumes. With several traps at, or near, their leak-off pressure, the likelihood of top-seal failure and gas leakage is prevalent. Such top-seal failure appears to occur intermittently, and in some instances is associated with gas chimneys. The main causes of pressure increase are thought to be volume increases associated with gas generation from source rocks, and thermal cracking of oil to gas. Top seal failure due to pressure buildup and salt diapirism has also resulted in a series of compositionally fractionated oils and gases. A novel technique has been developed whereby the geochemical character of a shallow (Tertiary) reservoired oil which has undergone fractionation can help de-risk the presence of hydrocarbon at depth within potential, deep (Mesozoic) reservoirs.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana