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AAPG Bulletin, V.
Central North Sea hydrocarbon systems: Generation, migration, entrapment, and thermal degradation of oil and gas Gary H. Isaksen1
1ExxonMobil Exploration Company, P.O. Box 4778, 233 Benmar, Houston, Texas 77210-4778; [email protected]
Gary Isaksen coordinates geoscientists and technical programs in ExxonMobil's Upstream Geoscience companies. Isaksen holds B.Sc. and M.Sc. degrees and a Ph.D. from the University of Bergen, Norway. Since he joined Exxon in 1985, he has worked on hydrocarbon system analyses in numerous sedimentary basins worlwide as well as the application of petroleum geochemistry to development and production.
I thank ExxonMobil International Ltd., United Kingdom, Shell Exploration, United Kingdom (especially Christian Bukovic), BP United Kingdom, and Total/Fina/Elf, United Kingdom, for permission to publish these results. This work is based on a regional central North Sea study carried out at Esso Exploration and Production, United Kingdom, during 1994–1995 and was presented at the AAPG Annual Meeting in New Orleans during April 16–19, 2000. Special thanks is extended to my colleagues on this project: Glen T. Cayley (project leader), Tore Husmo, Morten Rye-Larsen, Dave S. Walley, William A. Symington, and Scott K. Jonsgaard. I also appreciated the many discussions I had on central North Sea geology with coworkers Dave Converse, Paul Nicholson, Dan E. Helgeson, Duncan Erratt, Lochlann L. Magennis, and Geoffrey W. Farquharson. Finally, I thank the technical reviewers Alan R. Carroll, John Lorenz, and an anonymous reviewer for improving the manuscript.
The high-pressure and high-temperature (HPHT) areas of the central North Sea constitute an important hydrocarbon province. This includes the deep, Mesozoic reservoirs in United Kingdom quadrants 22, 23, 29, and 30. This study was undertaken to better understand oil and gas compositional histories in HPHT hydrocarbon systems and to help identify new exploration opportunities.
The Late Jurassic Kimmeridge Clay Formation has been the source for both oil and gas over the entire area, with additional gas charge from the humic coals of the Middle Jurassic 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 to 180C and formation pressures whose gradient to the surface exceeds 0.8 psi/ft (0.192 MPa/m). Several of these oil accumulations have undergone in-reservoir thermal cracking, resulting in a lighter, single-phase fluid, 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 is intermittent and, in some instances, is associated with gas chimneys. The main causes of pressure increase in Mesozoic sediments are thought to be volume increases associated with gas generation from source rocks, clay dehydration, and thermal cracking of oil. Top seal failure because of pressure buildup by salt diapirism and the buoyancy of large hydrocarbon columns has resulted in a series of compositionally fractionated oils and gases. A new technique is presented, whereby the geochemical character of a shallow (Tertiary) oil reservoir that has undergone fractionation can help lower the risk of detecting the presence of hydrocarbon at depth in potential, deep (Mesozoic) reservoirs.
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