--> ABSTRACT: Source and Timing of Diagenesis in North Sea, Upper Jurassic Reservoirs, by A. D. Carr and I. C. Scotchman; #90906(2001)

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A. D. Carr1 and I. C. Scotchman2

1Advanced Geochemical Systems Ltd., 1 Towles Fields, Burton on the Wolds, Leicestershire, LE12 5TD, UK email: [email protected]
2Statoil (UK) Ltd., 11a, Regent Street, London, SW1Y 4ST UK email: [email protected]

ABSTRACT: Source and Timing of Diagenesis in North Sea, Upper Jurassic Reservoirs

The Upper Jurassic sandstones (e.g. Brae, Fulmar and Piper) form the one of the main pre- Cretaceous reservoirs in the North Sea. These sandstones are interdigitated with the shales of the main source rock, the syn-rift Kimmeridge Clay Formation (KCF), and the link between the maturation of the source and diagenesis in the adjacent reservoirs has been made in a number of studies (e.g. Jeans and Fisher, 1986; McCants and Burley, 1996; Gluyas et al., 2000). A paragenetic sequence from a Fulmar Formation reservoir in the central Graben, although other cements including calcite, dolomite, feldspar, quartz, chalcedony, pyrite, ankerite and illite also occur. In addition early diagenetic (<1 km burial) carbonate concretions have been recorded in a number of Upper Jurassic reservoirs, e.g. Elgin and Franklin in the central Graben (Hendry et al., 2000a), and Miller in the Viking Graben (Gluyas et al., 2000). The main phase of diagenesis and secondary porosity generation however is thought to extend from end Cretaceous until Neogene (e.g. Saigal et al., 1992; Burley et al., 1989; Williamson et al., 1997; Gluyas et al., 2000; Hendry et al., 2000b). An example from the Fulmar Field shows that the Th temperatures in water saturated fluid inclusions occur in the range 85-125oC, and were modeled as being formed from late Miocene to present day (Saigal et al., 1992). Some of quartz cements predate the chalcedony cements, and were thought to be ‘early’ cements (Stewart, 1986), but the high temperature fluid inclusion data suggests that they are late cements. Fluid inclusion and isotope studies on quartz veins in the Fulmar Field showed a Th range of 85- 163oC, and led Macauley et al., (1997) to confirm the late formation of the cements. High salinity aqueous fluid inclusions (15-19% NaCl), and the presence of anhydrite inclusions in the quartz indicate that the quartz precipitated from evaporitic fluids derived from the underlying Permian (Macauley et al., 1997). A Permian source was also suggested by Burley et al. (1989) for the high salinity fluid inclusions (12-16% NaCl) and barite-sulphide cements in the Lowlander structure, Outer Moray Firth (McCants and Burley, 1996).

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado