Calum Ian Macaulay1,
R. Stuart Haszeldine1,
Colin M. Graham2,
Anthony E. Fallick3
(1) University of Edinburgh, Edinburgh, Scotland
(2) University of Edinburgh, Scotland
(3) Scottish Universities Research and Reactor Centre, East Kilbride, Scotland
Abstract: Silicon isotopes identify sponge spicules as source of silica cements in North Sea oilfield sandstones
The sources of quartz cements in
sandstones are difficult to constrain and in many sandstones are not
unambiguously identified from petrography. We have used silicon isotope
analyses to examine the possibility that biogenic silica has measurably
contributed to quartz cements in Upper Jurassic marine sandstones from the
northern North Sea. Previous workers have shown that biogenic silica in sponge
spicules has 30Si values as
much as 3.5‰ lower than silicate minerals in most rocks. Silicon isotope
compositions were measured in this study using an ion microprobe, which allows
high resolution (20µm) in-situ analysis of quartz overgrowths and growth zones
as well as detrital grains.
Sponge spicules and quartz
cements have been identified in the shallow marine sandstones of the Fulmar
Field, and in the deep water sandstones of several fields with reservoirs in
the Brae Formation. Sponge spicules are abundant in Fulmar and silica cements
occur as chalcedony, minor quartz overgrowths and rare quartz veins. Quartz
overgrowths are much more abundant in the Brae Formation but sponge spicules
are much less common. Results from the Fulmar field show detrital grains with 30Si ~ 0‰ (relative to NBS28) but
vein quartz with
30Si ~ -3‰
suggesting a sponge silica source. In contrast, analyses of quartz cements and
detrital quartz from the Permian aeolian sandstones from the southern North Sea
yielded values of
30Si ~ 0‰
for both. This confirms that silica cement has more than one source. Accurate
prediction of silica cement may need calibration by micro-analyses.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana