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