The Chemical Composition of Authigenic Illite within Two Sandstone Reservoirs as Analysed by ATEM

Analytical transmission electron microscopy (ATEM) was used to obtain chemical analyses of single illite crystals from Upper Carboniferous reservoir sandstones ofthe Bothamsall Oilfield, East Midlands, UK, and from Rotliegendes (Permian) sandstones of the North Sea. All samples were found to be highly aluminous, with only minor Mg and Fe, and to have near-ideal dioctahedral sheet otals. No significant var!ation in chemical composition of illite was found within the Bothamsall reservoir rocks, irrespective of paragenesis or strati-graphic horizon. Similar esults were obtained from the Rotliegendes samples. Variation was found, however, between Bothamsall and Rotliegendes analyses populations. The Rotliegendes illites were distinctly more K-rich, which is the result of greater charge deficiency in the octahedral sheet. These results indicate that all the illite in both reservoirs precipitated in equilibrium with the reservoir pore-fluid. Furthermore, they imply that the physico-chemical composition fthe pore-fluid id not evolve significantly between the different iUite generations in the Bothamsall samples. These data, when compared with published analyses of authigenic illite, indicate that he compositional field for illite in sandstones is restricted incomparison with that of illite in mudrocks.