Mark Wilkinson¹, R Stuart Haszeldine¹

(1) Edinburgh University, Edinburgh, United Kingdom

ABSTRACT: Hairy Illite in Oilfield Sandstones - A New Theory of Growth

Does illite date oil migration? Why doesn't illite apparently grow today? Fibrous illite is one of the most damaging clay mineral cements which can grow in a reservoir sandstone, because just a very few percent of pore-bridging fibres reduces permeability by orders of magnitude. Additionally, radiometric K-Ar ages of illite are assumed by many geologists to indicate the time of petroleum charge into the reservoir, although others propose that illite is depth-controlled and there is a thermodynamic temperature barrier to illite growth. The fundamental controls on illite growth are still not understood.

Compiled illite growth temperatures and age dates show that there is no critical temperature or depth control for growth: illite can grow as cool as 50° C or less, before or during oil migration. We propose that the growth of fibrous illite is controlled by nucleation kinetics, and not by thermodynamic or growth kinetic considerations as has been previously assumed. To grow a new crystal it is necessary to create a stable nucleus "seed". Crystal growth theory shows that this is more difficult for illite than for other common authigenic cements (quartz, calcite). Illite crystals are composed of many fundamental particles, which are self-limiting in size. Consequently, enabling a nucleus to grow into a large fibre (10s of micron-scale) requires many nucleation events through time, which is impossible under normal geological conditions. Hence illite growth can only take place during events such as oil emplacement or regional compaction, either by changing porefluid chemistry or by 'stirring' the porefluids.

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