Abstract

Pressure solution

If there were a grain-boundary solution phenomenon due solely to pressure it would be a function of depth of burial and have important consequences for the prediction of porosity reduction trends in hydrocarbon reservoirs. However, many of the interpenetrant grain contacts that have been illustrated by speakers at this meeting and termed 'pressure solution' require additional explanation. From our own work on both outcrop and core materials we have been unable to find any convincing correlation between burial depth and 'pressure solution'—for a variety of facies. Furthermore, Hancock (1978) does not claim to have seen pressure-solution phenomena although his sediments were recovered from a greater depth than any others described at this meeting.

We suspect that the nature of the pore fluids flushing through the sediment is very important, particularly where grain-boundary reactions have been catalysed by surfactant coatings such as clays and micas. Reservoir sandstones did not become 'closed systems' until hydrocarbon emplacement was completed. Before this occurred pore-fluid evolution was probably continuous.

Fibrous illite

In our own studies on offshore core material from probable Old Red Sandstone in the Buchan Field, we have found fibrous outgrowths on clays and micas (Pl 1, fig. 1) in fractures. In this case, therefore, the fibres post-dated fracture development.

We are not convinced that the fibres are simple illites (as has been proposed by several speakers) for the following reasons:

1. Using non-dispersive X-ray analysis it may be possible to show that the fibres are composed mainly of K2O,

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