Quartz Cementation in Sandstones: Reappraising the Controls
Richard. H. Worden
Department of Earth Sciences, University of Liverpool, Liverpool, L69 3GP, UK
Quartz cementation, seemingly a mineralogically and texturally uncomplicated process, has led to interesting disputes in recent years. The old allochemical models, linked to large-scale fluid flow, were superseded by simpler isochemical models. These latter models have been pushed to the point of predictive capability, at least partly because they may be linked to basin modelling. All models require assumptions and simplifications. Problems and disputes have arisen with the isochemical models since some of the assumptions and simplifications, essential to facilitate quartz cement prediction, have been treated as being fundamentally true. Some of the assumptions that have been employed are listed in Table 1 with an assessment of their veracity.
The move away from descriptive allochemical models to quantitative isochemical models is to be applauded but only if the assumptions involved in quantification are constantly reappraised. The drive to quantification has led to many older precepts being either ignored or actively challenged. Some of the assumptions seem to have become articles of faith that are followed regardless of substantial bodies of evidence. Scientific hegemony of a model or approach is only welcome if all assumptions and their corollaries can withstand constant examination.
Thus it is held that since quartz grains in oilfields are assumed to be water wet, and since it is assumed that quartz cementation is rate limited by precipitation, then quartz cementation must not be inhibited in oilfields. Well-documented case studies prove that this is corollary is incorrect.
The initial fabrication of quartz cementation growth rates involved an assumption that quartz growth was continuous from the time-equivalent of the lowest fluid inclusion temperature through to the present day, irrespective of gaps in the temperature record. The assumption may be reasonable but it is now commonly held that quartz growth must be continuous since the models tend to produce the right answer. The possibility of episodic but transiently faster quartz growth is now ignored even though it would lead to quite different kinetic output.
Chemical compaction was formerly known as pressure dissolution but the role of effective stress in intergranular dissolution has been questioned recently and temperature alone (±mica crystals at grain boundaries) has been assumed to drive chemical compaction. An interesting hypothetical spin-off of the temperature-driven chemical compaction model is that basin subsidence may be the result of heating and not increasing effective stress. However, it now seems as if effective stress is a prerequisite for intergranular dissolution since there is no real alternative chemical driving force. Basin subsidence due to heating remains a bizarre corollary of models that ignore effective stress.
There remain many possible sources of quartz cement in sedimentary basins (Figure 1). While it is possible that internal sources dominate the budget, there are still the supply and transport steps to consider as well precipitation. While silica may be locally sourced, influx of fluids may drive silica-release reactions. Addition of oil will influence the rates of processes but local wettability will always be an important control on the rate of quartz cement growth. Quartz growth maybe continuous or episodic – the case is not yet proven.
Figure 1: Detailing the key steps, the main options influencing the steps and some intrinsic controls that influence the process of quartz cementation. The bold items represent commonly assumed controls.
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