The influence of clay mineralogy on formation damage in North Sea reservoir sandstones: A review with illustrative examples

This paper critically reviews the clay mineralogy of reservoir sandstones in the North Sea, as assessed from peer-reviewed papers in the literature as well as from the authors' personal experience, in the particular context of formation damage. The most common clay minerals in these sandstones are well-crystallized kaolinite, mainly occurring as pore-filling vermiform and booklet-like aggregates, illitic clays, usually in the form of pore-filling networks of thin lath-like, filamentous or hairy particles, but less frequently as platy aggregates, and chlorite, most commonly found as pore-lining aggregates of interlocking, well-developed, bladed crystals. All these clay minerals are authigenic (diagenetic) in origin. Discrete smectite is rarely found in North Sea reservoir sandstones, even though associated pelitic rocks may contain an abundance of this type of clay mineral.The crystallization of kaolinite in these sandstones has been attributed to a variety of stages in the paragenetic sequence, particularly in relation to the authigenic formation of calcite and quartz. However, most evidence suggests that vermiform kaolinite is a product of early diagenesis at temperatures ranging from surface to 40 °C, either before or contemporaneously with carbonate cementation and before the formation of quartz overgrowths. Exceptions to this generalization may occur because of complex basin histories involving tectonic uplift and migration of fluids of varying chemistry. With increasing depth of burial and at higher temperatures, the kaolinite in these reservoir sandstones converts at least partially to dickite, which occurs in more blocky aggregates. Consideration of recent theoretical studies of kaolinite indicate that at pH values > 8 all face and edge surfaces will be uniformly negatively charged, strongly suggesting that in these circumstances the kaolinite will become disaggregated to form stable dispersions capable of migration where there is sufficient force of hydrodynamic flow.The illitic clay in North Sea reservoir sandstones is usually described in terms of two discrete phases, namely illite itself and mixed-layer illite–smectite (I/S). Evidence is presented to show that in all probability only one illitic phase exists in these sandstones and that the mixed-layer phase in reality consists only of very thin illite (< 5 nm in thickness). Such material when sedimented on glass slides adsorbs ethylene glycol between its thin particles and yields an XRD pattern identical to that of I/S, usually of a regularly ordered (R3) form. In this case, however, diffraction is an interparticle phenomenon and the “smectite” layers detected are more apparent than real. In the North Sea reservoir sandstones, illitic material is considered to exist in both pore-filling and pore-lining modes, with the latter forming at low temperatures very early in the paragenetic sequence, perhaps even in equilibrium with depositional pore waters. In contrast, the pore-filling illitic clay is thought to have formed at higher temperatures, in excess of 100 °C, following deeper burial. In this paper, it is argued that so-called pore-lining illite is sometimes an artefact of the customary drying procedure of the sandstone samples prior to examination by SEM or optical microscopy. Such procedures cause some or all of the delicate pore-filling illite filaments to shrink back against the pore walls so producing the appearance of a pore-lining mode. Evidence against a separate early phase of illite formation in pore-lining mode includes K–Ar dating showing that the age of North Sea illites is always much later than the stratigraphic age of the sediment, oxygen isotope evidence showing that North Sea illites have usually formed at relatively high temperatures (> 100 °C), SEM observations showing that putative pore-lining illite actually consists of dense masses of compacted illite fibres identical to those that fill pores, and finally the similar chemical compositions of both pore-lining and pore-filling illite showing them both to be of a muscovitic or phengitic nature. It is probable that the illitic clay in North Sea reservoir sandstones is highly dispersible and is prone to redistribution, particularly in the Na+-saturated state, and because of its very fine particulate form would therefore be readily mobilized by hydrodynamic forces.Finally, several illustrative examples are presented to show that clay migration during fluid injection is a probable cause of formation damage in North Sea reservoir sandstones. SEM observations following fluid treatments clearly show the breakup of kaolinite aggregates whereas the evidence for illite mobilization consists of the appearance of large holes in the network of pore-filling illite laths. In addition to this, analytical evidence shows that both kaolinite and illite particles have exited core samples following fluid or gas flow, thereby proving their mobility within the core.