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| Quartz with oil inclusions from Zhob, Balochistan, Pakistan |
Inclusions of small drops of oil may be trapped in quartz cement and show up well in fluorescent light. Fluid inclusion data from quartz helps to constrain the temperature for quartz cementation and in sandstone reservoirs it has been demonstrated quite clearly that quartz cementation continues after oil emplacements in a reservoir. The lowest fluid inclusion temperatures in quartz cement indicate an onset of quartz cementation close to 70–80◦C. In the Ula Field (North Sea basin), fluid inclusion temperatures in quartz cement range from 86◦C to 126◦C, the highest temperature being close to the present day reservoir temperature which is also the maximum burial depth and temperature. Fluid inclusions from 24 samples from 11 different reservoir units from the North Sea and Haltenbanken also show temperatures from about 80◦C to values close to the present reservoir temperatures. This shows that quartz cementation occurs as a continuous process at a rate controlled by the temperature. There is no evidence that quartz cementation is episodic, controlled by the supply of silica, or that quartz cementation stops after the sandstones have become oil-saturated. In a water-wet reservoir precipitation can still continue in the remaining water around the grains. At high oil saturation, the transport of silica by advection as well as by diffusion becomes much less efficient. The continued growth of quartz cements after oil emplacement results from the closed system nature of quartz cementation in sandstones. In an oil-wet system, however, quartz can not precipitate on the grain surfaces and oil or bitumen may become very effective coatings. Asphaltic oil and bitumen formed by bio-degradation or other processes may preserve good reservoir quality, particularly if the heavy oil only occurs as a grain coating. In summary, quartz cementation is controlled by the slow kinetics (high activation energy) for quartz cementation and normally a minimum temperature of 70–80◦C is required. This is however also dependent on the pH. In sedimentary basins marine porewater starts out with a pH close to 7 but quickly becomes more acid due to the build of CO2 and other reactions with the minerals present. At 3-4 km depth the pH may typically be 4.5–5 but at 120◦C the pH is close to neutral. The rate of quartz cementation is then lowered by the pH but increased by higher temperatures. At very high pH quartz cementation may occur at the surface and silcrete is fine-grained quartz formed in soils due to concentration of porewater by evaporation, and quartz is also forming in some alkaline African lakes. Authigenic illite consists of thin hair- or plate-like minerals and it is fairly obvious that they would have a detrimental effect on reservoir quality by reducing the permeability. SEM images are routinely taken from dried-out cores where the illite appearance is no longer representative of its morphology in the reservoir. When cores are dried without destroying the delicate illite morphology the pore space often looks like it has been filled with rockwool. Illite can often be seen to grow at the expense of kaolinite and may also form by alteration of smectite. Although authigenic illite may also be observed on fractures and other places where there are no obvious precursor minerals, it is most commonly found as a replacement of an earlier Al-rich mineral phase. Because of the low Al-solubility in porewater, illite will in most cases precipitate where the source of Al is available locally from a dissolving mineral. Calculations suggest that the solubility of aluminium is only about 1 ppm at 150◦C and that organic acids have little effect in terms of increasing its solubility. The formation of illite from smectite via mixed layered minerals is well known and occurs in sandstones in the temperature range of 70–100◦C. Sandstones with abundant smectite are poor reservoir rocks at the outset, and illitisation of such rocks may itself slightly improve reservoir quality as illite has a lower specific surface area than smectite. In better sorted and potentially good reservoir rocks kaolin minerals (kaolinite or dickite) are the most important precursors for illite. However, the formation of illite requires potassium, and K-feldspar is usually the only significant source present in the sediment.
KAlSi3O8 + Al2Si205(OH)4 = KAl3Si3O10(OH)2+ 2SiO2 +H2O
K-Feldspar + Kaolinite = Illite + Quartz
The reaction between K-feldspar and kaolinite occurs at about 130◦C and above this temperature these two minerals are no longer thermodynamically stable together. In the North Sea basin and at Haltenbanken this corresponds to a burial depth of about 3.7–4 km. A sharp increase in the illite content in sandstone reservoirs is observed. If the matrix is well-cemented the rate of diffusion is reduced, and the minerals are then able to co-exist at higher temperatures if they do not occur close together. In sandstones with little or no K-feldspar, however, kaolin remains stable at greater depth as it is not dissolved and replaced by illite. The formation of illite can therefore be predicted from the sandstone provenance with respect to K-feldspar supply, and from the early diagenesis and freshwater flushing with respect to the distribution of kaolinite. If a sandstone is derived from an albite-rich gneiss the K-feldspar content is likely to be too low and much of the kaolinite would then not be illitised. Similarly, not much illite will be formed in sandstones with little kaolinite or smectite. Both in Haltenbanken and the North Sea there are Jurassic reservoirs where plagioclase is the dominant feldspar and where the low K-feldspar content is unable to supply the necessary potassium for illitisation of kaolinite. The low illite content in such reservoirs preserves better permeability. This is a direct function of the provenance and could be due to erosion of albite gneisses rather than granitic gneisses. The distribution of authigenic illite in sedimentary basins like the North Sea and Haltenbanken shows that illite formation is strongly controlled by the present day burial depth and temperature. The increase in illite content at about 3.7–4.0 km is usually very sharp, indicating a temperature-controlled reaction rather than a high kinetic reaction rate when the association of kaolinite and K-feldspar becomes thermodynamically unstable. Basin loading from thick Pleistocene sequences in these areas suggests that the illite formed recently. K-Ar dating of illite gives variable ages for the formation of illite. This is probably because even very small amounts of detrital (older) mica or feldspar will produce too-old ages.
