Sandstones diagenesis and reservoir quality: Introduction and examples from
Austria
Speaker: An-Ho Huang
Abstract
Study on diagenetic processes of reservoir rocks is essential to understand reservoir qualities such as mineral cements, porosity, and roles of compactions. In this talk, I first review the general principal of sandstone diagenesis, then a case study from Austria is presented. The samples are cored from Miocene sandstones in the Vienna Basin. The succession of samples is from ~892 m to ~2800 m depth. Oils are present in the depths around 1670-1700 m, in the middle part of Badenian section. From X-ray Diffraction results, clay minerals in the < 2 μm fractions of sandstone samples consist of mixed layer illite/smectite (I/S), illite, kaolinite and chlorite. The mineral compositions in reservoir are identical in comparison to those non-reservoir samples. There are abundant discernable detrital carbonate lithic fragments in the shallower samples; they may presumably eroded from proximal mountain belts (e.g. Northern Calcareous Alps). On the other hand, pyrite framboid and glauconite are appeared in a few samples under the SEM image, they are authigenic minerals and reworked grains respectively. The sandstones in Vienna Basin contain a high primary interparticle porosity from greater than ~30% at 1000 m to less than ~15% at 2700 m depth. The secondary porosity here is resulted from feldspar and carbonate dissolutions, but the dissolution process does not led to an increase on net porosity. A plot of the amount of carbonate and ductile grains versus porosity displays a triangular spread of data points, suggesting that carbonate and ductile grains exert a strong control on porosity. Oil stained and non-oil stained sandstone share similar porosity distribution. The results indicate that, material flux and oil-inhibition may play insignificant roles on process of diagenesis or silicate dissolution and reprecipitation.
Reference
Gire, S., Worden, R.H., Johns, W.D., Kurzweil, H., 2008. Diagenesis and reservoir quality of Miocene sandstone in the Vienna Basin, Austria. Marine and Petroleum Geology, 25, 681-695.
Nadeau, P.H., Wilson, M.J., McHardy, W.J., and Tait, J.M., 1985. The conversion of smectite to illite during diagenesis: evidence from some illitic clays from bentonites and sandstones. Mineralogical magazine, 49, 393-400.