This study examines the one-dimensional stress–strain behaviour of sand at effective stresses as high as 50 MPa. Experiments were performed on 22 sands (approx. 150 tests) with different grain size, uniformity coefficient, angularity, density, grain mineralogy, and clay content. The results show that minor grain corner crushing starts at stresses of 2–8 MPa. The point of maximum curvature (yield point) in the porosity (n) versus logarithm of vertical effective stress (σ'v) curve defines the initiation of marked particle crushing. The stress at the yield point varies between 3 and 31 MPa depending on sand characteristics. A low yield stress is indicative of high porosity loss in the interval of intermediate stress (5–25 MPa). The yield stress is low when the grain size is large, grains are angular, grain strength is low, and uniformity coefficient is low. The lowest yield stress value occurs in the coarser carbonate sand, and the highest in the chert-rich sands. The sands rich in clays are highly compressible up to 25 MPa. At stresses higher than ~10 MPa, the coarser biogenic carbonate sands maintain higher porosities than the other sands. This can be explained by the fact that coarser biogenic carbonate sands have low yield stresses due to high angularity and low grain strength and initially there is local grain crushing at grain contacts. This increases the area of the grain contacts, so the coarser carbonate sands become less compressible at higher stresses. Within the high stress range (25–50 MPa) the porosity loss differences related to grain size, grain shape, grain mineralogy, and sand uniformity coefficient are significantly reduced. Hence the greater compressibility of lithic and carbonate sands becomes less evident in the high-stress interval as the grain size increases.Key words: sand, grain crushing, grain size, high stress, compression.

No abstract

No abstract

The Middle Jurassic Garn Formation of the Haltenbanken area has been studied using mineralogical and geochemical data from 21 wells, ranging in burial depths from 2.0 to 4.1 km relative to seafloor (RSF). K-feldspar and plagioclase contents show variations on a regional scale both laterally and as a function of burial depth. The content of pore-filling authigenic illite increases sharply, and the content of K-feldspar and kaolinite decreases in Garn sandstones presently at depths greater than 3.6-3.7 km RSF (120-130؇C). The depletion in Kfeldspar below 3.7 km RSF is not accompanied by lower potassium values in the bulk chemical composition (wt % K 2 O). This suggests that the potassium released during K-feldspar dissolution is retained in the sandstones and is precipitated as illite. The variations in bulk contents of potassium and sodium are therefore considered to be related principally to primary variations in sandstone mineralogy.The shallower sandstones (Ͻ 3.7 km RSF) with average wt % K 2 O greater than 0.95 (K/Al molar ratio Ͼ 1/3) have a K-feldspar:kaolinite ratio greater than one. The deeply buried (Ͼ 3.7 km RSF) sandstones with similar potassium contents contain excess K-feldspar and most of the kaolinite is illitized. However, deeply buried sandstones containing an average of 0.38 wt % K 2 O (K/Al molar ratio Ͻ 1/4) contain a significant amount of kaolinite but negligible K-feldspar. This suggests that the K-feldspar:kaolinite ratio before the onset of illitization was less than one, and hence that the kaolinite-illite reaction has been restricted by an insufficient supply of potassium (absence of K-feldspar). This illustrates how illitization of kaolinite depends upon K-feldspar as a local source of potassium. Prediction of illitization in sandstones, therefore, must be based on integration of models for provenance, facies, and early diagenesis in addition to burial and thermal history. The formation of pore-filling authigenic illite in these sandstones is an important influence on the total reservoir quality.

No abstract