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The mechanical behavior (structural stratigraphy) of the Upper Cretaceous Austin Chalk is established from the study of fracture intensity along its outcrop trend from Dallas to San Antonio and westward to Langtry, Texas, and in the subsurface from the study of core and/or fracture identification logs from 39 wells. Three mechanical-stratigraphic units are recognized as: (1) an upper, fractured massive chalk corresponding to the Bid House Chalk Member, (2) a middle, ductile chalk-marl corresponding to the Dessau Chalk and Burditt Marl Members, and (3) a lower, fractured massive chalk corresponding to the Atco Chalk Member.
Representative samples from these units were experimentally shortened dry, at 10, 17, 34, and 70-MPa confining pressure, 24°C (75°F), and at 2.5 × 10-4 s-1 to determine if the relative mechanical behavior observed at the surface could be extrapolated into the subsurface at different simulated depths of burial. The experimentally determined ductilities do parallel those determined from outcrop and subsurface studies. Through multiple linear regression analyses of strength versus intrinsic rock properties and environmental parameters, it appears that first porosity and then smectite-content are most strongly correlated with strength. For low-porosity specimens (9 to 13.5%) smectite present in amounts as little as 1% by volume has the highest correlat on with strength accounting for 83% of its variability. For example, the strength of specimens with 4% smectite is reduced by a factor of 2 compared to those with no smectite. The coefficient of internal friction at 70-MPa confining pressure decreases from 1.58 to 0.57 as the smectite content increases from 0 to 1 to 4%.
SEM photomicrographs of the experimentally deformed specimens show that smectite and other clays are distributed as small, discrete, concentrated masses throughout the chalk. They are smeared-out along the induced shear fracture surfaces where they are greatly reduced in grain-size. These observations suggest that the smectite acts mechanically as a "soft-inclusion," localizing shear failure and correspondingly weakening the material.
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