The influence of diagenetic microfabric on oil sands behaviour

The diagenetic processes of pressure solution and authigenic crystal overgrowth have altered the arenaceous oil sand materials of northeastern Alberta, creating sands with a decreased porosity and interlocking grain contacts. Examination of specimens in the scanning electron microscope indicates that a large number of the grain contacts in the materials have been altered from tangential to long and concavo-convex. Except for infrequent isolated bands, the materials are free from true grain-to-grain mineral cement.Strength, compressibility, and index tests (density, grain size) were performed in the laboratory on oil-free samples of the McMurray and Grand Rapids Formations, two of the Alberta oil-bearing strata. Results indicate that increased grain contact area and grain interlock cause a reduction in the compressibility and an increase in the shear strength of the materials.The influence of porosity, mineralogy, grain size, and degree of diagenetic alteration on the behaviour of granular materials is discussed, and a qualitative classification for degree of diagenetic alteration and its influence on shear strength is presented.The recognition of the geological processes reponsible for the unusual engineering behaviour of oil sands will provide a valuable predictive capacity for all friable sandstone behaviour. On the other hand, the relatively straightforward properties of relative density and compressibility can serve as effective measures of geological diagenetic history for future process quantification.