MIGRATION IN FRACTURE NETWORKS; AN ALTERNATIVE INTERPRETATION OF THE SUPPLY OF THE “GIANT” TAR ACCUMULATIONS IN ALBERTA, CANADA ‐ I†

Abstract

During secondary migration, the tendency towards a thermodynamic minimization of the hydrocarbon‐water interfacial energy can result in the confinement of petroleum in open fractures and coarser pores, in spite of the repeated conversion of gravitational energy into interfacial energy. A discussion of lithological influences leads to the prediction that migration in granular porosity is likely to end in trapping or dispersal, whereas migration in brittle, compact formations, which are little fractured, can allow the petroleum to reach distant structures.The source of supply for the “giant” tar accumulations of Alberta is then investigated. Chromatographic analyses strongly support a predominantly Triassic, basinal origin for the Pre‐Mesozoic Unconformity oils and, consequently, of the tars accumulated in the basal Cretaceous sands and Grosmont Paleozoic carbonates. A study of the spatial distribution of the thickness of oil‐stained rocks in the respective formations suggests that migration to the tar accumulations cannot have occured without interruption in the basal, transgressive Cretaceous sandstones, whereas continous migration (from the basin to the shelf) appears to have taken place in the weathered, carbonate Paleozoic sub‐crops.