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The four dominant mechanisms of primary and secondary hydrocarbon migration are diffusion, in solution, and as an oil or gas phase. The larger molecules move mainly by the latter two mechanisms with buoyancy as the driving force. The oils move laterally along bedding planes, unconformities, and sheet sands, and vertically through permeability pathways along fractures, faults, mud volcanoes, and igneous intrusions. Heavy oils usually result either from the leakage of accumulations near the surface or from the weathering of conventional oils during migration and accumulation. A few heavy oils form directly from organic-rich source rocks of early maturity. Some of the largest heavy-oil accumulations in the world, such as the Faja of eastern Venezuela, have resulted from long-distance migration covering a huge gathering area along the unconformities of foreland basins.

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