Habitat of Biodegraded Heavy Oils: Industrial Implications

Abstract

Heavy oil, extra-heavy oil and tar sands account for half of the petroleum resources of the world. Due to their high viscosity their production is a major technical and economical challenge. The understanding of the origin and geological habitat of these unconventional oils is crucial in order to optimize exploration and production operations. The vast majority of these heavy oils originates from the biodegradation of conventional oils by bacterial activity. The limiting factors of the involved biological processes (temperature, nutrients, etc.) are controlled by the geological situation. In this respect, foreland basins which harbour a large part of the current deposits of heavy oils correspond to particularly favourable conditions in promoting the biodegradation of large charges of oil. They are characterized by long distance lateral oil migration draining substantial volume of the petroleum system. This migration is supported by an adequate drainage system which exhibits a large lateral continuity and consists of the first syntectonic fluvial and fluvial-deltaic sediments filling the foreland basins. As a result of this migration the oil reaches shallow situation in the forebulge where the temperature is compatible with bacterial activity. The reservoirs associated with the “forebulge” are often high porosity and high permeability sand bodies, initially hosting large volume of water and facilitating the circulation of meteoric water helping in the nutrient availability. For the sake of production, the heterogeneities associated with these fluvial and fluvial-deltaic sediments have to be carefully considered. The geological models developed for the architecture and for the stratigraphic evolution of fluvial channels and incised valleys provide useful guidelines in order to characterize the reservoirs for production purposes in such geological setting. The technical difficulties in recovering these highly viscous fluids require to integrate more detailed reservoir description than usually needed when producing conventional oil plays. The Cretaceous reservoirs of the Mannville Formation in Canada is presented to exemplify the types of heterogeneities encountered in fluvial reservoirs, their rational, their effect on the heavy oil recovery and the impact of the geological knowledge on the production strategy.