A method for obtaining heavy oil is disclosed. The method includes mixing a material including heavy oil (e.g., oil sand) with a solvent including biodiesel to form a mixture and separating the mixture into a oil-enriched solvent phase and a residual sand phase. The method also can include introducing a solvent including biodiesel into an in-situ geological formation including heavy oil and collecting a mixture including biodiesel and heavy oil from the formation. For example, the mixture can be collected after the solvent travels through at least a portion of the formation by gravity. A method for producing biodiesel also is disclosed. The method includes microbially digesting asphaltenes to form a liquor including a fatty acid and reacting the fatty acid with an alcohol to produce biodiesel. This method can be used to convert petroleum asphaltenes and/or coal asphaltenes into biodiesel.