A series of pressure depletion experiments was conducted in a glass micromodel using five Venezuelan oils with the objective of studying their microscopic mechanism during primary production under a solution gas drive process. Photographs taken from a large area of the micromodel during depressurisation reveal a higher number of bubbles for heavy oils than for light oil. Image processing results show differences in the number of activated nucleation sites among three heavy oils that have similar degrees of critical supersaturation. Three steps in the heavy oil flow are also established.