ABSTRACT

A scanning electron microscope analysis of oil morphology and microfracture network in two oil source rocks reveals the role played by shale fabric in primary migration of oil. SEM micrographs of oil produced by hydrous pyrolysis experiments from two well known source rocks (Woodford Shale, Devonian-Mississippian, Oklahoma; Kimmeridge Clay Formation, Jurassic, Dorest, England) show the following sequence of initial oil formation and movement: (1) oil is initially generated within the shale matrix (at 350°C for 1 day heating-- Woodford; 300°C for 3 days heating--Kimmeridge); (2) initial oil generation and microfracture formation occur at the same stage; (3) oil droplets continue to move along a pressure gradient out of the matrix into adjacent open microfractures; and (4) ribbons of oil fill the microfractures and move along the fracture network. It is suggested that the primary migration phase through the source rock continues as oil is expelled. Observations indicate the importance of microfabric in influencing shale properties in the primary migration of oil.