Layering in Shales Controls Microfracturing at the Onset of Primary Migration in Source Rocks

Abstract The process of primary migration, which controls the transfer of hydrocarbons from source to reservoir rocks, necessitates the existence of fluid pathways in low permeability sedimentary formations. Primary migration starts with the maturation of organic matter which produces fluids that increase the effective stress locally. The interactions between local fluid production, microfracturing, stress conditions, and transport remain difficult to apprehend in shale source rocks. Here, we analyze these interactions using a coupled hydro‐mechanical numerical model based on the discrete element method. The model is used to simulate the effects of fluid production emanating from kerogen patches contained within a shale rock alternating kerogen‐poor and kerogen‐rich layers. We identify two microfracturing mechanisms that control fluid migration: (i) propagation of hydraulically driven fractures induced by kerogen maturation in kerogen‐rich layers and (ii) compression‐induced fracturing in kerogen‐poor layers caused by fluid overpressurization of the surrounding kerogen‐rich layers. The relative importance of these two mechanisms is discussed considering different elastic properties contrasts between the shale layers, as well as various stress conditions encountered in sedimentary basins, from normal to reverse faulting regimes. The layering in shales causes local stress redistribution that controls the prevalence of each mechanism over the other and the onset of microfracturing during kerogen maturation. The approach is applied to the Draupne formation, a major source rock in the Norwegian continental shelf in the North Sea.