Primary migration by oil-generation microfracturing in low-permeability source rocks: Application to the Austin chalk, Texas

Fracturing of low-permeability source rocks is induced by pore-pressure cha nges caused by the conversion of organic matter to less dense fluids (oil a nd gas); these fractures increase the permeability and provide pathways for hydrocarbon migration. An equation for the pressure change is derived usin g four major assumptions. (1) The permeability of the source rock is neglig ibly small (0.01 mu d; 10(-20) m(2)) so that the pore-pressure buildup by t he conversion is much faster than its dissipation by pore-fluid flow. (2) T he stress state is isotropic so that horizontal and vertical stresses are e qual. The source rock fails when the pore pressure equals the overburden pr essure. (3) The properties of the rock, organic matter, and fluids remain c onstant during oil generation. This assumption is valid when the change in depth (i.e., pressure and temperature) is small. (4) Only two reaction rate s are required for the conversions, a low-temperature reaction rate for the kerogen/oil conversion (E approximate to 24 kcal/mol, A approximate to 10( 14)/m.y.) and a high-temperature reaction rate for oil/gas conversion (E ap proximate to 52 kcal/mol, A approximate to 5.5 x 10(26)/m.y.). The equations for generation rate and pressure change are applied to the Au stin source rock by adjusting the several variables to fit geochemical data , core saturations, and observed levels of oil and gas production. This app lication demonstrates that the equations are easily applied in calculating depths of primary migration for low-permeability source rocks.