Seismic assessment of maturity and richness in carbonate source rocks

Evolution of intra-kerogen porosity is an important physical consequence of thermal maturation. The properties of the fluid within the organic porosity also evolve as a function of maturation. Evolution of organic porosity and pore fluid affects the elastic properties of the source rock. It has been shown in the past that the acoustic impedance of organic-rich source rocks decreases with both increasing organic fraction and increasing maturation. Recently, it was also shown that the relative differences in maturation-induced bulk and shear softening, or the bulk-to-shear ratio can be a potential maturation indicator. Fluid evolution with maturation adds complexity to the problem, because the pore-fluid can be different at different stages of thermal maturation. We show that it is possible to model the effects of fluid evolution using the knowledge that the pore-fluid typically goes from heavy oils in immature rocks to lighter oil, condensate, and finally to gas, as the rock matures. The elastic response of the source rock is a function of its organic content, organic porosity, and pore-fluid properties, where the latter two properties are determined by the degree to which the kerogen has matured. We present a rock physics forward model and inversion scheme to predict and estimate organic fraction and maturity from seismic in source rocks. Using pre-stack seismic data, we demonstrate the estimation of organic content and maturity from compressional and shear elasticity.