Upper and lower asphaltene weight fractions of fluid proximate ends of an oil column are obtained based on measured OD. Upper and lower maltene partial densities are obtained based on the asphaltene weight fractions. A maltene partial density distribution is obtained utilizing the maltene partial densities and a predetermined diffusion model. An asphaltene partial density distribution is obtained based on the maltene partial density distribution and an estimated mass density gradient. An asphaltene weight percentage is obtained based on the asphaltene partial density distribution and the mass density gradient. The asphaltene weight percentage distribution is converted to an OD distribution utilizing a predetermined correlation. An optimization then reduces differences between the OD distribution and the measured OD data to within a predetermined range to refine a biodegradation time of the predetermined diffusion model. A viscosity distribution may be obtained based on the optimized OD distribution.