Methods and systems for determining an estimated reservoir property using a determined desorption stage are disclosed. The method includes determining a fluid property and composition of a first fluid sample obtained from a reservoir, determining a measured relative volume of gas components and isotope ratios of gas components of the sample, and determining an equation of state. The method also includes obtaining a second and third sample at two later times, determining a composition, a measured relative volume of gas components, and isotope ratios of gas components of the later samples, and calibrating the equation of state utilizing the fluid composition and measured relative volume of gas components. The method further includes predicting a relative volume of gas components from the equation of state, determining a desorption stage, determining a critical pressure, an extent of desorption and a quantity of produced desorbed gas and determining the estimated reservoir property.