A computer-implemented method for determining an amount of hydrocarbon fluid present in a rock of a hydrocarbon-producing reservoir is provided. The rock comprises organic matter and porous and permeable inorganic matter. The method comprises the steps of receiving data relating to chemical and kinetic properties of the organic matter, rock lithology data, rock thickness and reservoir temperature and pressure data, inputting the received data into a computer-implemented model, and operating the model. The model operates to a) simulate hydrocarbon fluid generation in the rock based on the input data and thereby determine an amount of generated hydrocarbon fluid, b) generate predicted data, and c) determine a total amount of hydrocarbon fluid present in the rock based on the predicted data. The generated predicted data is indicative of i) an amount of the generated hydrocarbon fluid adsorbed onto a surface of the organic matter within the rock, ii) an amount of the generated hydrocarbon fluid present in the pores of the organic matter by determining the porosity of the organic matter, based on the chemical and kinetic properties of the organic matter, and iii) an amount of the generated hydrocarbon fluid present in the pores of the inorganic matter by determining the porosity of the inorganic matter, based on the rock lithology data. A corresponding system, a computer program and a computer readable medium are also provided.