Exploitation of heavy/extra heavy oil is becoming challenging as it conventionally requires injection of steam in high proportions, making it not environmentally friendly. In-situ upgrading technology (ISUT) is a novel alternative to the current heavy oil and bitumen production methods, which gives the benefits of both oil recovery and upgrading in one single stage. In this method, recovered vacuum residue (VR) from produced oil along with nano-catalyst and hydrogen will be injected in the reservoir, where upgrading reactions take place. For the first time, a comprehensive kinetic analysis of mild hydrocracking reactions inside a carbonate rock is implemented, which considerably enhances predictive simulations of the technology. With the aid of a continuous experimental setup, nano-catalyst deposition into the rock is also assessed in carbonate cores. Furthermore, a five-lumped kinetic model is developed and matched against experimental data with an average error of 6%. Significant viscosity reduction of 99.8% and API gravity increase of 8 degrees API are observed for the most severe condition with 35% VR conversion, while asphaltene stability criterion is met. It is recurrently omitted in most in-reservoir upgrading reports and the literature the essentially relevant information about oil stability limits. Non-catalytic conversion of heavy oils, disregarding the energy source employed, is most certain to produce unstable asphaltenes and/or coke before reaching surface transportability viscosity requirements, with additional mid-long term reservoir impairment consequences. It is included in this paper the required confirmation that the in-situ upgraded oil is fully stable when transportable viscosity values are reached.