Previous pressure-volume calculations during oil cracking to gas, based on the conventional model that presupposes oil cracking to be completed by approximately 150 degrees C, underestimate the potential for gas accumulation in petroleum reservoirs. In this article, a compositional kinetic model of gas generation from oil cracking is suggested based on pyrolysis data using sealed gold tubes, and the pressure-volume changes are recalculated based on the new kinetic model under various geological conditions. The kinetic modeling of oil cracking confirms that crude oil begins cracking at about 160 degrees C for a heating rate of 2 degrees C/m.y., and that the oil-cracking process has two distinct stages with significant differences in gas composition. The first stage is characterized by dominant C2-5 wet gases, whereas the second is characterized by the recracking of C2-5 wet gases to methane and pyrobitumen, leading to a progressive increasing dryness of the gas. The pressure-volume-temperature simulations of oil cracking to gas show that initial oil saturation, temperature-pressure gradients, and openness of reservoirs are important geological factors that control gas accumulation in original petroleum reservoirs. For a reservoir that is geologically open and saturated with 100% oil, gas spills out of the trap at 196 degrees C. The gas loss at 240 degrees C is almost 50% of the total gas, far lower than the 75% based on the conventional model of oil destruction. With lower oil saturation, the gas loss `engineering.,His major research interests are applying molecular modeling and experimental simulation techniques to energy-related problems.