Gas generated and expelled from coals often results in economic gas accumulations. As a consequence, it is important to consider the theoretical aspects of these processes and to develop methods that can be used to assess coal as a potential source rock. On the basis of results of modeling hydrocarbon generation, expulsion, and retention in coals, two comprehensive indexes are proposed for assessing the quality and the nature of coal as a potential source rock. Theoretical charts of the two indexes are used to assess coals as potential source rocks in the Turpan-Hami Basin in northwestern China.