It is common sense that a deeper well implies higher temperature in the exploration of deep geothermal resources, especially with hot dry rock (HDR) geothermal resources, which are generally exploited in terms of enhanced geothermal systems (EGS). However, temperature is always different even at the same depth in the upper crust due to different heat sources. This paper summarizes the heat sources and classifies them into two types and five sub-types: crust-origin (partial melting, non-magma-generated tectonic events and radiogenic heat production), and mantle-origin (magma and heat conducted from the mantle). A review of global EGS sites is presented related to the five sub-types of heat sources. According to our new catalog, 71% of EGS sites host mantle-origin heat sources. The temperature logging curves indicate that EGS sites which host mantle-origin magma heat sources have the highest temperature. Therefore, high heat flow (>100 mW/m2) regions with mantle-origin magma heat sources should be highlighted for the future exploration of EGS. The principle to identify the heat source is elucidated by applying geophysical and geochemical methods including noble gas isotope geochemistry and lithospheric thermal structure analysis. This analytical work will be helpful for the future exploration and assessment of HDR geothermal resources.