Oilfield Review Summer 2011: 23, no. 2. Copyright © 2011 Schlumberger. For help in preparation of this article, thanks to Nicholas Drenzek, Cambridge, Massachusetts, USA. FLAIR is a mark of Schlumberger. Rock-Eval is a mark of the Institut Français du Pétrole. Rock-Eval 6 is a mark of Vinci Technologies SA. Every oil or gas play originates from source rock. The viability of each play—conventional or unconventional, oil or gas—depends on its source rock. Without this source of petroleum, all other components and processes needed to exploit a play become irrelevant. A source rock can be broadly defined as any fine-grained, organic-rich rock that is capable of generating petroleum, given sufficient exposure to heat and pressure. Its petroleum-generating potential is directly related to its volume, organic richness and thermal maturity. Although its volume—a function of thickness and areal extent—must not be discounted, this article focuses on the other two characteristics. Organic richness refers to the amount and type of organic matter contained within the rock. Thermal maturity refers to a source rock’s exposure to heat over time. Heat increases as the rock is buried deeper beneath successive layers of sediment. It is the thermal transformation of organic matter that causes a source rock to generate petroleum.