Oil cracking-the thermal breakdown of heavy hydrocarbons to smaller ones-takes place within oil-bearing rock formations at depths commonly accessed by commercial oil wells. The process ultimately converts oil into gas and pyrobitumen, and thus limits the occurrence of petroleum and the success of exploration. Thermal cracking of liquid petroleum increases with depth until it reaches completion at the so-called ＇oil deadline＇ which is generally placed(1,2) at around 5 km depth and at temperatures of 150-175 degrees C. However, cracking experiments(3-8) and the discovery of relatively ＇hot＇ oil reservoirsg(9,10) imply that petroleum is thermally more stable than previously assumed; in fact it has been suggested that liquid petroleum might persist at temperatures reaching(6,11-13) or even exceeding(3,14) 200 degrees C. But reliable estimates of the extent of oil cracking and the depth at which it occurs in any given reservoir are difficult to obtain. Here we demonstrate that the relative abundance of diamondoids, a class of petroleum compounds whose unique thermal stability leads to their progressive concentration during cracking(15), can be used to identify the occurrence and estimate the extent of oil destruction and the oil deadline in a particular basin. We are also able to identify oils consisting of mixtures of high- and low-maturity components, demonstrating that our method yields valuable information on the cracking and mixing processes affecting petroleum systems.