Rate constants are presently not established well enough to allow accurate theoretical quantification of organic matter evolution. Instead, a rapid empirical method for determining source rock maturity in relatively unexplored basins as a function of sediment age and thermal history is developed. From a compilation of published worldwide data, the oil threshold temperature may be empirically described in a non-Arrhenius relationship as a function of sediment age: T = 164.4 - 19.39 ln t, where T is threshold temperature in °C and t is time in 10⁶ years. The depth to the oil ceiling in a sedimentary basin is then D_oc = 100[(T - T_s)/(dT/dZ)], where D_oc is depth in meters, T is derived from the preceding relationship, T_s is the average surface temperature, and dT/dZ is the geothermal gradient in °C/100 m. Similarly, the depth in meters to the oil floor, D_of, may be calculated from D_of = 100[(150 - T_s)/(dT/dZ)].

Sedimentary geothermal gradients can vary in time and space. These nonsteady-state and/or tectonically perturbed basin thermal histories are more complicated and must be considered in order to apply maturation models correctly. For sedimentary basins floored by oceanic crust less than 120 m.y. old, the geothermal gradient may be modeled as dT/dZ = 113/(K^radicalt), where K is the thermal conductivity. Basins in convergent tectonic settings have characteristic geothermal gradients that vary as a function of tectonic setting.