The thermal evolution of sedimentary basins and its effect on the maturation of hydrocarbons

Abstract

The maturation of hydrocarbon source rocks depends on a range of factors, including the primary rock type and its original content of organic matter (kerogens); the history of sedimentation and burial (depth); the local geothermal gradient (temperature); and duration of sedimentation (time). The standard approach to modelling this process assumes an evolving burial history, but in a basin with a steady-state geotherm, both in the sediment column and in the underlying basement rocks. This first-order approach neglects any effects of changes in the geothermal profile resulting from the timing and history of the particular mechanism that formed the sedimentary basin, and hence may lead to systematic overestimation or underestimation of hydrocarbon maturation in such rocks. This systematic effect may be small when sedimentation rates are low, but can be significant in basins with sedimentation rates that are rapid when compared with the rate of heat transport and re-equilibration in the system. Here we describe two analytical 1-D mathematical models for diffusive heat transport during sediment deposition, which is rapid compared with the thermal relaxation rate. The two models describe sedimentary events that can be considered either effectively instantaneous (sudden) or continuous. In both cases we calculate a time-dependent geothermal profile and the resulting maturation index of particular source rocks, given the combined effects of the thermal and burial histories. The models take account both of the transient cooling effect of the cold sediment blanket and of the steady-state warming from the underlying basement. After calibration with borehole data, the method allows a reconstruction of the initial geothermal gradient at the start of the sedimentation event. The model has been applied to data derived from two wells drilled in two different tectonic settings: the Pannonian Basin and the Central North Sea. In both cases there is a good agreement between the model predictions of the maturation index and empirical observations based on the vitrinite reflectance technique applied to borehole samples of actual source rocks. In contrast, the steady-state geotherm models give a systematic overestimation of both the maximum temperature and the degree of maturation reached by the sediments by the end of the sedimentary event; this overestimation is particularly evident for the case of the Pannonian Basin. Such systematic effects could have significant implications both for the interpretation of past thermotectonic events, and for oilfield appraisal.