The variation of temperature with time and hydrocarbon maturation in sedimentary basins formed by extension

When a sedimentary basin is formed by extension both the subsidence history and the temperature gradient can be calculated. Hence the time-temperature history of any stratum can be obtained. Three types of behaviour occur. The temperature of material which never reaches depths greater than about 4 km increases at all times. The time-temperature history of material which finally sinks to greater depths reaches a maximum during the subsidence history. The time at which this maximum occurs decreases as the depth increases. At greater depths the maximum temperature occurs at the time the basin is formed. The time-temperature history is then used to calculate Lopatin's maturation index for each stratum as a function of time, and the index plotted as a function of depth and time for different amounts of extension. The maximum temperature at a given depth, and hence Lopatin's index, is almost independent of the amount of extension when the age of the basin is greater than about 150 Ma. Similar calculations taking account of sediment compaction show the same behaviour, though the maturity of the material at a given depth is increased. Because both Lopatin's index and its calibration are empirical it is important to develop new maturation indices based on individual molecular reactions whose activation energies can be determined in the laboratory.