The Golden Zone

A source rock, a reservoir rock, a cap rock, a trap, and adequate heat are the five essential requirements for a commercial accumulation of petroleum. Heat is required to generate the petroleum from kerogen disseminated within the source bed. The relative importance of heat, time, and pressure is still a matter of debate. It is generally agreed, however, that there are certain critical temperature boundaries: If the temperature is less than 60 C, the source rock is immature; if the temperature ranges between 60 C and 120 C, oil generations can occur; if the temperature ranges between 120 C and 220 C, gas generations can occur; and if the temperature is above 220 C, kerogen is reduced to graphite (over-mature).

From this remark on heat it is clear that temperature controls oil and gas generation. This has long been known. New research (Bjorkum and Nadeau, 1998; Nadeau et al., 2005), however, shows that most of the hydrocarbons in a sedimentary basin--both oil and gas--are located at a depth at which the temperature is between 60 C to 120 C: this is the Golden Zone. This new understanding represents perhaps one of the main advances in petroleum geological

thinking in recent years. The theory which is now in the process of becoming firmly rooted in the petroleum community is, in principle, applicable in all types of sedimentary basins. The Golden Zone lies at different depths in different basins, depending on geothermal gradient. It is, for example, shallow in the Bombay High off India and deep in the Gulf of Mexico.

The Golden Zone concept, which has been developed from theoretical considerations, is supported empirically. By plotting the distribution of oil and gas resources against temperature (not depth), a universal pattern emerges. Independent of the way sedimentary basins formed, their history of development, and age, a distribution is obtained that shows an accumulation of oil and gas between 60 C to 120 C. Surprisingly, gas also occurs in precisely the same temperature interval as oil, despite the fact that gas forms much deeper and at different temperatures compared to oil. Data from across the world show that the volume of hydrocarbons in reservoirs falls exponentially with temperature when temperature rises above 100 C to 120 C, and the total volume of oil and gas that geoscientists can find at temperatures above 120 C is only about 10% of the total amount in a given basin.

The importance of this discovery in terms of evaluating world oil and gas resources and the status of depletion cannot be exaggerated. The challenge now is for geoscientists to invent remote sensing techniques for estimating the geothermal gradient.

http://www.sciencedaily.com/releases/2007/10/071020082836.htm