Thermal maturity is a key parameter for the determination of potentially prospective shale oil and gas accumulations at initial stages of exploration. Based on thermal maturity values, source rocks are categorized as thermally immature, mature or post-mature rocks in terms of capability to generate hydrocarbons.
Immature (i.e. thermally unaltered) source rocks, that are still at relatively small depths, may generate (biogenic) natural gas which is formed by certain bacteria species as a result of their metabolic activity. Normally, these processes occur at temperatures below 50oC, and the degree of organic matter alteration, as expressed by reflectance index, is not in excess of 0.5% Ro. Conventional biogenic gas fields have been exploited for many years in the Carpathian Foredeep (Southeastern Poland).
As the source rock is buried, for example under an increasingly thicker overburden, the processes of bacterial organic matter alteration subside, while temperatures and pressures of the rock increase and gradually trigger chemical changes that lead to the generation of oil and smaller quantities of natural gas. A rock which had been heated so as to generate crude oil is called mature rock in the oil generation phase or in the so-called “oil window”, and its alteration degree, expressed as vitrinite reflectance index, ranges from 0.5 to 1.2% Ro.
If buried deeper, the source rock will generate mostly (thermogenic) natural gas. Accordingly, a mix of the previously generated oil and of the “newly generated” natural gas (the so-called condensate) is expected to occur in the rock.
If the rock medium is even deeper buried, organic matter will generate (thermogenic) natural gas only. Moreover, any oil previously accumulated in the rock will be transformed into (thermogenic) natural gas. This degree of thermal alteration called the “gas window” occurs at temperatures in the order of 120 – 150oC and is expressed by reflectance index values ranging from approx. 1.2 to 2.0% Ro.
The process will continue until alteration processes reach the degree at which the source rock becomes overmature and its hydrocarbon generative potential is exhausted (>3.0% Ro). This occurs at temperatures in excess of 200oC.
It should be noted that the oil and gas generation cycle, as presented above, is much simplified, and petroleum basins vary in terms of hydrocarbon generation conditions. In fact, each basin has its specific thermodynamical conditions, different type of organic matter distributed within the rocks, different rock-building minerals, different burial rates and several other factors.
authors: Izabella Grotek, Marcin Janas