Effect of maturity on carbon isotope ratios of oils and condensates

For modelling isotopic variations in oils it is convenient to differentiate the effects of oil generation ( 100–150°C) from the effects of oil to gas cracking ( 150–180°C). During generation, δ¹³C of kerogen may increase by up to 1% due to release of isotopically light oil and gas, although most kerogens show little or no chan δ¹³C of the generated oil increases by between 0 and 1% (av. 0.5%) due to mixing of isotopically heavy oil with an initial isotopically light unbound fraction, possibly of bacterial origin. The change occurs mostly over the first 20% of generation. During oil to gas cracking, kinetic isotope effects become important and the effect on δ¹³C of the remaining oil can be modelled as a Rayleigh process. δ¹³C increases by 1.5% by 50% cracking. Insufficient data are available to calibrate the effects at higher levels of cracking, and modelling these variations is hindered by a lack of understanding of the mechanism of pyrobitumen formation. However, increases greater than about 4% are unlikely to be observed. With increasing maturity, the low molecular weight fractions become isotopically heavy faster than the high molecular weight fractions. As a result, any separation of the low molecular weight fraction into a gas phase (“condensate formation”) will produce an isotopic difference between oil and condensate that depends on maturity. In the early stages of generation the condensate may be up to 1% lighter than the remaining oil. With increasing maturity, this difference at first decreases and then increases in the opposite sense. By half way through oil to gas cracking the condensate may be 1.5% heavier than the residual liquid. More subtle rearrangement reactions may result in small, but significant, changes to the shape of the isotope “type-curves” when different oil fractions are compared.