Suppression and retardation of vitrinite reflectance, Part 2. Derivation and testing of a kinetic model for suppression

The presence of suppressed and retarded vitrinite reflectance (VR) data introduces a number of difficulties into the prediction of hydrocarbon generation in sedimentary basins. Although the effects of suppression can be removed from measured YR values manually, a kinetic model for suppressed vitrinite maturation would enable both suppressed and unsuppressed VB values to be predicted using thermal histories derived from basin modelling. The evaluation of hydrocarbon generation from suppressed and unsuppressed vitrinite shows that both have similar reaction kinetics. While hydrocarbon generation involves the rupture of the bonds holding volatiles into the vitrinite structure, increases in VR are mainly produced by aromatisation and condensation reactions which take place after volatiles have been expelled. The reactions involved in hydrocarbon generation are different from those responsible for increases in VR, and it is not therefore appropriate to derive kinetic models of vitrinite maturation from laboratory hydrocarbon generation experiments. During the maturation of normal (unsuppressed) vitrinite, the volatiles generated are expelled via the microporous network; the expulsion efficiency is not limited by the capacity of the microporous network. In hydrogen-rich (suppressed) vitrinites, excess volatiles saturate the microporous network, restricting further aromatisation and condensation processes within the vitrinite, which results in suppression of VB. Kinetically, this has been modelled by using a variable pre-exponential or "A" value. Two versions of a kinetic model of vitrinite maturation (SMod-1 and SMod-2) have been prepared, based on the amount of suppression predicted by HI-VR calibration models published by Lo (1993) and Samuelsson and Middleton (1998). Two case studies, involving wells Bunga Orkid-1 (Malay Basin) and 20/3-4 (Outer Moray Firth, North Sea), are discussed. Both wells contain suppressed VR values; well 20/3-4 is also overpressured and contains VR data that are both retarded and suppressed. The application of the SMod model to the wells enables heatflow histories derived from tectonic (rift) histories to be used for the prediction of VR data, although in the case of well 20/3-4 the use of a pressure retardation model was also required. Complementary evidence to support the use of the heatflow history applied to well 20/3-4 is provided by palaeotemperature data obtained from diagenetic concretions.