Kinetic modelling of petroleum formation and cracking: implications from the high pressure/high temperature Elgin Field (UK, North Sea)

In this paper we use a case study to test the simulation of generation, migration and cracking of petroleum fluids by a 2D compositional basin simulator using kinetic models based on chemical classes of compounds. The selected area is the high pressure (1100 bar) high temperature (190°C) Elgin Field in the North Sea. This study is mainly focused on kinetic modelling of secondary cracking because in this zone paraffinic condensates with almost 50% C6+ alkanes were recently discovered, whereas only gas (C5−) was expected, given the high temperature conditions. The detailed hydrocarbon composition observed today in the Fulmar sands reservoir of the Elgin Field is compared with the results of several simulations aimed at testing the influence of Jurassic source rocks, retention properties, kinetic parameters and temperature variations on the predicted composition of fluids. The influence of pressure was neglected in the modelling on the basis of results published in the literature. Results show that in this North Sea area, primary cracking of the kerogen into oil is completed whichever source rock is considered, and that secondary cracking of accumulated petroleum compounds in the reservoir is presently in progress. Under these conditions, source rock type and retention during primary and secondary migration have little influence on the present oil composition. The occurrence of a monophasic paraffinic fluid in the Elgin reservoir, despite the high temperature, is due to its very recent increase from 160°C to over 180°C in the last million years. The fluid composition evolves rapidly and our compositional kinetic scheme accounts correctly for the presence of C6+ hydrocarbons in these geological conditions. First results using the compositional kinetic scheme of Behar et al. [Behar, F., Kressman, S., Rudkiewicz, J.L., Vandenbroucke, M., 1991. Experimental simulation in a confined system and kinetic modelling of kerogen and oil cracking. Organic Geochemistry 19(1–3), 173–189] for secondary cracking showed a reasonably good fit between calculated and observed detailed fluid composition, except for methylated aromatics which are overestimated and methane which is underestimated. A recent comparison of kinetic parameters obtained from pyrolysis experiments on model compounds showed that it is not realistic to assume, as done previously, the same frequency factor for all chemical classes of compounds. The higher stability of methylated aromatics relative to saturates, observed under laboratory conditions, would be extended to geological conditions. A second set of simulations using frequency factors and activation energies obtained on these model compounds gave a better fit for methylated aromatics. However, methane is still strongly underestimated, and reasons which could explain this discrepancy are discussed.