Compositional Modeling of Petroleum Generation and Expulsion: Trial Application to a Local Mass Balance in the Smorbukk Sor Field, Haltenbanken Area, Norway (1)

We present an application of a compositional kinetic model of petroleum generation, degradation into gas and residue, and simultaneous expulsion. Petroleum composition is described by four carbon fractions (C[1], C[2]-C[5], C[6]-C[14], C[15+]) and expulsion is related to a petroleum saturation threshold. Expulsion is important because rapid expulsion of petroleum from source-rocks favors oil, whereas late expulsion increases the cracking reactions and favors gas accumulation. Such a model is tested against field data, for the first time, by applying it to the Smorbukk Sor field [previously named Smorbukk (Greek) beta], located in the Norwegian Sea. Geometrical and one-dimensional thermal reconstructions are performed, followed by a computation of a mass balance of expelled and unexpelled products for each source rock throughout the drainage area. Comparison of the model results is made with Rock-Eval data, vitrinite reflectance, and actual composition of accumulations. Lower Jurassic coals were the main source expelling gas and oil, mainly since 5 Ma. As the present accumulation accounts for less than 10% of total expelled petroleum, major petroleum losses probably occurred through the structural spillpoint. This migration scheme is supported by the simila composition of the reservoired petroleum and the petroleum expelled since 0.5 Ma. Although various uncertainties are possible, this study shows that our model has the capability to quantify expulsion consistently with the observed amounts of unexpelled petroleum in mature source rocks and with the reported composition of the accumulation.