Initial geochemical screening of the Upper Jurassic Mandal formation (TOC, Rock-Eval) identifies a very prone petroleum source rock that is situated in the maturity window to generate petroleum. Nevertheless, composition and amounts of predicted hydrocarbons reveals significant discrepancies to actual accumulations encountered in identified trap structures. Petroleum generation and expulsion models for petroleum source rocks are relatively simplistic, often combining kinetic descriptions of oil (often C5+) and gas (C1-4) generation with arbitrary sorption threshold values and a basic permeability description of the source rock. While generally satisfactory for assessing bulk volumes generated and expelled from a source rock such approaches omit a range of chemical and physical details of the processes and effects involved, which may be critical for predicting the amount and type of retained and expelled fluids, and changes of these amounts and compositions as a function of maturity. Barren petroleum accumulations in potential trap structures could likely be attributed to delayed expulsion due to adsorption thresholds varying from commonly applied sorption models (e.g. Pepper and Corvi, 1995). Brief comparisons drawn to several Upper Jurassic source rocks as the Draupne formation indicate high lateral variability in deposition and preservation of organic matter of this Upper Jurassic main source rock of the central North Sea petroleum system.
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