Molecular memory effects recording the accumulation history of petroleum reservoirs: A case study of the Heidrun Field, offshore Norway

The immobile portion of the oil occupying the pore space of reservoir sandstones, which is mostly represented by visible impregnations, is shown by the present study to contain geochemical signals that record their filling history. The clue to unravelling these signals is sequential extraction of such impregnations of residual oil by a high-pressure solvent flow-through cell in which the solvent percolates through the intact and primary pore system of the rock sample. Gas chromatography–mass spectroscopy (GC–MS) analysis of residual oils extracted from samples of certain reservoir sandstones within the Heidrun Field, offshore Norway, reveal significant contrasts in the chemical maturity of the first and the last fractions recovered (as high as 0.69 versus 0.50 for the MPI-1 ratio, corresponding to 0.79% versus 0.67% Rc). The hypothesis is proposed that by using this extraction technique oil impregnations from different portions of the pore system (i.e. from large versus small pores) are obtained separately. This interpretation is especially supported by the regular occurrence of reservoir sandstones in which the residual oil exhibits maturity contrasts between those fractions extracted from large and small pores of the same rock. Within the Heidrun Field the distribution of reservoir sandstones with such maturity contrasts is strictly controlled by their position in the oil column and the location of their fault compartment. These contrasts are interpreted to reflect the maturity levels of end members of the petroleum charges, which have filled this trap during an extended period of geological time. Reservoir sandstone samples containing such end-member charges are limited to the uppermost portion of the oil column. Lower in the oil column, the pore system of all reservoirs contains oil impregnations of a uniform maturity level, regardless of their pore-size origin. This distribution is believed to indicate an early stage of the accumulation history of the Heidrun Field, characterised by the formation of a palaeo-oil/water contact well above the present one. Moreover, the four fault compartments studied, each controlled by bordering faults with different sealing capacities, were not filled during the same stages of the accumulation history. In summary, the present study has shown that oil impregnations, depending on their location within the pore system of individual reservoir sandstones, can retain molecular memory effects, which in turn allow the accumul