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The Skellefte district in northern Sweden consists mainly of pyrite-rich massive sulphide deposits intercalated in an early Proterozoic volcano-sedimentary sequence. The volcanic activity was predominantly submarine, and gave rise to abundant felsic rocks with minor mafic volcanic rocks and rare andesitic volcanism. A few dykes and volcanic flows of high Mg-Cr basalts, probably komatiitic, are also found within the volcanic pile.The geochemistry of the volcanic rocks is of subalkaline character and displays slightly tholeiitic to calcalkaline differentiation trends. The Ti, Zr, Y, Cr and rare-earth contents of the basaltic and andesitic rocks are consistent with a subduction related volcanic arc setting.The bimodal nature of the volcanic suite, the presence of highly primitive basaltic rocks and the abundance of felsic pyroclastic material suggests that rifting occurred within the volcanic arc. Long periods of tensional stress predominated and favoured the extrusion of large volumes of felsic pyroclastics and the formation of tectono-volcanic depressions. The convergence in time and space of a thick volcanic sequence with high permeability, a high geothermal gradient related to the rising of basaltic magma, and synvolcanic faulting resulted in extensive hydrothermal activity. The hydrothermal activity caused regional alteration of the volcanic rocks and locally deposition of pyrite-rich massive sulphide deposits within local, tectonically controlled, depressions.

Stochastic modeling in reservoir description should be classified with respect to the type of variables involved in the study: continuous, mosaic or event spatial variables. The calcite cemented units in a sand stone reservoir is considered as a mosaic variable, and the stochastic model is based on Markov random field theory. The model and parameter values, constrained by the well observations, provide the base for generating several realizations of the reservoir description. These realizations are homogenized and the production potential is evaluated by a preliminary numerical production simulation. The set of production profiles corresponding to the reservoir realizations will reflect the uncertainty due to lack of exact geological knowledge about the reservoir. The importance of establishing an automatic line of production for uncertainty evaluation is emphasized.

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