Rudy Swennen, K.U.Leuven (Belgium)
Julie Dewit, Earth and Environmental Sciences K.U.Leuven (Belgium)
marijke Huysmans, Earth and Environmental Sciences K.U.Leuven (Belgium)
David Hunt, StatoilHydro ASA (Norway)
Near the village of Ranero (Cantabrian Mountains, N-Spain) impressive coarse crystalline dolomite bodies occur, either cementing up to 50 m wide faults, or as cement infill in a limestone breccia body 200 by 150m wide. However, the most interesting dolomite body develops along the Ranero fault, along which different dolomite pulses have been recognised in the Pozalagua quarry. Along this major fault as well as its satellite faults, several dolomite bodies, up to a 0.5 square kilometres in size developed in platform carbonates. The development of the latter clearly is structurally controlled.
Within the latter dolomite bodies several types of coarse crystalline dolomites have been recognised such a zebra dolomites, zebroid dolomites and different types of dolomite breccias with as major variation the degree of breccia fragment assimilation by the coarse crystalline dolomite. Lithology exerts a major control on porosity and permeability with highest values recorded in the zebra/zebroid dolomites. Porosity varied between 0-11%. Its distribution was studied by microfocus X-ray tomography. Based on regional variations in lithology, trace element geochemistry (Ca, Mg, Fe, Mn, Sr, Na and K) and stable carbon and oxygen isotope characteristics which have been assessed by a multiple point geostatistical analysis the major controlling aspects with regard to structural setting and fluid flow have been addressed. Clearly one of the controlling aspects on porosity relates to overdolomitisation, explaining the loss of porosity due to cementation by dolomite. A late pore blocking calcite also lower the overall porosity.