Fault-controlled and stratabound dolostones in the Late Aptian-earliest Albian Benassal Formation (Maestrat Basin, E Spain): petrology and geochemistry constrains

Fault-controlled hydrothermal dolomitization of the Late Aptian to Early Albian Benassal Fm shallow water carbonates resulted in the seismic-scale stratabound dolostone geobodies that characterize the Benicàssim case study (Maestrat Basin, E Spain). Petrological and geochemical data indicate that dolomite cement (DC1) filling intergranular porosity in grain-dominated facies constituted the initial stage of replacement. The bulk of the dolostone is formed by a replacive nonplanar-a to planar-s dolomite (RD1) crystal mosaic with very low porosity and characteristic retentive fabric. Neomorphic recrystallization of RD1 to form replacive dolomite RD2 occurred by successive dolomitizing fluid flow. The replacement sequence DC1-RD1-RD2 is characterized by a depletion in the oxygen isotopic composition (mean δ18O(V-PDB) values from -6.92, to -8.55, to -9.86‰), which is interpreted to result from progressively higher temperature fluids. Clear dolomite overgrowths (overdolomitization) precipitated during the last stage of the replacement. Strontium isotopic composition suggests that the most likely origin of magnesium was Cretaceous seawater-derived brines that were heated and enriched in radiogenic strontium and iron while circulating through the Paleozoic basement and/or Permo-Triassic red beds. Burial curves and analytical data indicate that the replacement took place at burial depths between 500 and 750 m, which correspond to the Late Cretaceous post-rift stage or early Tertiary extension of the Maestrat Basin, and by hydrothermal fluids likely exceeding temperatures of 80ºC.Following the partial dolomitization of the host rock, porosity considerably increased in dolostones by burial corrosion related to the circulation of acidic fluids likely derived from the emplacement of the Mississippi Valley-Type deposits. Acidic fluids, probably overpressured, circulated along faults, fractures and stylolites. Saddle dolomite and ore-stage calcite cement filled most of the newly created vuggy porosity. Subsequent to MVT mineralization, precipitation of calcite cements resulted from the migration of meteoric-derived fluids during uplift and subaerial exposure. This late calcite cement destroyed most of the dolostone porosity and constitutes the main cause for the present day poor reservoir quality of the Benassal Fm dolostones.