Controls on Hydrothermal Sub-stratiform Dolomitization of an Outcrop Analogue: The Benicassim Case Study (Maestrat Basin, E Spain)

Gomez-Rivas, Enrique ¹; Martín-Martín, Juan Diego ²; Corbella, Mercè ¹; Stafford, Sherry L.³; Lee, Alex G.K. ⁴; Travé, Anna ²; Salas, Ramón ²; Teixell, Antonio ¹; Vergés, Jaume ⁵; Griera, Albert ¹; Cardellach, Esteve ¹
(1) Departament de Geologia, Universitat Autònoma de Barcelona, Barcelona, Spain. (2) Departament de Petrologia, Geoquímica i Prospecció Geològica, Universitat de Barcelona, Barcelona, Spain. (3) ExxonMobil Upstream Research Company, Houston, TX. (4) ExxonMobil Corporate Strategic Research, Annandale, NJ. (5) Institute of Earth Sciences Jaume Almera, Barcelona, Spain.

Hydrothermal dolomitization is one of the most important processes in enhancing or degrading carbonate porosity and permeability. Burial, high-temperature hydrothermal dolomite forms because of the interaction of one or more solutions, mainly seawater-derived or deep brines, with limestones. The Early Cretaceous Benicassim ramp (Maestrat basin, E Spain) is an excellent outcrop analog for partially dolomitized reservoirs. In this area, seismic-scale sub-stratiform hydrothermal dolomitized bodies extend for several kilometers away from large-scale faults. The Benicassim ramp is taken as a case study to characterize dolomite events, evaluate possible Mg sources (as well as reactivity of fluids at various temperatures) and, finally, perform 3D geochemical, fluid flow and heat flow numerical simulations of the dolomitizing process.

Petrography reveals that porosity was considerably reduced by early calcite cementation before the replacement stage. The grain-dominated facies are preferentially replaced, while the mud-rich facies are rarely dolomitized. The paragenesis is formed by two phases of replacive dolomite and a late phase of saddle dolomite associated to galena mineral deposits. The geochemical data (fluid inclusion microthermometry, C, O, and Sr isotopes) indicate that the main dolomitization event was produced by a high temperature (> 80 C) brine that interacted with K-rich rocks, probably in the underlying Permian-Triassic rocks and the Paleozoic basement.

Geochemical, fluid flow, and heat flow numerical simulations rule out a rapid release of overpressured and overheated interstitial fluids, in recurrent pulses through large-scale faults, as the unique dolomitizing fluid flow mechanism in Benicassim. Such fluids would not maintain the high temperature of the Benicassim dolomite, as thermal conduction dominates heat advection over large time scales.

Instead, convection due to temperature differences within the basin is the most plausible mechanism to account for the dolomitization of the Benicassim limestone. A long-term open and deep convection system in the whole basin would have pumped dolomitizing fluids at high temperature and during a long enough period of time to cause the dolomitization reaction. A permeability contrast of two orders of magnitude is required to drive lateral flow preferentially along high permeability layers to form sharp, sub-stratiform dolomitization fronts.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.