Reactivity of dolomitizing fluids and Mg source evaluation of fault-controlled dolomitization at the Benicàssim outcrop analogue (Maestrat basin, E Spain)

Abstract

The mechanisms responsible for the formation of huge volumes of dolomitized rocks associated with faults are not well understood. We present a case study for high-temperature dolomitization of an Early Cretaceous (Aptian-Albian) ramp in Benicàssim (Maestrat basin, E Spain). In this area, seismic-scale fault-controlled stratabound dolostone bodies extend over several kilometres away from large-scale faults. This work aims at evaluating different Mg sources for dolomitization, estimating the reactivity of dolomitizing fluids at variable temperature and quantifying the required versus available fluid volumes to account for the Benicàssim dolostones. Field relationships, stable 13 C and 18 O isotopes, as well as radiogenic 87 Sr/ 86 Sr isotopes, indicate that dolomitization at Benicàssim was produced by a high temperature fluid (> 80ºC). 13 C and 18 O isotopic compositions for dolomite vary from +0.5 and +2.9 ‰ V-PDB and from +21.1 and +24.3 V-SMOW, respectively. A Mg source analysis reveals that the most likely dolomitizing fluid was seawater-derived brine that interacted with underlying Triassic red beds and Paleozoic basement. Geochemical models suggest that evolved seawater can be considerably more reactive than high-salinity brines, and that the maximum reactivity occurs at about 100ºC. Mass-balance calculations indicate that interstitial fluids with high pressure and/or high temperature relative to the normal geothermal gradient cannot account for the volume of dolomite at Benicàssim. Instead a pervasive fluid circulation mechanism, like thermal convection, is required to provide a sufficient volume of dolomitizing fluid, which most likely occurred during the Late Cretaceous post-rift stage of the Maestrat Basin. This study illustrates the importance of fluid budget quantification to critically evaluate genetic models for dolomitization and other diagenetic processes.

Activation of stylolites as conduits for overpressured fluid flow in dolomitized platform carbonates

“…The Benassal Fm is partially dolomitized in close association with the aforementioned regional faults, constituting a world-class case study of fault-controlled dolomitization and associated Mississippi Valley-Type (MVT) mineralization recently reported in the literature (Martín-Martín et al 2013Corbella et al 2014;Gomez-Rivas et al 2014b). These previous studies reported dolostone bodies of tens of meters thick that extend several km away from seismic-scale faults with a dominant stratabound distribution.…”

“…This paper reports the role of stylolites in the burial dolomitization and subsequent hydrothermal corrosion that affects the Benassal Fm (Late Aptian-early Albian) platform carbonates located in Benicàssim (Maestrat Basin, E Spain). Previous works dealing with the dolostones of the Benicàssim area focused on the study of the replacement process (Martín-Martín et al 2013;Corbella et al 2014;Gomez-Rivas et al 2014b;Martín-Martín et al 2015), and concentrated on well-exposed stratabound dolostones cropping out in areas relatively far away from the seismic-scale faults that acted as feeders for dolomitizing fluids. However, the areas closer to the fault zones are not equally well documented, although these represent areas of high interest.…”

Fundamental controls on fluid flow in carbonates: current workflows to emerging technologies

“…Although truly quantitative predictions using RTM are still difficult to achieve (e.g. Katz et al 2011), compare and contrast studies allow us to link depositional environments and climate variations to patterns of early diagenesis (Whitaker & Xiao 2010;Whitaker et al 2014) or to quantify how strata and structures around three-dimensional (3D) faults influence resulting patterns of dolomite geobody characteristics Gomez-Rivas et al 2014). The outcome of RTM models can then guide the modelling of geobodies, porosity and permeability evolution in between wells, ensuring that the static geological model is self-consistent.…”

“…The application of RTM to fault systems in 3D remains limited but is critical to understanding the ways by which the combined impacts of strata and structures influence the resulting patterns of dolomite geobody characteristics. A few 3D studies have provided insights into the rock types and structural associations that strongly influence reservoir quality trends around faults Gomez-Rivas et al 2014) (Fig. 8).…”

“…For example, brine reflux modelling by Al-Helal et al (2012) shed light on why in some settings reflux preferentially dolomitizes muddy sediments but elsewhere favours grainstones. In a study of fault-associated dolomitization, Gomez-Rivas et al (2014) was able to define a minimum difference in permeability to control fluid-flow paths into specific layers. This enabled the authors to differentiate preferred scenarios for fluid fluxes and heat flow to achieve observed patterns and characterization of dolomitization.…”