The creation of calcite microcrystals and microporosity through deep burial basinal flow processes driven by plate margin obduction – A realistic model?

Calcite microcrystals and associated microporosity are ubiquitous and extensively developed in Jurassic and Cretaceous carbonate sequences in the Middle East. Clumped isotope analyses of calcite microcrystals in the Lower Cretaceous Thamama-B strata in UAE reservoirs indicate temperatures of 60–90 °C and burial of 1.5–2.5 km suggesting formation synchronous with and updip of Late Cretaceous ophiolite obduction at the Eastern Arabian continental margin. Assuming that recrystallization of precursor calcite to calcite microcrystals requires initially undersaturation to drive dissolution/re-precipitation a basin-scale 2D reactive transport model (RTM) was constructed. The model is constrained by hydro-mechanical simulations and used to quantitatively evaluate the hypothesis that the formation of calcite microcrystals and associated microporosity is driven by expulsion of compaction fluids during rapid burial. The combined influence of fluid flux and cooling results in trace net calcite dissolution (porosity increase <0.1 vol %) focused at depths of 2.7–5.1 km. The presence of even minor amounts of minerals with common ions (dolomite and anhydrite) induces additional dissolution but does not change its' spatial distribution. Whilst RTMs only yield a minimum estimate of the degree of recrystallization that likely occurs driven by calcite disequilibrium, simulations suggest these reactions occur at temperatures of 95–170 °C, markedly higher than those estimated from clumped isotopes of the calcite microcrystals. Mixing of fluids leaked from underlying strata up faults into the lateral flow system could play an important role in burial diagenesis, with the slow leakage dissolving up to ten times greater mass of calcite than a shorter pulse of equivalent fluid volume. A previously unrecognized effect of the introduction of H 2 S(aq)-rich fluids, derived from accelerated thermal maturation or thermochemical sulphate reduction (TSR), into formation water with high CO 2 (aq) concentration is CO 2 degassing that drives net calcite precipitation (<3 vol %). This suite of numerical simulations suggest that calcite disequilibrium will have occurred in fluids expelled during ophiolite obduction, but that any associated recrystallization will have occurred at depths greater than those inferred from temperatures measured in the calcite microcrystals. Recrystallization at shallower depths may have been associated by fluid-mixing around the injection points, but reaction kinetics suggests that laterally pervasive alteration would require very rapid flow rates. These results suggest that alternative mechanism(s) are needed to be considered to explain the extensively developed calcite microcrystals and associated microporosity in Mesozoic carbonates of the Middle East. • We investigate whether calcite microcrystals and associated microporosity can be generated in limestone sequences under burial conditions as a result of regional migration of basinal brine caused by a plate collision. • We propose a quantitative approach to evaluate the diagenetic effects of compactional flow using burial fluid expulsion estimates from geomechanical modelling as the injection of fluids in reactive transport simulations. • Cooling of compaction fluid expelled after rapid burial generates calcite dissolution, and thus the potential for recrystallization, with minor net porosity increase over a wide range of burial temperature and pressure conditions with varying CO 2 (aq) concentrations and shows only marginal effect of the presence of gas phase (CO 2) and minerals with common ions (dolomite and anhydrite). • These reactions occur at temperatures higher than suggested from clumped isotopes measurements from calcite microcrystals that appear to have developed synchronous with Late Cretaceous ophiolite obduction onto the Eastern Arabian continental margin. • Mixing between compaction fluid and fluid released from underlying units under dynamic flow conditions shows that the introduction of H 2 S(aq)-rich fluids and mixing with high CO 2 (aq) concentration formation water could induce CO 2 degassing and calcite precipitation. [ABSTRACT FROM AUTHOR]