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

Abstract

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 H2S(aq)-rich fluids, derived from accelerated thermal maturation or thermochemical sulphate reduction (TSR), into formation water with high CO2(aq) concentration is CO2 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.