Geological significance of early cements: insight from laboratory simulated diagenesis

Little is known about the evolution of petrophysical properties (porosity, permeability, mechanical resistance) associated with the growth of microstructures produced during the early stages of diagenesis. To gain a better understanding of these processes, we set out to recreate the microstructures resulting from early carbonate diagenesis in the laboratory. We reproduce a meteoric phreatic environment by heating a sorted aragonitic ooid sand saturated with distilled water. During an initial phase of mineralogical stabilization, we rapidly observe internal dissolution of the ooids and the formation of a calcitic rim at the periphery of the grains. Dissolution occurs along the laminated structures of the ooids. The outer calcite crystals are regular in shape and size. This is followed by a second phase, in which the internal structure of the oolite disappears. Large, sparitic calcite crystals appear in place of the internal laminations. These two phases can be explained in terms of the evolution of CaCO₃ saturation in the fluid, and by a competition between nucleation and crystal growth. In the geological record, those structures are often interpreted as the beginning of burial diagenesis. We show that those features can appear very early in the system and are likely to be the cause of the chemical and thermodynamic isolation of the ooids from the fluid in the pore space. We can also conclude from these observations that the creation of internal porosity inside grains is very early, and can appear in a stationary fluid, at shallow depth.