Carbonates present in siliciclastic sequences, although in low abundance, can play a significant role in controlling the porosity-permeability field of reservoir units. This is mainly because organic aragonite
present at time of deposition will tend to dissolve and reprecipitate as a cement phase. Diffusion processes at the pore scale will control the growth of individual carbonate concretions, and their evential merging into more continuous cemented layers.
Here, we use the novel clumped isotope technique to study carbonate precipitation in siliciclastic reservoirs, notably the Cretaceous Mancos Shale of Colorado and Utah. One of the main focus of our effort has been to understand what controls the depletion of 18O in the iron-rich dolomite/ankerites of concretions in the Mancos Shale, i.e. whether it is triggered by mixing of meteoric fluids, or if precipitation occured at high temperatures. Furthermore, we study the sepatarian fracture infills of the concretions to understand their genesis.
The advantage of clumped isotopes for this application is that the temperature derived from clumped isotope is independent of the 18O composition of the diagenetic fluid, thus giving an unambigous answer to our question.