Interaction of stratigraphic and sedimentological heterogeneities with flow in carbonate ramp reservoirs: impact of fluid properties and production strategy
Septarian carbonate concretions contain carbonate precipitated during progressive growth of the concretion and subsequent fracture-filling. As such, they have been used to track variations in _13C and _18O of pore waters during diagenesis and to define diagenetic zones in clastic rocks. However, the _18O value of the carbonate is dependent on precipitation temperature and the _18O value of the pore fluid from which it precipitated. Interpretations must assume one of these parameters, both of which are highly variable through time in diagenetic settings. Carbonate clumped isotopes of the cement can provide independent estimates of temperature of precipitation, allowing the pore-water _18O to be back-calculated. Here, we use this technique on carbonate concretions and fracture fills of the Upper Cretaceous Prairie Canyon Member, Mancos Shale, Colorado. We sampled concretions from two permeable horizons separated by a 5 m shale layer, with one permeable horizon containing concretions with septarian fractures. We show cores precipitated at cooler temperatures (31_°C, _660 m burial depth) than the rims (68_°C (_1980 m burial depth) and relate that to the _13Ccarbonate values to suggest the concretion core precipitated in the methanogenic zone, with increasing input from thermogenically produced CO2. The two concretion-bearing horizons have different back-calculated _18Oporewater values (mean ‰ and 1.13‰ VSMOW) for cements formed at the same temperature and similar _13C values, suggesting the shale layer present between the two horizons acted as a barrier to fluid mixing. Additionally, the _18Ocarbonate of the septarian fractures (‰ VPBD) are due to precipitation at high temperatures (102 to 115_°C) from a fluid with a mean _18Oporewater of 0.32‰ (VSMOW). Therefore, we can tie in the cementation history of the formation to temporal and spatial variations in _18Oporewater.