Calcite‐cemented concretions in non‐marine sandstones: An integrated study of outcrop sedimentology, petrography and clumped isotopes

Abstract

Calcite‐cemented concretions can reduce reservoir quality and form important low‐permeability baffles for fluid flow in sandstone hydrocarbon carrier beds/reservoirs. Understanding the origin and distribution patterns of concretions has important implications for characterizing reservoir heterogeneity and developing fields. The origin of calcite concretions in non‐marine sandstones lacking detrital carbonate to source the cement is poorly studied. The practical difficulty of obtaining the precipitation temperature of the concretions prevents the precise determination of the source of cement components, solute flux, pore water type and the geochemical microenvironments in which cement precipitated. Carbonate clumped isotope thermometry can give unique constraints to the temperature of concretionary cement precipitation. This study focused on outcrops of the Lower Cretaceous non‐marine Qingshuihe Formation in the Urho area, on the north‐western margin of the Junggar Basin (West China). Four depositional facies associations were recognized, corresponding to channel, mid‐channel bar, abandoned channel and overbank floodplain environments. Calcite concretions occur only in sandstones and include spherical, ellipsoidal and tabular forms. They are isolated and/or mutually aligned in layers that tend to be either at the top, base, or within channel and mid‐channel sand bodies. The clumped isotope temperatures for the concretionary calcite range from 31 to 45°C with a standard error of <3°C. The calcite has highly variable, low δ13C values ranging from −16.91 ± 0.01 to −2.93 ± 0.02‰ (Vienna Pee Dee Belemnite). The 13C‐depleted bicarbonate source was linked to biodegradation of migrated oils at shallow burial owing to tectonic uplift and erosion. Calcite δ18O values are very consistent and fall between −12.30 ± 0.04 to −9.79 ± 0.31‰ (Vienna Pee Dee Belemnite). Introduction of meteoric water was a dominant mechanism for the pronounced 18O‐depletion in the oxygen isotopes of pore water, along with water–rock interaction during alteration of volcaniclastic materials in sandstones. Meteoric groundwater flushed the Qingshuihe sandstones as confined aquifers down the regional palaeo‐slope, biodegrading the migrated oils and enabling a sufficient solute flux for precipitation of concretions.