Multiple organic–inorganic interactions and influences on heterogeneous carbonate‐cementation patterns: Example from Silurian deeply buried sandstones, central Tarim Basin, north‐western China

Abstract

AbstractThis study investigates the importance of multiple organic–inorganic interactions on heterogeneous carbonate cementation patterns in Silurian deeply buried sandstones, central Tarim Basin, north‐western China, to evaluate their effects on reservoir quality. Petrographic observations and mineral geochemistry identify two stages of carbonate cementation: (i) eogenetic, poikilotopic blocky calcite precipitated at 20 to 50°C (δ13CVPDB from −0.3‰ to +6.8‰; δ18OVPDB between −6.6‰ and −2.9‰) and dolomite precipitated at 40 to 62°C (δ13CVPDB from +1.9‰ to +5.3‰; δ18OVPDB between −6.2‰ and −2.1‰); and (ii) mesogenetic, isolated pore‐filling ankerite formed at 70 to 120°C (δ13CVPDB from −14.4‰ to −6.6‰; δ18OVPDB between −11.6‰ and −7.8‰). The eogenetic carbonates are predominantly distributed as carbonate‐cemented beds or concretions along sandstone–mudstone contacts and were derived mainly from microbial methanogenesis of organic matter and dissolution of carbonate minerals in adjacent calcareous mudstones. The mesogenetic ankerite cements occur predominantly as carbonate‐cemented beds, concretions and patches, and are more concentrated in the central sections of sandstone bodies. It is inferred that ankerite cements were sourced from dissolution of eogenetic calcite and dolomite via in situ generation of organic CO2 related to thermal decarboxylation of organic acids within sandstones. Kinetic modelling results coupled with petrographic observations illustrate that mesogenetic dissolution of eogenetic calcite and dolomite cements did not enhance total reservoir porosity due to re‐precipitation of ankerite cements on a very local scale within sandstone bodies. The development of extensively carbonate‐cemented geometries (beds, concretions and patches) created by different generations of carbonate cements derived from external sources, led to reservoir heterogeneity and significant destruction of sandstone reservoir quality especially during diagenesis. The results of this study demonstrate the importance of multi‐stage, intra‐formational mass transfer induced by concentration gradients during organic–inorganic interactions in mediating a variety of spatial patterns of carbonate cementation under different diagenetic regimes. Better insights of these organic–inorganic interactions in a coupled sandstone–mudstone system could improve predictive models of heterogeneous diagenetic alterations in deeply buried sandstones of similar origin.