Origin of calcite cements and their impact on reservoir heterogeneity in the Triassic Yanchang Formation, Ordos Basin, China: A combined petrological and geochemical study

Abstract

Abstract Calcite cement is a typical product of fluid–rock interaction in petroleum reservoirs, and its migration has a major impact on reservoir heterogeneity. A case study of a closed diagenetic system was carried out on the Upper Triassic Yanchang Formation of Ordos Basin, China—the most successful exploration region for tight sandstone oil in China. The investigated sandstones are mainly lithic arkoses (Q31F42L27). Three occurrences of calcite cement are the most important diagenetic minerals affecting reservoir quality: early calcite completely filling intergranular volumes; late calcite filling pores after chlorite coating, distributed in oil layers, or locally replacing feldspar; and crystal-stock calcite typically distributed in dry layers. δ13C values of the sandstones are in the range −9.69‰ to −2.05‰, indicating that calcite is derived mainly from thermodynamic decarboxylation of organic matter. Dark mudstone interbedded with sandstone and hydrocarbon emplacement are the two crucial sources of organic fluid, providing sufficient Ca2+, Mn2+, Fe2+, and Mg2+ for calcite precipitation through interaction with feldspar and other silicates, such as dissolution and replacement together with transformation of smectite to illite. Manganese was continuously enriched in calcite during burial diagenesis. Driven by compaction of interbedded mudstone and fluid overpressure, calcite migrated widely into calcite-cemented zones (CCZs) where fluid–rock interaction was strong, such as in the margins of sandstone bodies, intrastratal sandstones with original high-permeability zones, and thin sandstones, thereby forming an important organic CO2 sink. Potential high-quality reservoirs are distributed mainly in the central part of the closed system, restricted by calcite-cemented zones, and are an exploration target. Our data suggest that frequently alternating sandstone and mudstone sequences comprise a complete organic CO2 source/sink system, controlling the migration of calcite cement in a closed diagenetic system and reservoir heterogeneity by fluid–rock interaction.