Geochemical characteristics of tectonic fracture-filling calcite in Yanchang formation of Longdong area and its relationship with hydrocarbon fluid flow

Key words:  fracture-filling calcite  trace element  stable isotope  hydrocarbon fluid  Ordos Basin

Abstract:

The origin of calcite veins and their relationship with hydrocarbon fluid flow were studied by analyzing trace element and stable isotope, which occurred extensively in the widely distributed tectonic fractures of Yanchang formation in Longdong area of Ordos Basin. The results show that calcite veins in sandstones and mudstones are all ferrocalcite with relatively high Fe and Mn content and low Mg content, and their δ¹³C ranges from -6.6‰ to 2.7‰ (PDB) and δ¹⁸O ranges from -23.0‰ to -14.9‰ (PDB). Combining these data with geological features, it is indicated that the enrichment of Mn in calcite veins is attributed to corrosion of volcanic tuff in Chang 7 member by hydrocarbon fluid, and the formation of these calcite veins is associated with carbon dioxide of rich ¹³C generated by microbial methanogenesis and that of poor ¹³C generated by thermal decarboxylation. The calcite veins mainly formed in the Early Cretaceous during oil generation and expulsion of source rocks in Chang 7 member, and their formation was prior to or coeval with extensive hydrocarbon fluid flow. The tectonic fractures formed in the Late Jurassic are significant fluid flow pathway and play an important role for oil expulsion from source rocks and migration through low permeability sandstones.