Vein calcite in cretaceous carbonate reservoirs of Abu Dhabi: Record of origin of fluids and diagenetic conditions

Petrographic, geochemical analyses (major, trace and rare-earth elements and C, O- and Sr-isotopes), and fluid-inclusion microthermometry study of vein calcites in primarily Cretaceous reservoir carbonate rocks in oilfields from Abu Dhabi, UAE, helped to unravel the formation conditions and origin of diagenetic fluids. The vein calcites have diverse crystal shapes (equant blocky, bladed and fibrous) and sizes (up to 3 cm across) and display complex cross-cutting relationships. These calcites display a wide range of stable carbon (delta C-13(V-PDB) = -11.1 parts per thousand to +9.6 parts per thousand) and oxygen (delta O-18(V-PDB) = -12.7 parts per thousand to -2.3 parts per thousand) isotopic compositions, indicating formation under diverse geological conditions. The oxygen isotopic composition suggests that the precipitation of vein calcites occurred at temperatures of ca. 30-100 degrees C assuming that the fluids had delta O-18(V-SMOW) values of marine pore waters which evolved to basinal brines (i.e. -1.2 parts per thousand and +2 parts per thousand, respectively). These inferred temperatures corroborate the fluid-inclusion microthermometry, which revealed a predominantly single, whole liquid phase that suggests entrapment temperature less than ca. 50 degrees C. A few two-phase fluid inclusions in equant and bladed calcite indicate precipitation at ca. 68-100 degrees C and salinity of ca. 3.9-10.1 wt.% equivalent NaCl). The carbon isotopic signatures suggest derivation of dissolved carbon from the host marine carbonates and, less commonly, from the degradation of organic matter by methanogenic and sulfate-reducing bacteria. The diverse sources of carbon coupled with the inferred wide range of precipitation temperatures presumably account for the lack of correlation between the carbon and oxygen isotopes. The derivation of dissolved carbon mainly from the host rocks is in good agreement with the marine Sr isotopic ratios of these calcites (Sr-87/Sr-86 = 0.70744 to 0.70766). Rare-earth element patterns suggest significant fluid interaction with upper continental crustal rocks as evidenced by negative Eu anomalies and LREE-enriched patterns. (C) 2010 Elsevier B.V. All rights reserved.