Download Vein calcite in cretaceous carbonate reservoirs of Abu Dhabi

Journal of Geochemical Exploration Volume 106, Issues 1–3, July–September 2010, Pages
156–170
GEOFLUIDS VI: Recent Advances in Research on Fluids in Geological Processes
Sixth International GEOFLUIDS Conference
Vein calcite in cretaceous carbonate reservoirs of Abu Dhabi:
Record of origin of fluids and diagenetic conditions
S Morad, IS Al-Aasm, M Sirat, MM Sattar
Abstract
Petrographic, geochemical analyses (major, trace and rare-earth elements and C, O- and Srisotopes), 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 (δ13CV-PDB = − 11.1‰ to + 9.6‰) and oxygen (δ18OV-PDB = − 12.7‰ to
− 2.3‰) 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 °C assuming that the fluids had δ18OV-SMOW values of marine
pore waters which evolved to basinal brines (i.e. − 1.2‰ and + 2‰, 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 °C. A few twophase fluid inclusions in equant and bladed calcite indicate precipitation at ca. 68–100 °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 (87Sr/86Sr = 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.