DIAGENETIC HISTORY OF LATE PALAEOCENE POTENTIAL CARBONATE RESERVOIR ROCKS, KOPET-DAGH BASIN, NE IRAN
A. Mahboubi1, R. Moussavi-Harami1,2, R.L. Brenner2 and L.A. Gonzalez2
1Department of Geology, Faculty of Sciences, Mashhad University, Mashhad 91384, Iran.
e-mail: harami@science1.um.ac.ir or rmoussav@blue.weeg.uiowa.edu
2Department of Geoscience, University of Iowa, Iowa City, IA 52242-1379, USA.
e-mail: robert-brenner@uiowa.edu.
The Upper Palaeocene (Thanetian) Chehel-Kaman Formation in the Kopet-Dagh Basin of NE Iran is principally composed of carbonates with minor siliciclastics and evaporites. Six stratigraphic sections were measured and more than 1,000 samples were collected for petrographic, stable isotope geochemistry (carbon and oxygen isotopes) and trace element analyses. Four major carbonate lithofacies (and 13 subfacies) have been identified in previous studies, and reflect deposition in a shallow-marine environment.
Petrographic analyses indicate that the Chehel-Kaman Formation carbonates have undergone a complex diagenetic history which includes compaction, cementation, micritization, dissolution, silicification, dolomitization, neomorphism and fracturing. d 18O and d 13C values in Chehel-Kaman Formation limestones range between +0.8 and -15.1‰ PDB, and -2.82 and +3.5 ‰ PDB respectively. These variations are interpreted to reflect meteoric and burial diagenetic processes. Variations in trace-element concentrations (Fe and Mn increased while Na decreased) also indicated the effects of meteoric flushing. The limestones appear to have been formed at about 28°C.
Dolomites were divided into d1 (finely-crystalline) and d2 (coarsely-crystalline) types. Petrographic and geochemical results indicated that the d1 dolomites formed under sabkha conditions from a parental solution at around 26°C, while d2 dolomites formed during burial diagenesis with much hotter pore fluids at around 72°C. The paragenetic sequence indicates that primary porosity decreased during early stages of diagenesis, although secondary porosity was subsequently created improving the reservoir quality of the carbonates.