CHEMOSTRATIGRAPHY OF CENOMANIAN-TURONIAN CARBONATES OF THE SARVAK FORMATION, SOUTHERN IRAN

E. Hajikazemi^1*, I. S. Al-Aasm^{2 4} and M. Coniglio³

¹Department of Geology, Iranian Offshore Oil Company, Tehran, Iran, PO Box 5591.

* Corresponding author: ekazemi@iooc.co.ir

²Department of Earth and Environmental Sciences, University of Windsor, Ontario, N9B 3P4 Canada.

³Department of Earth and Environmental Sciences, University of Waterloo, Ontario, N2L 3G1, Canada.

⁴The Petroleum Institute, Abu Dhabi, UAE.

Stable-isotope and trace-element analyses from six surface and subsurface stratigraphic sections of the mid-Cretaceous Sarvak Formation in southern and offshore Iran confirm the presence of the regional-scale Turonian unconformity and of more local Cenomanian-Turonian unconformities. The geochemical results indicate the presence of previously unrecognized and/or undifferentiated subaerial exposure surfaces. Sarvak Formation carbonates at or near palaeoexposure surfaces show varying degrees of diagenetic modification, and more extensive alteration is associated with longer periods of exposure. The subaerial exposure and associated diagenetic processes greatly influenced reservoir quality and amplified karstification and evolution of porosity in the Upper Sarvak Formation.

The palaeo-exposure surfaces are identified by their negative δ¹³C values (as low as -6.4‰) and negative δ¹⁸O values (as low as -9.4‰), together with low Sr concentrations and relatively high ⁸⁷Sr/⁸⁶Sr ratios. These geochemical characteristics are interpreted to be the result of the interaction of the Sarvak Formation carbonates with meteoric waters charged with atmospheric CO₂. The meteoric waters also caused karstification and soil formation which in some places extends a few metres below the exposure surfaces. Depleted carbon values were not recorded in areas where palaeosols are not well developed or where the uppermost layers of the Sarvak Formation have been removed by erosion.