Depositional Environment, Diagenetic History and Chemostratigraphy of the Mid-Cretaceous Sarvak Carbonates in Southern Iran

This study presents integrated petrography, stable carbon and oxygen and also strontium isotopes and major and trace element data for the Cenomanian-Turonian carbonates of southern Iran. These data provide new insights into the diagenetic history and reservoir characteristics of the prolific Sarvak Formation and chemostratigraphy of Mid-Cretaceous strata in the Tethyan region.;Chemostratigraphy established from several sections confirms the presence of the regional Turonian and local Cenomanian-Turonian unconformities and reveals the presence of previously unrecognized subaerial exposure surfaces. The paleoexposure surfaces are recognized by their negative delta 13C and delta18O values (as low as -6.4 and -9.40/00 VPDB, respectively) coupled with low Sr concentrations and higher 87Sr/86Sr ratios. These exposure surfaces have dual effects on reservoir quality: a constructive effect, which enhances the porosity through extensive dissolution of allochems and a destructive effect which includes occlusion of some of the porosity by meteoric cement precipitation.;Positive delta13C values in the various carbonate phases reflect values of sea water coeval with Oceanic Anoxic Event (OAB) and Mid-Cenomanian Event (MCE). The delta13C excursions identified in Sarvak carbonate have been correlated globally.;The interaction of the Sarvak carbonates with meteoric waters charged with atmospheric CO2, caused dissolution, karstification and palaeosol and bauxite formation below the exposure surfaces in some localities. The latter is likely an indicator of a warm and humid climate during that time. Calcite cementation is the predominant contributor to the porosity loss.;The Cenomanian-Turonian Sarvak Formation forms one of the main hydrocarbon reservoirs in southern Iran and the Persian Gulf. The Sarvak carbonates are underlain by Kazhdumi Shales while a regional unconformity marks the top of the formation. These carbonates were deposited in a ramp setting. Detailed investigation using: well-logs, pertrography and geochemistry indicate that facies and diagenetic processes influenced reservoir quality.;The delta18O and delta13C values of various generations of calcite cements overlap to a large extent with calcite matrix and rudist shells, indicating their equilibration with fluids of similar isotopic composition (i.e. marine fluids). More negative delta18 O values suggest a significant meteoric component. The progressive decrease in delta18O values of some of the calcite cements (e.g. -12.3‰) and matrix-replacive dolomites could be related to precipitation at higher ambient temperatures. Their occurrence adjacent to stylolites suggests the formation during burial diagenesis. Partial dolomitization in some mud/wackestone intervals with shale interbeds is another factor in enhancement of the secondary porosity and reservoir development.