Microfacies and geochemistry of the Middle Cretaceous Sarvak Formation in Zagros Basin, Izeh Zone, SW Iran

The Sarvak Formation (Albian to Cenomanian), forming part of the Bangestan Group, comprises a thick succession of carbonate rocks in the Zagros Basin of Southern Iran. The Sarvak Formation forms one of the main hydrocarbon reservoirs of this region. Three sections in the northern (252 m thickness) and southern flanks (348 m thickness) of the Mish and Anneh anticlines (245 m thickness) in the Izeh Zone of SW Iran were selected for this study. Petrographic analysis led us to define 10 microfacies units deposited in four facies belts: (i) tidal flat, (ii) lagoonal, (iii) shoal, and (iv) open marine. All of these form part of a platform ramp environment. Geochemical investigations (limited to outcrops of the Mish anticline) aided in understanding geochemical and diagenetic characteristics of carbonate rocks from the Sarvak Formation. Variations of oxygen and carbon isotope data from limestones of the Sarvak Formation indicate near primary marine phreatic and partially meteoric digenesis for both flanks of the Mish anticline, respectively. Lower abundances of Mn and Fe suggest that the carbonates at both sites appear to have stabilized under mainly oxic conditions. The Sarvak carbonates from the North Flank of the Mish anticline indicate higher rock–water interaction and a closed diagenetic system in comparison to the South Flank of the Mish anticline. Vertical and lateral variations in stratigraphic thicknesses in selected outcrops can be related to the presence of syn-sedimentary faults in the Zagros Basin that resulted in regional instability or due to rates of sedimentation with lower rates of deposition in the basin during Middle Cretaceous times.