Karstification characteristics of the Cenomanian-Turonian Mishrif Formation in the Missan Oil Fields, southeastern Iraq, and their effects on reservoirs

The Middle Cretaceous Mishrif Formation of the Cenomanian-Turonian age is an important Middle Eastern reservoir. Previous studies have shown that carbonates in the Mishrif Formation are affected by karstification and are concentrated at the top of the formation. However, there is a lack of systematic research on the effects of karstification on the formation’s reservoir characteristics. Based on core samples, scanning electron microscopy of thin sections, and geochemical analysis of C, O, and Sr isotopes from wells in the Missan Oil Fields, unconformity was identified at the top of the B21 subzone of the Mishrif Formation. Core and cast-thin-section observations indicate that there is a significant lithological difference above and below the unconformity, and dissolution pores and voids are well-developed beneath it. Vadose silt fills the intragranular pores and geopetal fabric is developed inside some mouldic pores beneath the unconformity surface. The isotopic values of 87Sr/86Sr (0.707270–0.707722) and δ18O (− 5.94 ‰— −2.32 ‰) suggest that the top of the B21 subzone of the Mishrif Formation has been severely affected by syngenetic karst. Karstification is the key to high-quality reservoir formation and the reservoir’s oil-bearing heterogeneity. This study will inform oil and gas exploration in the Missan Oil Fields and in other areas with similar reservoir characteristics.