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.

Cenomanian-Turonian时代的中白垩统Mishrif组是中东重要的储层。先前的研究表明，Mishrif 组中的碳酸盐受岩溶作用的影响，集中在地层顶部。但目前缺乏对岩溶作用对地层储层特征影响的系统研究。根据岩心样品、薄片扫描电镜分析以及对 Missan 油田井的 C、O 和 Sr 同位素地球化学分析，在 Mishrif 组 B21 亚带顶部确定了不整合面。岩心和铸薄片观察表明，不整合面上下岩性差异显着，下方溶蚀孔洞发育良好。包气粉砂填满粒内孔隙，不整合面下的一些模孔内发育土瓣织物。的同位素值⁸⁷ Sr/ ⁸⁶ Sr（0.707270～0.707722）和δ ¹⁸ O（- 5.94 ‰——-2.32 ‰）表明Mishrif组B21亚带顶部受到了同生岩溶的严重影响。岩溶作用是形成优质储层和储层含油非均质性的关键。这项研究将为米桑油田和其他具有类似储层特征的地区的油气勘探提供信息。