伊拉克东南部白垩系塞诺曼阶—土伦阶下部巨厚型碳酸盐岩油藏具有强非均质性,储层特征受沉积作用控制.应用岩心和铸体薄片资料,结合区域沉积背景,基于岩石结构、颗粒组分和生物碎屑类型等特征识别沉积微相,根据微相组合建立研究区碳酸盐岩沉积模式,并分析了沉积环境及其演化.结果表明:①伊拉克东南部白垩系塞诺曼阶—土伦阶下部发育28种碳酸盐岩微相,发育潮上坪、台内滩、潟湖、潮道、礁滩、滩间、浅水斜坡、深水斜坡和深水陆棚等9种沉积环境.②塞诺曼阶—土伦阶下部处于碳酸盐缓坡沉积环境.早期水体开阔,以深水陆棚和深水斜坡为主,随着全球海平面下降,逐渐演化为浅水环境,发育大规模浅水斜坡、礁滩和潮道,礁滩垂向加积阻挡了水体的正常循环,晚期逐渐演化为局限环境,以大规模的潟湖和台内滩为主.建立了研究区塞诺曼阶—土伦阶下部缓坡—弱镶边碳酸盐台地沉积模式,为储层预测和成因分析奠定了地质基础.

The Cretaceous is an important oil and gas producing layer in the Middle East. The Cenomanian-lower Turonian develops thick carbonate strata, including the Ahmadi Formation, Ruamila Formation, and Mishrif Formation, of which the Mishrif Formation is an important oil-bearing interval in Iraq. The Upper Cretaceous Cenomanian-lower Turonian is a complete second-order sequence. In the early Cenomanian, the tectonic range in southern Iraq was relatively small, and the sedimentation was dominated by carbonate ramp. In the mid-Cenomanian, the central part of Iraq continued to subside, while the southern part was in a tectonic uplift with shallow water, high land, and high subsidence deposition rates. From the late Cenomanian to the early Turonian, the sea level continued to decline, and with the further strengthening of tectonic action, the scale of the paleo-uplift continued to expand. The carbonate sediments filled the landforms, resulting in gentle topographic changes, and the reefs and shoals gradually formed barriers, resulting in poor water exchange between southern Iraq and the open sea. The Cenomanian-early Turonian giant-thick carbonate reservoirs of Cretaceous in southeastern Iraq are highly heterogeneous, and the reservoir characteristics are controlled by sedimentation, but the research on microfacies and sedimentary models is weak. Based on core and cast thin section, combined with regional sedimentary evolution, the study of microfacies and sedimentary models is carried out according to the analysis of rock structure, particle composition and bioclastic. According to microfacies combination, sedimentary model is established, and the depositional environment and its evolution were recognized. The results show that: (1) 28 types of carbonate microfacies are developed in the Cretaceous Cenomanian-early Turonian in southeastern Iraq. The rock types in the study area include mudstone, mudstone-wackstone, wackstone, packstone, grainstone, and rudstone, reflecting the depositional energy from weak to strong. Bioclasts mainly includes rudist, bivalves, echinoderms, enthic foraminifera, algae, sponges, ostracods, gastropods, etc., as well as a large number of unidentifiable fine-grained debris. Each biological debris reflects specific sedimentary hydrodynamic conditions. Phenomena such as bioturbation and bird-eye structures can be identified and 9 types of sedimentary environments, i. e., upper tidal flat, intra-platform shoal, lagoon, intershoal, tidal channel, reef-shoal, shallow-water lope, deep-water slope and deep-water shelf, are identified. Different facies zones were systematically summarized concerning the location, marine condition, microfacies combination, bioclastics, and rock structure. (2) The Cenomanian-early Turonian in the study area is developed in carbonate ramp. At the beginning of Cenomanian, the water body was open and dominated by deep-water shelf and deep-water slopes. With the decline of global sea level, it gradually evolved into a shallow-water environment with large-scale shallow-water slopes, reef-shoal and tidal channels. The vertical accumulation of reef-shoals blocked the water circulation, and it gradually evolved into a restricted environment in the late stage, dominated by large-scale lagoons and intra-platform shoal. Finally, a gentle ramp-weakly rimmed carbonate platform sedimentary model is established, which lays a geological foundation for reservoir prediction and genetic analysis.