| 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, mudstonewackstone, 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 stablished, which lays a geological foundation for reservoir prediction and genetic analysis. |
