Marine nodular limestone reservoirs are generally developed in the Middle East, some oilfields such as Halfaya, Ahdeb, Azadegan have reserves of more than 4 billion tons and the capacity to produce more than 400 million tons. Strengthening the study on the genesis of nodular limestone reservoirs is of great significance to improve the accuracy of characterization. In this paper, the research object is the reservoir of the Lower Cretaceous Khasib Formation in Iraq A Oilfield. Based on core, thin section, logging and other basic data, this paper studies on reservoir characteristics and sedimentary diagenesis, and clarifies the genetic mechanism of nodular limestone and its control on reservoir. The results show that: (1) Nodular limestone reservoirs are developed in various carbonate rocks such as green algal bioclastic limestone, micritic bioclastic limestone and bioclastic sandy limestone. Most cores of Khasib limestone reservoirs are characterized by nodule, which can be divided into white and dark patches according to the color. The white patch reservoir is dense and oil-free, the dark patch reservoir has good physical properties and is oil-bearing. (2) The intergranular pores of bioclastic in white patches are almost completely cemented by bright calcite, while the cementation of bright calcite in dark patches is weak and inter granular pores are developed. (3) In the sedimentary discontinuous period, the form of hardground of early diagenesis is mainly divided into four stages: biological disturbance, nodule growth, nodule fusion, and exposure dissolution. It is the main controlling factor for the formation of nodular limestone reservoirs. The late diagenesis inherits and further strengthens the reservoir heterogeneity formed by early diagenesis. (4) The more mature the hardground is, the more obvious the improvement of reservoir physical properties is. Among them, the high permeability layer developed in Khasib Formation belongs to the mature hardground. After the dissolution by atmospheric water, the pore structure is further improved. The anti-compaction effect of the hardground in the late diagenetic stage preserves the high permeability channel in the mature hardground. This study clarifies the reservoir characteristics and genetic mechanism of nodular limestone reservoirs, which is of great significance to improve the characterization accuracy of nodular limestone reservoirs and formulate reasonable development (adjustment) technology policy.