Based on the sedimentary structure, paleontology and lithology combinations of cores, combined with the analysis of logging facies and seismic facies, the sedimentary environment and facies of Lower Cretaceous in M Oilfield of Santos Basin were studied, and a new sedimentary facies classification scheme was proposed. The research shows that: it is an open littoral-shallow lake sedimentary environment with freshwater of Formation I in the study area, which includes microfacies of coquina shoal, granular shoal, inter-shoals, shallow lake marl, shoal margin, and slump fan, it is mound-shoal type littoral-shallow lake sedimentary environment with salt water sediments of Formation B, which includes algal mound, granular shoal, mound margin, shoal margin, inter-mounds, and algal marl flat. From the bottom to the top, the water body gradually become shallow, showing a sedimentary pattern that changes from the coquina shoal granular shoal dominated in Formation I to algal mound-granular shoal dominated in Formation B. The stratigraphy of lacustrine carbonate rocks in the study area is mainly controlled by paleogeomorphology, lake level and water medium. Structural subsidence and lake level control the accommodating space, and water media controls the types of organisms, which in turn controls the types of sedimentary facies. These are the main controlling factors for sedimentary. The high-energy facies such as algal mound and coquina shoal are mainly developed in the paleo-uplifts, with large thickness and wide distribution, and they are distributed in a near north-south and northeast-southwest direction. Low-energy facies such as inter-mounds, inter-shoals, and shallow lake marl are mainly developed in lower parts of paleogeomorphology.