Reconstructed by several tectonic activities,the preservation conditions for oil and gas reservoirs of marine strata in southern China is severely disturbed.The distribution characteristics of hydrodynamic fields are of great importance to the formation and evolution of the oil-gas reservoirs.Through the analysis on hydrogeological/hydrogeochemical characteristics of several exploration areas of China,a close relationship between the hydrogeological/hydrogeochemical characteristics and the hydrocarbon preservation conditions is discussed in this paper.In the uplift and denudation stage,the formation compaction stopped,buried sedimentary water was driven out by meteoric water,therefore,the deposition and burial stage becomes the main period during which the formation,migration and accumulation of hydrocarbon occurred.The preserved meteoric water occurring near unconformable surfaces can be an indicator of the reservoir's being damaged and poor preservation conditions,and also can be an important indicator that the organic material has not yet began secondary hydrocarbon generation.As the buried depth increases,stratal open extent decreases.The exchanging-retarded zone is favorable to the preservation of oil and gas as it is not influenced by surface water,while the free exchanging zone,strongly affected by surface water,offers a poor environment for petroleum preservation.Based upon above,a relatively systematical hydrogeochemical index system is established for distinguishing hydrocarbon preservation conditions in this paper.