This paper utilizes multi-methods,such as core observation,slice identification,isotope analysis,trace element analysis,fluid inclusion technique and so on,to study the causes of the dolomite in the Nanpu Sag and the origins of the dolomite reservoir.The study results show that the forming environment of dolomite in the Nanpu Sag is a fresh-water lake environment,and the dolomite is the product of dolomitization which is caused by volcanic thermal fluids in the Early Dongying period.With the development of intergranular pores in the dolomitization process,a lot of dissolved pores/vugs and fractures were formed by denudation in the later periods because of the influence of thermal fluids including the associated fluids of volcanic activities and the expelled hydrocarbon fluids of the source rocks.On the whole,these secondary dissolution spaces greatly enhance the reservoir ability of the dolomite,and there are enough reservoir spaces in the dolomite in the Nanpu Sag. This paper utilizes multi-methods, such as core observation, slice identification, isotope analysis, trace element analysis, fluid inclusion technique and so on, to to study the causes of the dolomite in the Nanpu Sag and the origins of the dolomite reservoir. results show that that forming environment of dolomite in the Nanpu Sag is a fresh-water lake environment, and the dolomite is the product of dolomitization which is caused by volcanic thermal fluids in the Early Dongying period .With the development of intergranular pores in the dolomitization process, a lot of dissolved pores / vugs and fractures were formed by denudation in the later periods because of the influence of thermal fluids including the associated fluids of volcanic activities and the expelled hydrocarbon fluids of the source rocks.On the whole, these second dissolution spaces greatly enhance the reservoir ability of the dolomite, and there are enough reservoir spaces in the dolomite in the Nanpu Sag.