The petroleum exploration of deep-buried reservoir in lower Es3 in the north zone of Chezhen sag faces the problem that the distribution of favorable reservoirs is not clear. For further exploration，through core，casting thin sections，scanning electron microscope and other chemical examination data，the formation mechanism and distribution laws of favorable reservoir in lower Es3 in the north zone of Chezhen sag was studied. Results show that there are much differences in rock composition and gravels components in Dawangbei-Guojuzi area and Chexi area in the eastern and western of the study area respectively. Sand-conglomerate reservoirs in the Dawangbei-Guojuzi area are mainly controlled by Archeozoic granite gneiss parent rock in the area of eastern Chenzikou uplift，which are characterized by relatively high composition maturity and gravel components of granite gneiss. While sand-conglomerate reservoirs in Chexi area are mainly controlled by lower Palaeozoic carbonate parent rock in the area of western Chenzikou uplift，which are characterized by relatively low composition maturity and gravel components of carbonate rocks. The differences of parent rock property cause the differences of compressive strength of sand-conglomerate bodies，thus they have impacts on the preservation of primary pores and the development of secondary porosity. It’s the key to the formation of deep-buried favorable reservoir in the study area. Taking into account of sedimentary types，parent rock property and abnormal high pressure，it is believed that the favorable reservoir is mainly distributed in Dawangbei-Guojuzi area which has high formation pressure coefficient and is affected by granite gneiss parent rock.