In order to determine the geochemical characteristics and paleosedimentary environment of the Lower Cretaceous Yixian Formation and Jiufotang Formation in the northern part of the Naiman sag, petrographic analysis was conducted on N10 and NA-2 wells located near the sedimentary center in the north. Samples of mudstone and evaporite were tested for major and trace elements, carbon and oxygen isotopes, organic carbon, pyrolysis, and group composition. The paleoredox conditions, paleosalinity, paleowater depth, paleoclimate, and lake water sealing characteristics of the study area were systematically analyzed. The study shows that they were volcanic facies in the early sedimentary period of Yixian Formation in the northern part of the Naiman sag, mainly fan delta-lacustrine depositional-lake facies sedimentary systems in the middle and late sedimentary period, and local shore-shallow lacustrine facies of different types of evaporites in the late sedimentary period. Sensitive elements or element ratios such as Cu/Zn, Sr/Ba, Sr/Cu, Rb/Sr, Fe₂O₃/MnO, Ga, Sr indicate that the climate of Yixian Formation changed from semi-humid-semi-arid to arid from the middle period to the end period, the water environment also changed from semi-closed-semi-open with partial oxidation to completely closed with weak oxidation-weak reduction, and the sedimentary geochemistry environment changed from freshwater-brackish deep lake to brackish shallow lake. The Jiufotang Formation is a semi-enclosed deep lake sedimentation of oxidized fresh water with a climate similar to that of the middle of the Yixian Formation. The sedimentary environment during the middle and late periods of the Yixian Formation and the Jiufotang Formation were suitable for biological reproduction, which were favorable for the development of argillaceous source rocks. And for the development of pores and intercrystalline pores of thick-layer evaporites, which can become good reservoir spaces, they are favorable for oil and gas enrichment.