Geochemical characteristics of Neogene mudstone in Guanzhong Basin: Provenance and paleosedimentary environment restoration

At present, the geothermal resources developed and utilized in the Guanzhong Basin are mainly Cenozoic sandstone and glutenite pore−fissure geothermal resources. The development and utilization horizons are mainly Neogene thermal reservoirs. The occurrence characteristics of geothermal resources, the distribution of favorable areas for geothermal resources development, and the amount of geothermal resources are closely related to the provenance and paleosedimentary environment. Identifying the characteristics of Neogene mudstone provenance and paleosedimentary environment is conducive to indicating the characteristics of sandstone provenance and paleosedimentary environment in the same sedimentary period. It has practical guiding significance for the determination of key horizons for the development and utilization of geothermal resources in the Guanzhong Basin, the prediction of favorable areas, and the calculation of resources. The geochemical characteristics of elements in sedimentary rocks record important provenance and sedimentary environment information. According to the characteristics of major, trace and rare earth elements of Neogene mudstone core samples in Guanzhong Basin, the provenance and sedimentary environment were comprehensively studied. The results show that the tectonic background of the source area of Neogene mudstone in Guanzhong Basin is mainly active continental margin, and the source rocks are mainly syenogranite of Taibai rock mass and biotite monzonitic granite of Huashan rock mass in the southern margin of Guanzhong Basin, which contain some intermediate−basic igneous rocks. The chemical alteration index (CIA) ranges from 61.34 to 76.78, with an average value of 70.78, reflecting that the Neogene mudstone has experienced moderate weathering, indicating that the Gaoling Group and the Lantian−Bahe Formation were in a warm and humid climate during the deposition period, and the Zhangjiapo Formation was deposited. The climate changed from warm and humid climate to cold and dry climate; geochemical indicators comprehensively reflect that the Neogene mudstone deposition period is mainly in a reducing environment, and the paleosalinity of the water body is generally in a freshwater and brackish water environment.