Abstract:

The study of the microcharacterization and macroscopic distribution of layered geothermal reservoirs is an important basis for the fine evaluation and scale development of geothermal resources in sandstone geothermal thermal reservoirs. Neogene sandstone geothermal reservoir, which is widely developed in Xi’an Sag, Guanzhong Basin, is consisted of Gaoling Group, Lantian-Bahe Formation and Zhangjiapo Formation from the bottom to the top. Based on the logging and core sample testing data of 16 typical wells in Xianyang, Xi’an, Zhouzhi and other areas in Xi’an Sag, this paper systematically discusses the reservoir characteristics and the relationship between reservoir characteristics and water-yielding capacity. The geothermal reservoir is composed of fluvial-delta-lacustrine interactive sand and mudstone deposition, with positive rhythm characteristics. The reservoir rock is mainly feldspar lithic sandstone, and the reservoir space is dominated by dissolution pores and primary pores. The average porosity of Zhangjiapo Formation is 30.02%, and the average permeability is 451.82×10^-3 μm², which belongs to the medium-high-porosity and medium-high-permeability geothermal reservoir. The average porosity of Lantian-Bahe Formation is 20.43%, and the average permeability is 140.7×10^-3 μm², which belongs to the medium-high-porosity and medium-low-permeability geothermal reservoir. The average porosity of Gaoling Group is 17.28%, and the average permeability is 109.5×10^-3 μm², which belongs to the medium-low-porosity and medium-low-permeability geothermal reservoir. The temperature is generally linearly increased with depth, showing the characteristics of conduction geothermal resources, and the typical temperature-depth curves can be divided into two types of linear and folding patterns. Meanwhile, some wells located in the Weihe Fracture Zone have partial-vertical pattern of temperature-depth curves, showing the characteristics of conduction and convection. The reservoir sands are widely distributed and well hydrated with a large overall thickness, while the single sand thickness is generally less than 5 meters. The sandbody development of the Lantian-Bahe Formation and Gaoling Group is better than that of the Zhangjiapo Formation both vertically and horizontally. Reservoir conditions affect the water-yielding capacity of geothermal wells, and physical properties are the most important factors influencing the actual water production of sand bodies. The water yield of the monolayer is mainly affected by physical properties and argillaceous content. The better physical properties and lower argillaceous content can lead to a higher water yield of monolayer sand. Considering the physical properties, thickness, temperature and other factors, Lantian-Bahe Formation is the optimal layer for development, and its scientific and sustainable development is an important guarantee for the development of the regional geothermal industry.

Key words: Xi’an Sag, sandstone geothermal reservoir, reservoir characteristics, geothermal field, water-yielding capacity