| Abstract |
Ningbo, who belongs to continental volcanic zone of Zhejiang Southeast region, its geothermal exploration is basically a blank area. In order to meet the development of the local economy, a lot of work has been performed by geologists recently including successfully drilling geothermal wells in Dragon Lake and East Lake which fill in deep volcanic geothermal development gaps of Ningbo eastern regions: seven to eight geothermal wells were drilled during 2004 in Shenzhen, Ninghai country, Ningbo, of which four were successful, and the largest one has more than 1000 m3 water per day; the first deep well was drilled with 1230m, 41.5℃ and 252m3 water everyday, in Xi'ao, East Lake area in 2006; Also in the cover of thick volcanic rocks about 3000m in Dragon Lake area, a geothermal well with 2000m was completed in March 2012. The study area is the eastern part of Ningbo, and this article is based on the Ningbo – Shenzhen geothermal area, locating in the fourth tectonic unit called Xinchang- Dinghai fault uplift, and covered by pyroclastic rocks (tuffaceous siltstone) and a small amount of lava from Cretaceous to Jurassic strata basically within several kilometers underground. With low porosity, very weak permeability, low thermal conductivity of rock, these formations can be a good cap rock with superior insulation properties. Temperature gradient is around 2.8℃/100m in the area. It is more difficult to find layered reservoir due to its non- geothermal anomaly. Therefore the key choice is to find out thermal conductivity and heat faults. Although volcanic rocks are distributed throughout the study area, it has been indicated that the origin of geothermal doesn't concern with volcanic rocks and might be subject to deep geothermal exploration theory. Higher porosity formations or faults have been acknowledged based on the geophysical methods to confirm the geothermal reservoir structure. This paper puts emphasis upon several methods, such as Controlled Source Audio-frequency Magnetotelluric(CSAMT), Radon and Mercury Measurement and High-precision Gravity Measurements, which played an important role in exploring geothermal in Dragon Lake area. Integrated geophysical methods are much better on Searching for geothermal than a single geophysical method in this region. It manifests enriching the extraction of geological and abnormal information, improving the accuracy of interpretation by single geophysical method, reducing the false judge about anomalies, and enhancing geothermal exploration outline for the study area for the future. Indicating the chemical type, this analysis is combined with chemistry data, such as geothermal wells, springs, and surface water, confirming the origin of geothermal fluid, explaining hydrogeochemical characteristics by some features of ion, and estimating geothermal reservoir temperature by geothermometer method. Through the geothermal resource assessment, including release of heat, water quality and water use, we predict hot water migrated to the surface mixed with cold water and the geothermal temperature was in the low level, small and medium geothermal field. Hot water can be used as medical care, bath and so on. Meanwhile by speculating higher occurrence of hot temperatures in deep formation, this area has some prospect of development of geothermal resources. |