羊八井地热田地热地质条件及其对超临界地热资源勘探的启示

Geothermal geological conditions in the Yangbajing geothermal field and its enlightenment to the exploration of supercritical geothermal resources

随着能源结构调整以及“双碳”目标的推进,高温地热资源,尤其是超临界地热资源的勘探已成为地热学研究的焦点。羊八井地热田作为我国目前已知具有最高温度(329.8℃)的地热田,其是否存在超临界地热资源是一个关键问题。为此,通过综合梳理羊八井地热田的地热地质条件,分析地热田地热流体的来源及其演化过程,多方法评估限定地热田的储层温度,明确地热田的热源属性和特征,建立了超临界地热资源的流体C—He同位素指标体系,并评价其对超临界地热资源勘探的启示。研究结果表明:①羊八井地热田高CO_(2)含量与海相碳酸盐岩和变质沉积物的热成因脱碳有关,He主要来源于地壳中的放射性衰变,岩浆挥发份占比不到3%;②羊八井地热田至少有3个储层,第3个储层是地热母流体的来源,温度约为320℃,第2个储层的温度约为250±10℃,第1个储层的温度为150±15℃;③羊八井地热田深部存在岩浆房,距离地表15~20 km,为该地热田高温地热提供热源;④羊八井地热田的岩浆房为花岗质岩浆,埋深较大而体积较小,与世界上已探明的超临界地热资源的地热田在物质来源、热源特性等方面具有显著差异。结论认为:①羊八井地热田不具备形成超临界地热资源的地热地质条件;②该项研究成果从地热地质角度建立的超临界地热资源判别体系对我国超临界地热资源的勘探具有重要的启示意义。

With the transformation of energy structure and the promotion of the"dual carbon"goal,the exploration of high-temperature geothermal resources,especially supercritical geothermal resources,has become a focus of geothermal researches.The Yangbajing geothermal field is well-known with the highest temperature(329.8°C)in China,and one of its key problems is whether there are supercritical geothermal resources.In this regard,the source and evolution process of geothermal fluids in geothermal fields are first analyzed by reviewing the geothermal geological conditions of this geothermal field comprehensively.Then,the reservoir temperatures limiting geothermal fields are evaluated by means of multiple methods,and the properties and characteristics of heat sources in geothermal fields are clarified.Finally,the fluid C–He isotope index system of supercritical geothermal resources is established and its enlightenment to the exploration of supercritical geothermal resources is evaluated.And the following research results are obtained.First,the high CO_(2) content in the Yangbajing geothermal field is related to the thermogenic decarbonization of marine carbonate rocks and metamorphic sediments,and the He is mainly sourced from radioactive decay in the crust while magma volatiles accounts for less than 3%.Second,there are at least three reservoirs in this field,among which,the third one is the source of parent geothermal fluids with the temperature of 320°C and the second and the first one have the temperature of 250±10°C and 150±15°C,respectively.Third,there is a magma chamber at the depth of 15–20 km below the surface,which provides heat source for this high-temperature geothermal field.Fourth,the magma chamber of this field is granitic magma with great burial depth and smaller volume,which is significantly different from the other proved geothermal fields with supercritical geothermal resources around the world in terms of of material sources and heat source characteristics.In conclusion,there are no sufficient geothermal geological conditions for the formation of supercritical geothermal resources in the Yangbajing geothermal field.Moreover,the discrimination system of supercritical geothermal resources established from the perspective of geothermal geology is of great enlightenment significance to the exploration of supercritical geothermal resources in China.