裂谷火山型地热系统——奥卡瑞地热田基本特征

Characteristics of Olkaria Geothermal Field,a Rift Volcanic Geothermal System

东非裂谷是世界著名的四大高温地热带之一,肯尼亚奥卡瑞地热田位于东非裂谷带中部,是典型的大陆裂谷火山型高温地热田.奥卡瑞地区断裂系统十分复杂,按走向可分为5组,其中以北西向为主要断裂,与裂谷走向一致.地热田整体面积约100km^2,目前投入生产的主要是东北区、东区和穹顶区,已落实热储面积约42km^2,主要有3个热源分别对东区、西区和穹顶区供热.奥卡瑞地热田是肯尼亚目前已知的热储温度最高的地热田,热储埋深为500~3000m,最高温度370℃,平均温度240℃.奥卡瑞高温地热系统的形成,与该地区晚第四纪强烈的火山活动有关,热田的盖层是最近一次火山活动的喷发物,以流纹岩为主,透水性差且裂缝不发育,使盖层成为良好的隔水层.热田热储主要为第四系火山岩,其中西区以凝灰岩为主,东区以粗面岩为主.地热水来源主要为大气降水,部分为渗入来源.综合分析认为,奥卡瑞地热田是一个由新近浅层侵入岩浆提供热源,由断裂、裂缝和有一定孔渗条件的火山岩构成热储,蚀变火山岩、流纹岩提供盖层,断裂作为热流体交换通道的火山型高温地热系统.

The East African Rift is one of the four world-famous high-temperature geothermal belts.Located in the middle of the East African Rift zone,Olkaria geothermal field in Kenya is a typical volcanic high-temperature geothermal field in continental rift.The complex fault system in Olkaria area can be divided into 5 groups according to the strike,which is mainly NW,consistent with the trend of the Rift.The total area of the geothermal field is about 100 km^2,and currently the Northeast area,the East area and the Dome area are put into production,with a geothermal reservoir area of about 42 km2.There are mainly 3 heat sources to heat the East,the West and the Dome areas respectively.Olkaria is a geothermal field with the highest geothermal reservoir temperature known in Kenya,with a geothermal reservoir depth of 500-3000 meters,a maximum temperature of 370℃ and an average temperature of 240℃.The formation of Olkaria high-temperature geothermal system is related to the intense volcanic activities in the Late Quaternary in this area.The cap rock of the geothermal field is the tephar of the latest volcanic activity,mainly rhyolite,with poor permeability and undeveloped fractures,making the cap rock a good aquifer.The geothermal reservoir is mainly Quaternary volcanic rocks.The West area is mainly Tuff and the East area is mainly Trachyte.Geothermal water comes mainly from atmospheric precipitation and partly from infiltration.All in all,Olkaria geothermal field is a volcanic high-temperature geothermal system.Its heat is supplied by newly intruded magma,its geothermal reservoir is composed of cracks,fractures and volcanic rocks with certain porosity and permeability conditions,its cap rock is supplied by altered volcanic rocks and rhyolite,and its fault system serves as the thermal fluid exchange channel.