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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Fluid geochemistry and soil gas fluxes (CO_2-CH_4-H_2S) at a promissory Hot Dry Rock Geothermal System: The Acoculco caldera, Mexico
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Fluid geochemistry and soil gas fluxes (CO_2-CH_4-H_2S) at a promissory Hot Dry Rock Geothermal System: The Acoculco caldera, Mexico

机译：活跃的热干岩地热系统的流体地球化学和土壤气体通量（CO_2-CH_4-H_2S）：墨西哥Acoculco破火山口

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The Acoculco caldera has been recognized by the Mexican Federal Electricity Company (CFE) as a Hot Dry Rock Geothermal System (HDR) and could be a potential candidate for developing an Enhanced Geothermal System (EGS). Apart from hydrothermally altered rocks, geothermal manifestations within the Acoculco caldera are scarce. Close to ambient temperature bubbling springs and soil degassing are reported inside the caldera while a few springs discharge warm water on the periphery of the caldera. In this study, we infer the origin of fluids and we characterize for the first time the soil degassing dynamic. Chemical and isotopic (δ~(18)O-δD) analyses of spring waters indicate a meteoric origin and the dissolution of CO_2 and H_2S gases, while gas chemical and isotopic compositions (N_2/He, ~3He/~4He, ~(13)C, ~(15)N) reveal a magmatic contribution with both MORB- and arc-type signatures which could be explained by an extension regime created by local and regional fault systems. Gas geothermometry results are in agreement with temperature measured during well drilling (260℃-300℃). Absence of well-developed water reservoir at depth impedes re-equilibration of gases upon surface. A multi-gas flux survey including CO_2, CH_4 and H_2S measurements was performed within the caldera. Using the graphical statistical analysis (GSA) approach, CO_2 flux measurements were classified in two populations. Population A, representing 95% of measured fluxes is characterized by low values (mean: 18 g m~(- 2) day~(-1)) while the remaining 5% fluxes belonging to Population B are much higher (mean: 5543 g m~(-2） day~(-1)). This low degassing rate probably reflects the low permeability of the system, a consequence of the intense hydrothermal alteration observed in the upper 800 m of volcanic rocks. An attempt to interpret the origin and transport mechanism of these fluxes is proposed by means of flux ratios as well as by numerical modeling. Measurements with CO_2/CH_4 and CO_2/H_2S flux ratios similar to mass ratios of sampled gases were considered as reflecting advective transport. A numerical model of CO_2 migration in the subsoil system under fully water and gas saturated conditions was performed using the TOUGH2 code in order to reproduce semi-quantitatively field measurements. The main results show that high flux values produced by advective geothermal degassing can be very localized and that low and heterogeneous permeability conditions can induce low advective CO_2 flux values. Therefore, in this case the populations discriminated by the GSA method should not be interpreted in terms of origin and/or transport mechanism but rather in terms of permeability conditions.
机译：Acoculco破火山口已被墨西哥联邦电力公司（CFE）认可为热干岩地热系统（HDR），并且可能成为开发增强型地热系统（EGS）的潜在候选者。除了热液蚀变的岩石外，Acoculco破火山口内的地热表现很少。据报道，火山口内部接近于室温冒泡的弹簧和土壤脱气，而少数温泉则在火山口的外围排放热水。在这项研究中，我们推断出流体的来源，并且首次表征了土壤脱气的动态。泉水的化学和同位素（δ〜（18）O-δD）分析表明，大气成因和CO_2和H_2S气体的溶解，而气体化学和同位素组成（N_2 / He，〜3He /〜4He，〜（13 C，〜（15）N）揭示了具有MORB型和弧型特征的岩浆作用，可以用局部和区域断层系统形成的扩展机制来解释。气体地热测定结果​​与钻井过程中测得的温度（260℃-300℃）一致。缺乏深度发达的储水层阻碍了地表气体的重新平衡。在破火山口内进行了包括CO_2，CH_4和H_2S测量在内的多气体通量测量。使用图形统计分析（GSA）方法，将CO_2通量测量值分为两个种群。代表95％的测量通量的种群A具有较低的值（平均值：18 gm〜（-2）天〜（-1）），而属于种群B的其余5％通量则更高（平均值：5543 gm〜 （-2）天〜（-1））。如此低的脱气率可能反映了该系统的低渗透性，这是由于在800 m以上的火山岩中观测到强烈的热液蚀变的结果。通过通量比以及数值模拟的方法，提出了这些通量的机理，认为CO_2 / CH_4和CO_2 / H_2S通量比的近似与采样气体的质量比有关的测量反映了对流运移。为了再现半定量现场测量结果，使用TOUGH2代码在完全水和气饱和的条件下进行了地下土壤系统研究，主要结果表明，对流地热脱气产生的高通量值可以非常局部化，并且低渗透率和非均质渗透率条件会导致低平流CO_2通量值。因此，在这种情况下，不应根据来源和/或运输机制来解释通过GSA方法区分的种群，而应根据渗透性条件来解释。

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