U-Th dating of travertines on the Colorado Plateau : implications for the leakage of geologically stored CO2
U-Th dating of travertines on the Colorado Plateau : implications for the leakage of geologically stored CO2
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科罗拉多高原石灰华的 U-Th 测年：对地质储存的二氧化碳泄漏的影响
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发表时间：
2010
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通讯作者：
N. Burnside
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作者：
N. Burnside
为了避免大气中二氧化碳含量上升造成的破坏性气候后果，并将当前大气中二氧化碳浓度降低到工业化前的水平，必须通过在发电厂捕获二氧化碳并将其储存数千年来减少人为二氧化碳排放。深层地质构造内的地下储存，例如枯竭的天然气和油田或深层盐水层，是最容易理解的二氧化碳储存解决方案。为了使这种方法获得更多公众和政治上的接受，重要的是要描述人为封存项目发生泄漏时可能导致的二氧化碳释放的潜在原因、数量和速率。
这项研究考察了科罗拉多高原的两个地点，那里有断层和活跃泄漏的二氧化碳储存库为失败的人为封存地点提供了天然的类似物。所讨论的两个地点，小大沃什断层和北部盐沃什地堑断层位于犹他州悖论盆地的北端，由于浅层、北倾的格林河背斜与一组东西走向的正断层并置，代表了经典的三向圈闭。除了每个地区活跃的泄漏点外，还有大量石灰华化石沉积物。沿着 Little Grand Wash 断层，古代土丘仅限于断层痕迹，而与 Salt Wash 地堑北部断层相关的古代石灰华土丘数量要多得多，并且出现在断层下盘约 530 m 处。这种更加分散的流动模式是由于地表无承压含水层单元的露头造成的。大多数这些石灰华土丘中的总共 45 个 U-Th 年代数据提供了一个独特的数据集。 Little Grand Wash 和北部 Salt Wash 地堑断层最古老的沉积物年龄分别为 113,912 ± 604 和 413,474 ± 15,127 年。重复年龄显示出合理的再现性，结果的分析误差约为年龄的 1%。石灰华高程测量值与其辐射年龄的结合给出了每个地点的切割率。 Little Grand Wash 断层的 0.342 m/ka 速率与 Green River 切割直接相关，并且与之前在科罗拉多高原的工作一致，为描述该省的隆起特征提供了进一步的数据点。对于盐洗地堑的北部断层，流经该地区的支流的流速为 0.168 m/ka，提供了一种可靠的方法来估计未标明土丘的年龄。
石灰华土丘的放射性测年和切割率年龄估计的结果表明，泄漏可以持续 10 万年的时间尺度，而单个土丘的高分辨率 U-Th 测年表明，单点的泄漏可以持续至少约 11,000 年。一系列石灰华年龄表明，随着时间的推移，渗漏到地表的位置不断变化，而一个位置存在三个不同年龄的土堆表明，路径可以在约 45,000 年的时间内反复重复使用。没有证据表明石灰华沉积具有时间周期性，但石灰华土丘的初始铀化学成分的局部相似性表明存在明显的泄漏空间模式。最初的渗漏位于格林河背斜的轴向轨迹附近，随后的渗漏从该中心点沿着断层面向东西方向扩散。流体流动路径到地表的切换可以通过三种主要机制来解释：矿化、与二氧化碳相关的流体流动的三相干扰以及地震触发的作用于断层水文的动态应变变化。这些机制在每个区域都有不同的影响 - 表明仅限于损伤带裂缝（小大洗断层）的系统中流体流动切换的行为与通过非承压含水层（盐洗地堑的北部断层）泄漏的系统不同。
将石灰华年龄与其体积估计相结合，得出了最坏的情况，即 Little Grand Wash 断层的 CO2 泄漏量为 6.2 x 10^6 ± 1.7 x 10^6 吨，Salt Wash 地堑北部断层的 CO2 泄漏量为 7.4 x 10^6 ± 2 x 10^6 吨。根据这些总数，每个故障的时间平均泄漏率估计为 55 ± 15 吨/年和 47 ± 13 吨/年。活跃沉淀的水晶间歇泉石灰华（人为勘探钻探的结果）的泄漏率估计为 3,153 ± 851 吨/年。这些总的和现代的速率为盖层破坏和灾难性井眼破坏引起的泄漏提供了类似物。将它们应用于 Weyburn 和 Gorgon 等大型封存场地表明，对于盖层破坏，这些储层的完全泄漏将在 10^5-10^6 年的时间尺度内发生，而对于单井的灾难性破坏，这些储层的完全泄漏可能会在短短 10^3 - 10^4 年的时间内发生。这一发现对于成功监测人为储存地点具有重要意义。
In order to avoid the damaging climatic consequences of rising atmospheric CO2, and reduce current atmospheric CO2 concentrations to pre-industrial levels, anthropogenic CO2 emissions must be mitigated by capturing CO2 at power plants and storing it for thousands of years. Underground storage within deep geological formations, such as depleted gas and oil fields or deep saline aquifers, is the best understood solution for storage of CO2. In order for this method to gain more public and political acceptance it is important to characterise the potential causes, quantities and rates of CO2 release that could result if leakage were to occur from anthropogenic storage projects.
This study examines two sites in the Colorado Plateau where faulted and actively leaking CO2 reservoirs provide natural analogues for failed anthropogenic storage sites. The two sites in question, the Little Grand Wash and northern Salt Wash graben faults are situated at the northern end of the Paradox Basin in Utah and represent classic three way traps due to juxtaposition of the shallow, north plunging Green River anticline against a set of east-west trending normal faults. In addition to active leakage sites in each area there are numerous fossilised travertine deposits. Along the Little Grand Wash fault the ancient mounds are restricted to the fault trace whereas ancient travertine mounds associated with the northern fault of the Salt Wash graben are far more numerous and occur up to ~530 m into the footwall of the fault. This more diffuse pattern of flow is due to the outcropping of unconfined aquifer units at the surface. A total of 45 U-Th dates from the majority of these travertine mounds provides a unique data set. The oldest deposits from the Little Grand Wash and northern Salt Wash graben faults produced ages of 113,912 ± 604 and 413,474 ± 15,127 years respectively. Repeat ages show reasonable reproducibility and analytical errors on results are of the order of 1% of the ages. The coupling of travertine elevation measurements with their radiometric ages gives an incision rate for each site. A rate of 0.342 m/ka for the Little Grand Wash fault relates directly to Green River incision and agrees with previous work on the Colorado Plateau, providing a further data point for characterisation of uplift of the province. For the northern fault of the Salt Wash graben a rate of 0.168 m/ka for the tributaries running through the area gives a robust method with which to estimate ages for un-dated mounds.
The results of radiometric dating and incision rate age estimation of travertine mounds shows that leakage can last for timescales of 100,000’s of years, while high resolution U-Th dating of an individual mound demonstrated that leakage from a single point can last for a minimum of ~11,000 years. A range of travertine ages show that leakage to the surface has constantly switched location through time, while the presence of three mounds of distinct age at one location demonstrate that pathways can become repeatedly re-used over periods of ~45,000 years. There is no evidence of temporal periodicity in travertine deposition but there is a distinct spatial pattern of leakage as shown by localised similarities in the initial uranium chemistries of travertine mounds. Initial leakage is proximally located to the axial trace of the Green River anticline and subsequent leakage spreads from this central point along the fault plane in both east and west directions. The switching of fluid flow pathways to the surface can be explained by three main mechanisms: mineralisation, 3-phase interference of CO2 related fluid flow and seismically triggered alteration in dynamic strain acting upon the hydrology of the faults. These mechanisms have differing influences in each area - demonstrating that the behaviour of fluid flow switching in a system confined to damage zone fractures (Little Grand Wash fault) is different to a system leaking through an unconfined aquifer (northern fault of the Salt Wash graben).
Coupling of travertine ages with estimates of their volumes provided a total worse case scenario for quantity of CO2 leakage of 6.2 x 10^6 ± 1.7 x 10^6 tonnes for the Little Grand Wash fault and 7.4 x 10^6 ± 2 x 10^6 tonnes for the northern fault of the Salt Wash graben. From these totals time averaged leakage rates of 55 ± 15 and 47 ± 13 tonnes/year were estimated for each fault. The leakage rate for the actively precipitating Crystal Geyser travertine (which is the result of anthropogenic exploration drilling) is estimated to be 3,153 ± 851 tonnes/year. These total and modern rates provide analogues for leakage via caprock failure and catastrophic wellbore failure. Applying them to large scale storage sites such as Weyburn and Gorgon revealed that for caprock failure complete leakage of these reservoirs will take place over timescales of 10^5-10^6 years, while for catastrophic failure of a single well complete leakage of these reservoirs could occur over as little as 10^3 – 10^4 years. This finding has important implications for the successful monitoring of anthropogenic storage sites.