Scientific Drilling into the San Andreas Fault: Fluid Transport Analysis - Rare Gas "Ages" of Matrix Porosity Fluids in Drill Core

对圣安德烈亚斯断层的科学钻探：流体传输分析-钻芯中基质孔隙性流体的稀有气体“年龄”Martin Stute，Peter Schlosser，Gisela Winckler首席研究员摘要目前支持的圣安德烈亚斯断裂带(SAFZ)低摩擦滑动的解释是在非常高的压力下存在流体。由于流体输运在时间和空间上都有很大的变化，现场测量断裂带钻孔中的压力梯度和渗透率将确定当前的条件，但几乎不能提供断层增压循环(几十年)期间的压力/输运模式的指示。这项由哥伦比亚大学Lamont-Doherty地球观测站的研究人员进行的研究测试了一种方法，利用从SAFZ深处获取的岩心中回收的孔隙流体中稀有气体浓度的分布来确定流体流入/流出断裂带的方法。铀和钍衰变系列元素，岩石中的钾产生4He和40Ar，以及核成因(21，22Ne)和裂变成因(132,134,136Xe)稀有气体。从稀有气体浓度得出的流体“年龄”取决于产量和进入孔隙流体的释放率。可以模拟这些气体在孔隙流体中的垂直和水平剖面，以确定流体流动的速度和方向。这些分析的样本是通过对从SAFZ不同深度的EarthScope/SAFOD项目框架内回收的新钻芯进行分抽样获得的。由于稀有气体从岩石中流失的时间从几小时到几天不等，因此对钻芯进行亚采样以去除外层(已经失去稀有气体)，然后将内部部分放置在野外的高真空容器中。在几周到几个月的时间内释放到顶部空间的稀有气体由质谱仪和总孔隙流体重量确定。全套测量的稀有气体同位素(3，4He，20，21，22Ne，36，38，40Ar，84，86Kr，129,131,132,134,136Xe)被用来从大气稀有气体组分中分离出非大气的(产生于岩石中并聚集在孔隙流体中的)。这项研究还调查了岩石中稀有气体的浓度和产生稀有气体的元素的浓度，以确定基质孔隙流体中过量稀有气体丰度的源函数。上述一些方法在浅层含水层研究和少数深部钻探中得到了大量应用，但在深部科学钻探方面从未全部使用过。这项研究将建立一种方法，为流体流动的来源、方向和大小提供关键信息，这些信息是超静压流体压力在几十年时间尺度上导致SAFZ沿线低摩擦滑动的假设的基础。这对理解SAFZ和地震预测具有广泛的影响，影响到加州数百万人。这项研究的更广泛影响包括培养一名研究生，以及将这项研究的结果纳入哥伦比亚大学的地球科学课程。这项工作也纳入了哥伦比亚危险与风险研究中心(CHRR)的研究和教育活动，该中心的使命是促进自然和环境危险的预测科学，并将科学与危险风险评估和管理相结合。

0454514StuteScientific Drilling into the San Andreas Fault: Fluid Transport Analysis - Rare Gas "Ages" of Matrix Porosity Fluids in Drill CoreMartin Stute, Peter Schlosser, Gisela Winckler Principal InvestigatorsAbstractThe currently favored explanation for the low friction slip along the San Andreas Fault Zone (SAFZ) is the presence of fluids under very high pressure. Since fluid transport is subject to wide variation in both time and space, the in situ measurement of pressure gradients and permeability in a fault zone drill hole will establish the current conditions but provide little indication of pressure/transport patterns over the time interval for pressurization cycles of the fault (decades). This study conducted by researchers from the Lamont-Doherty Earth Observatory of Columbia University tests a methodology to determine the flow of fluids into/from the fault zone using the distribution of rare gas concentrations in pore fluids recovered from cores obtained at depth in the SAFZ. Uranium and Thorium decay-series-elements, and Potassium in rocks produce 4He and 40Ar as well as nucleogenic (21,22Ne) and fissiogenic (132,134,136Xe) rare gases. Fluid 'ages' derived from rare gas concentrations depend on the production rates and the release rates into the pore fluids. Vertical and horizontal profiles of these gases in pore fluids can be modeled to determine fluid flow velocities and directions. Samples for these analyses are being obtained by sub-sampling fresh drill core recovered in the framework of the EARTHSCOPE/SAFOD project from various depths in the SAFZ. Because rare gas loss from the rock occurs over a timescales of hours to days, drill core is sub-sampled to remove the outer rind (which has already lost rare gases) and the internal sections are then placed in high vacuum containers in the field. The rare gases released to the headspace over a period of weeks to months are determined by mass spectrometry and the total pore fluid by weight. The entire suite of measured noble gas isotopes (3,4He, 20, 21, 22Ne, 36, 38, 40Ar, 84,86Kr, and 129,131, 132, 134, 136Xe) is used to separate the non-atmospheric (produced in the rocks and accumulated in pore fluids) from the atmospheric noble gas components. The study also investigates the concentrations of rare gases in the rocks and of the elements producing them to determine the source function for excess rare gas abundances in matrix pore fluids. Some of the approaches described above have seen numerous applications in studies of shallow aquifers and in a few cases of deep drilling, but it has never been used in its entirety in the context of deep scientific drilling. This study will establish the methodology providing key information for the source, direction and magnitude of fluid flow that underlie the hypothesis that superhydrostatic fluid pressures are responsible for low friction slip along the SAFZ on timescales of decades. This has wide ranging implications for the understanding the SAFZ and earth quake prediction, affecting millions of people in California. Broader impacts of the study include the training of a graduate student, and the integration of results of this study in the Earth Science curriculum at Columbia University. This work is also integrated in the research and educational activities of Columbia's Center for Hazards and Risk Research (CHRR) whose mission it is to advance the predictive science of natural and environmental hazards and the integration of science with hazard risk assessment and management.