Integrated geological, geophysical, and hydrological study of field-scale fault-zone cementation and permeability

由于断层可以成为地下流体流动的障碍，它们影响地下水供应、污染物输送、石油生产和地下废物储存。天然胶结物通常沿断裂带沉淀；这些胶结物可以显著地抑制流体流过断层。然而，确定断裂带胶结物的分布是困难的。因此，在估计断层对流体流动的影响时，通常不考虑断裂带胶结作用。本研究旨在利用天然胶结物独特的电学特性来表征一个异常暴露的断裂带的胶结物的三维空间分布。然后，通过从断层附近的井中抽取水，验证断裂带胶结程度和胶结连续性是流体流动的关键控制因素的假设。该项目的结果可用于更好地了解断层含水层中的污染物运移、油气藏特征以及与废水注入相关的诱发地震活动。本科生和研究生将参与该项目的所有阶段，这将加强新墨西哥理工大学（NMT）对代表性不足的少数民族的教育，这是一所专注于科学、技术、工程和数学（STEM）领域的西班牙裔服务机构。此外，本项目使用的地球物理设备和地下水井将成为NMT和其他大学主办的夏季野外课程的长期教学资源。该项目的目标是描述断层的现场渗透率，并将断层的水文地质行为与断裂带胶结程度的空间变化联系起来。主要的假设是：1)由于上/下剖面的粒度变化比沿断层走向的变化更大，因此胶结程度的下倾变化比沿断层走向的变化更明显；2)在野外尺度上，广泛的胶结并不会降低断层间的渗透率（即忽略断裂带胶结潜在影响的水文地质模型的事实假设）。拟开展的项目包括：1)沿断层露面方向确定胶结程度和程度；2)收集断层横断面上的电阻率和电荷率数据，推导断层浅部断裂带胶结程度与归一化电荷率之间的经验关系，并利用这些关系绘制断层深层胶结程度的估计图；3)对断层进行深部取心，确认胶结程度；4)在强胶结段和弱胶结段进行试井，以确定断层的渗透率。该研究将产生基于地质的地球物理观测结果，提供独特的胶结模式3D地图，从陆地表面到~30米深度，在Loma Blanca断层走向700米的横断面上。在地质和地球物理观测提供的背景下解释断层上的试井，将有助于对断层带渗透率进行整体观察，改进断层水文地质影响的预测模型。

Because faults can be barriers to underground fluid flow, they influence groundwater supply, contaminant transport, petroleum production, and underground waste storage. Natural cements commonly precipitate along fault zones; these cements can dramatically inhibit fluid flow across a fault. However, determining the distribution of fault zone cements is difficult. As a result, fault-zone cementation is typically not accounted for in estimates of the impact of faults on fluid flow. This study aims to take advantage of unique electrical properties of natural cements to characterize the three-dimensional spatial distribution of cement for an exceptionally well-exposed fault zone. Then, the hypothesis that the degree and continuity of fault-zone cementation is a key control on fluid flow will be tested by drawing water from wells adjacent to the fault. The results of this project can be applied to better understand contaminant transport in faulted aquifers, the characterization of hydrocarbon reservoirs, and induced seismicity associated with wastewater injection. Undergraduate and graduate students will be involved with all stages of the project, which will enhance education for underrepresented minorities at New Mexico Tech (NMT), a Hispanic-serving institution with a focus on science, technology, engineering, and mathematics (STEM) fields. In addition, the geophysical equipment and groundwater wells used in this project will be long-lasting teaching resources both for NMT and for summer field courses sponsored by other universities.The objectives of this project are to characterize field-scale permeability across a fault and link the hydrogeologic behavior of the fault to spatial variations in the degree of fault-zone cementation. The central hypotheses are: 1) there are more pronounced variations in the degree of cementation down-dip than along the strike of the fault because the up/down-section variations in grain size are larger than those along strike, and 2) extensive cementation does not reduce cross-fault permeability at the field-scale (i.e. the de facto hypothesis for hydrogeologic models that neglect the potential effects of fault-zone cementation). The proposed project comprises: 1) characterizing the degree and extent of cementation along the surface exposure of the fault; 2) collecting resistivity and chargeability data in transects across the fault, deriving empirical relationships between the degree of fault-zone cementation and normalized chargeability for shallow portions of the fault, and using those relationships to map the estimated degree of cementation for deeper portions of the fault; 3) coring the fault at depth to confirm the degree of cementation; and 4) conducting well tests to determine permeability across the fault in both strongly- and weakly-cemented sections. The study will generate geologically based geophysical observations, providing a unique 3D map of cementation patterns from the land surface to ~30 m depth, in transects covering 700 m along the strike of the Loma Blanca fault. Interpreting well tests across the fault in the context provided by the geologic and geophysical observations will allow for the development of a holistic view of fault-zone permeability, improving predictive models of the hydrogeologic impacts of faults.