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

由于断层可能成为地下流体流动的障碍，它们会影响地下水供应、污染物运输、石油生产和地下废物储存。天然胶结物通常沿断裂带沉淀；这些胶结物可以极大地抑制流体通过断层的流动。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，通常不考虑断裂带的胶结作用。这项研究旨在利用天然水泥独特的电学性质来表征异常暴露的断裂带水泥的三维空间分布。然后，将通过从断层附近的井中取水来检验断裂带胶结作用的程度和连续性是流体流动的关键控制的假设。该项目的结果可用于更好地了解污染物在断裂含水层中的运移、油气藏的特征以及与污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，该项目将加强新墨西哥理工学院(NMT)未被充分代表的少数族裔的教育，NMT是一家专注于科学、技术、工程和数学(STEM)领域的拉美裔服务机构。此外，该项目中使用的地球物理设备和地下水井将成为NMT和其他大学赞助的夏季野外课程的长期教学资源。该项目的目标是表征跨断层的野外渗透率，并将断层的水文地质行为与断层胶结程度的空间变化联系起来。中心假设是：1)胶结程度向下的变化比沿断层走向的变化更明显，因为向上/向下的粒度变化大于沿走向的变化；2)广泛的胶结作用并不降低野外尺度上的跨断层渗透率(即忽略断层胶结作用潜在影响的水文地质模型的事实假说)。拟议的项目包括：1)表征沿断层地表暴露的胶结程度和程度；2)收集跨断层横断面的电阻率和可充电性数据，推导出断层带胶结程度与断层浅层归一化充电性之间的经验关系，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，以确定胶结程度；以及4)进行油井测试，以确定强胶结和弱胶结部分的穿过断层的渗透率。这项研究将产生基于地质的地球物理观测，提供一张独特的三维胶结模式图，从陆地表面到约30米深，沿Loma Blanca断层走向横断面覆盖700米。在地质和地球物理观测提供的背景下解释跨断层的试井将有助于发展对断层带渗透率的整体看法，改进对断层的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.