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

由于断层可能是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。天然胶结物通常沿着断层带沉淀;这些胶结物可显著抑制流体流过断层。然而，确定断层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，通常不考虑断层带胶结作用。本研究的目的是利用天然胶结物独特的电性质来表征一个异常暴露的断层带的胶结物的三维空间分布。然后，将通过从断层附近的威尔斯井中抽水来检验断层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。该项目的结果可以应用于更好地了解污染物在断层含水层中的传输，油气藏的表征，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，这将加强新墨西哥州技术（NMT），一个西班牙裔服务机构，重点是科学，技术，工程和数学（STEM）领域代表性不足的少数民族的教育。此外，本项目中使用的地球物理设备和地下威尔斯井将成为NMT和其他大学赞助的夏季野外课程的长期教学资源。本项目的目标是描述跨断层的野外渗透率，并将断层的水文地质行为与断层带胶结程度的空间变化联系起来。核心假设是：1）向下倾斜的胶结程度比沿着断层走向的胶结程度有更明显的变化，因为粒度的上/下截面变化大于沿着走向的变化，以及2）广泛的胶结作用不会降低油田尺度上的跨断层渗透性（即，忽略断层带胶结作用潜在影响的水文地质模型的事实假设）。拟议的项目包括：1）表征沿着断层的表面暴露的胶结程度和范围; 2）收集横跨断层的横断面中的电阻率和充电率数据，导出断层的浅部的断层带胶结程度和归一化充电率之间的经验关系，并使用这些关系来绘制断层的较深部的估计胶结程度; 3）在断层深处取芯，以确定胶结程度; 4）进行井测试，以确定强胶结和弱胶结段中断层的渗透率。该研究将产生基于地质的地球物理观测结果，提供从地表到~30 m深度的胶结模式的独特3D地图，在沿Loma Blanca断层走向沿着700 m的横断面上。在地质和地球物理观测提供的背景下，对整个断层的试井进行解释，将有助于对断层带渗透性进行全面了解，从而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.