RVDC-1.01-026RPT UE-27a Borehole End of Well Report - eDCRO_111854 - eDCRO_111864 (1).pdf

Document No: RVDC-1.01-026RPT Revision 0

Rock Valley Direct Comparison (RV/DC) UE-27a Borehole End of Well Report

August 2026

Prepared for:

U.S. Department of Energy

National Nuclear Security Administration Defense Nuclear Nonproliferation Research and Development

Prepared by:

Geosciences Group

Defense Nuclear Nonproliferation Division Global Security Directorate

Mission Support and Test Services, LLC Nevada National Security Site

Acknowledgements

This Source Physics Experiment (SPE) research was funded by the National Nuclear Security Administration, Defense Nuclear Nonproliferation Research and Development (NNSA DNN R&D). The authors acknowledge important interdisciplinary collaboration with scientists and engineers from LANL, LLNL, NNSS, SNL and UNR-NSL.

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Table of Contents

1 Introduction

1.1 Project Description

1.2 Project Organization

1.3 Location and Significant Nearby Features

1.4 Objectives

1.5 Project Summary

2 Drilling Summary

2.1 Drilling History

2.2 Borehole Deviation

2.3 Drilling Problems

2.4 Fluid Management

3 Geologic Data Collection

3.1 Introduction

3.2 Coring Processing and Logging

3.3 Geophysical Log Data

4 Geology

4.1 Geologic Setting

4.2 Stratigraphy and Lithology

4.2.1 Introduction

4.2.2 Stratigraphic Overview

4.3 Alteration

4.4 Core Recovery and Rock Quality

4.5 Geotechnical Interval Log

4.6 Structure

4.7 Predicted and Actual Geology

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5 Hydrology

5.1 Groundwater Sampling

5.2 Preliminary Depth to Groundwater Table

6 Completion and Instrumentation

6.1 Completion Description

6.2 Instrumentation Description

7 Recommendations and Lessons Learned

7.1 Recommendations

7.2 Lessons Learned

8 Acronym/Abbreviation List

9 Cited Documents

10 References

Appendix A: Geologic Core Graphical Logs

Appendix B: Geophysical Logs

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List of Figures

Figure 1: Reference map showing location of Borehole UE-27a at the Nevada National Security Site. Inset map outline shows the extents of Figures 2 and 7

Figure 2: Reference map of the Rock Valley Region, showing topography and historic boreholes.

Test Well F (TWF) is labeled as United States Geological Survey (USGS) HTH F on this figure

Figure 3: Map showing the drill site set-up at the RV/DC testbed, Area 27. The Corehole location is to the right (east) of the drill rig

Figure 4: Diagram of UE-27a. Borehole diameters are shown on the left and casing diameters on the right

Figure 5: Modified drilling subcontractor's whiteboard showing sections of significance to drilling operations. Right: Charted drilling progress showing depth over time, GWS is groundwater sampling

Figure 6: Deviation survey for UE-27a. The left two columns show the azimuth and inclination in degrees, plotted by depth. The right shows a planar view of the borehole deviation with concentric circles showing feet from the collar location

Figure 7: General geologic map of the Rock Valley region showing GFM units. The map differentiates Quaternary and Tertiary Alluvium; however, the GFM groups Quaternary and Tertiary Alluvium into a single, undifferentiated layer. A-A’ Cross section shown in Figure 8

Figure 8: A-A' Cross Section showing geologic interpretation of the subsurface. The approximate location of UE-27a is projected. Units are same as Figure 7

Figure 9: Geologic data collected from UE-27a from collar (top) to TD (bottom). Detailed core logs are in Appendix A. New abbreviations: fractures (fx), Modified Rock Index (MRI).

Figure 10: Strip log showing predicted and actual depths for stratigraphy and GFM Layers at Corehole UE-27a

Figure 11: Schematic diagram showing the position of the DFOS cable and geophone array

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List of Tables

Table 1: Corehole UE-27a Data Summary

Table 2: Abridged Drill Hole Statistics for Corehole UE-27a. North American Datum (NAD), Universal Transverse Mercator (UTM), North American Vertical Datum (NAVD), etc. . 18

Table 3: Corehole UE-27a Geophysical Log Summary. Intervals are not always continuous and gaps are described in the text

Table 4: Key to Stratigraphic Units and Symbols Encountered in Corehole UE-27a

Table 5: Core Photos of Formations in UE-27a

Table 6: UE-27a Completed Casing String

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1 Introduction

1.1 Project Description

Corehole UE-27a was drilled for the U.S. Department of Energy, National Nuclear Security Administration Nevada (NNSA), Office of Defense Nuclear Nonproliferation (DNN) in support of the Source Physics Experiments (SPE) at the Nevada National Security Site (NNSS). Corehole UE-27a is the first borehole drilled for Rock Valley Direct Comparison (RV/DC) Venture, the third phase of the SPE series.

The aim of SPE is to improve U.S. arms control and nuclear nonproliferation verification and monitoring capabilities. The scientific plan is to generate data that records signals from a series of chemical explosions, contributing to the development and validation of seismo-acoustic modeling codes (Snelson et al., 2014). RV/DC will directly compare signatures of earthquakes and chemical explosions at the same depth and media, providing critical data to enhance and validate physics-based modeling codes that will reproduce key observables in new areas and emplacement conditions (Walter et al., 2012). Rock Valley was chosen as the site for the RV/DC experiments due to a series of unusually shallow (1–3 km deep) earthquakes that occurred on the Rock Valley Fault Zone (RVFZ) in 1993 (Smith et al., 2000). The hypocenter of these earthquakes will be targeted as the location for the chemical explosions, which will allow for the direct comparison of seismo-acoustic data. UE-27a is the first of several boreholes planned for the RV/DC Venture.

Corehole UE-27a (Corehole, CH, or UE-27a) is located on the NNSS, within Rock Valley in Area 27 (Figure 1). The purpose of UE-27a is to characterize the geology of the subsurface at the Rock Valley testbed and provide samples for measurement of physical, petrophysical properties, and obtain real-time seismic data to characterize the seismicity. The entire length of the borehole, from the ground surface (0 feet [ft] depth) to the total depth (TD; 6,857 ft; 2090 m depth) below ground surface (BGS) was drilled to recover core. This is the deepest borehole drilled at the NNSS in over 20 years, and the deepest continuously cored borehole ever drilled at the NNSS. Detailed geologic information about the subsurface at Rock Valley will support RV/DC efforts, including updating the models for the modeling and simulation framework, improving the Near- and Far-Field Rock Valley Geologic Framework Models (GFMs), and planning for future drilling operations.

Downhole geophysical surveys were conducted at intervals during and after drilling to document borehole conditions; these included seismic wave velocity, rock density, porosity, and temperature. After drilling was completed, groundwater samples were collected and an instrumentation system was installed in the Corehole that includes eight levels of geophones, distributed acoustic sensing (DAS) fiber, and distributed temperature sensing (DTS) fiber.

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Figure 1: Reference map showing location of Borehole UE-27a at the Nevada National Security Site. Inset map outline shows the extents of Figures 2 and 7.

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1.2 Project Organization

The RV/DC venture consists of four institutions led by a Venture Management Team (VMT), which includes members from Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL), Sandia National Laboratories (SNL), and the NNSS. The VMT is responsible for developing scope and schedule, establishing guidance for execution policies, and providing support to the Venture Manager for the RV/DC Venture.

1.3 Location and Significant Nearby Features

The RV/DC testbed is located in Area 27, in the south-central portion of the NNSS, approximately 8.2 mi (13.2 km) northwest of Mercury. The testbed is set in an alluvial plain that gently slopes down to the south toward the Specter Range and is bounded on the north by the Skull Mountain range (Figure 2).

Subsurface control in Rock Valley is sparse, with Test Well F (TWF, labeled HTH F in Figure 2) representing the nearest deep subsurface penetration into the Paleozoic section. More details on available geologic data are provided in the “Geologic Setting” and “Geologic Data Collection” sections.

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Figure 2: Reference map of the Rock Valley Region, showing topography and historic boreholes.

Test Well F (TWF) is labeled as United States Geological Survey (USGS) HTH F on this figure.

1.4 Objectives

Corehole UE-27a is a deep exploratory borehole, the primary purpose of which is to provide data for geologic site characterization and near real-time seismic data. The entire length of UE- 27a was cored to facilitate this and to provide rock samples for testing. Geologic information

Document No: RVDC-1.01-026RPT Page 11 of 51 from this borehole will assist in providing more accurate interpretations of the subsurface stratigraphy and structure at Rock Valley, which is not well understood due to the lack of historic drilling in the area and the inherent structural complexity of the region.

The scientific objectives and requirements for RV/DC are described in RVDC-1.04-003PLN and RVDC-1.04-016PLN. Applicable objectives and requirements supported by drilling UE-27a are listed below:

• Geologic characterization of the RV/DC testbed [RVDC-SR-1003, RVDC-SR-1015, RVDC-

SR-1016; RVDC-SO-1001, RVDC-SO-1006]

o Provide data for geologic interpretation of the stratigraphy and lithology of the subsurface at Rock Valley.

o Sample and examine groundwater chemistry for geophysical insights of the Rock Valley fault and any constraints on chemical explosive emplacement and residency.

o Enable further analysis of the Rock Valley Fault Zone.

o Measure the temperatures at the planned future working point depths of the future chemical explosives to help in planning their design, composition and residency time limits before execution.

o Support emplacement of chemical explosive source in competent and homogeneous rock within the Paleozoic section.

o Update the RV/DC models for the modeling and simulation framework.

o Update the Rock Valley Geologic Framework Model (GFM).

o Measure material properties and petrophysical data obtained from core samples.

• Install downhole instrumentation to acquire additional time series data to better characterize the background shallow and normal depth seismicity, as well as to support a network of sensors to record signals from chemical explosions and earthquakes

[RVDC-SR-1010; RVDC-SO-1004, RVDC-SO-1006]

1.5 Project Summary

Drilling of Corehole UE-27a started on March 19, 2025, and concluded on June 18, 2025, having drilled to a depth of 6,857 ft BGS. Despite several issues with stuck rods and borehole stability during drilling, mostly from the inherently challenging geology within the RVFZ, the hole was completed 30 days ahead of schedule. The Corehole was drilled through the contact between Cenozoic and Paleozoic rocks, and was completed within interbedded limestone and shale, interpreted as Paleozoic Carrara Formation. Multiple strings of casing were installed to

Document No: RVDC-1.01-026RPT Page 12 of 51 maintain borehole stability for drilling and instrumentation. Table 1 contains general information on the borehole.

The borehole is cased down to 4,189 ft (1,277 m) BGS, although this may have slipped to 4,205 ft (1,282 m) due to separation in the casing during geophysical logging. A distributed fiber optic sensing (DFOS) cable that includes an array of eight 3-component geophones was installed in UE-27a after completion and water sampling.

Geologic data collected during drilling included continuously cored rock from ground surface to TD. Downhole geophysical surveys were performed during and after drilling. These logs will assist in geologic characterization.

Corehole UE-27a is collared in alluvium and drilled into Miocene-aged tuffaceous sandstones and mudstones of the Rocks of Pavits Spring at 110 ft (34 m) BGS. The lower contact with Winapi Wash is placed at 2,247 ft BGS, marked by a transition to carbonate-dominated sedimentary rocks with marlstone as well as subordinate sandstone and mudstone. The contact between Cenozoic sedimentary rocks and Paleozoic carbonate rocks was encountered at 3,881

ft. The uppermost Paleozoic unit is interpreted to be the Bonanza King Formation. At 6,224 ft BGS this transitioned to the Carrara Formation. UE-27a reached TD within the Carrara Formation. A fluid-level depth of 1,406 ft BGS was measured in the borehole by the USGS on July 1, 2025.

Table 1: Corehole UE-27a Data Summary

Hole Name Corehole UE-27a

Site Coordinates1 578216.43 E, 4064062.99 N

Surface Elevation2 (meters) 1151.56 m

Drilled Depth 6,857 ft (2090 m)

Fluid Level Elevation3 2372.1 ft (723 m) above Mean Sea Level, measured on 6/1/2025

Surface Geology Alluvium

1Coordinate System: NAD83(2011) UTM Zone 11N, EPSG (European Petroleum Survey Group):6340 2Vertical Coordinate System: NAVD88 computed using the GEOID18 model, EPSG:9229 3Depth to water below ground surface is approximately 1,406 ft as measured by USGS.

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2 Drilling Summary

2.1 Drilling History

The drilling company Major Drilling (MD) was subcontracted by the NNSS to drill UE-27a. The drill site was prepared by the NNSS to accommodate MD’s requirements and to provide adequate space for science, operations, drilling, and core-handling facilities (Figure 3).

Field operations at the GZ (Ground Zero) drill pad began on March 19, 2025, when MD began setting up their Boart Longyear LF-350 drill rig, and positioned related equipment and supplies at the site. NNSS geologists began setting up their core trailer and equipment on March 20, 2025.

MD started coring on March 27, 2025. Drilling operations were 24 hours a day, 7 days a week.

MD cored in 10 ft runs using a wireline core catcher; in difficult sections the runs were temporarily decreased to 5 ft. A diagram showing drilled borehole dimensions is provided in Figure 4, and official as-built drawings are within RVDC-1.01-001DWG. Daily reports submitted by MD were uploaded to the RV/DC Data Catalog. A summary of the drilling information is shown in Table 2.

Coring began with a PQTT-size core bit with a 4.9 in. (12.4 cm) outer diameter (OD), which produces PQ core (3.3 in. diameter; 8.4 cm) (Figure 4). MD drilled to the depth of 57.0 ft (17.4 m) and reamed the borehole with three sets of tricones with progressively larger sizes:

5-7/8 in. (5.9 in.; 15 cm), 9.75 in. (25 cm), and 12.25 in. (31.1 cm). Surface casing with an OD of 6-5/8 in. (6.63 in.; 16.8 cm) and an inner diameter (ID) of 6 in. (15.2 cm) was set to 52 ft (15.8 m) BGS and cemented into place on March 28, 2025. Drilling resumed on March 30, 2025.

Drill fluid returns were lost at near the depth of 120 ft (36.6 m) and circulation was never regained. See Section 2.4 Fluid Management for fluid management information.

On April 1, 2025, at the drilled depth of 526 ft (160.3 m), the borehole bridged, causing the drill string to get stuck at 70 ft (21.3 m) BGS. MD reamed the hole with a 5.75 in. (14.6 cm) tricone bit down to a depth of 280 ft (85.3 m). To mitigate future borehole stability issues, the crew set PWT casing (5.5 in., 14 cm OD; 5 in., 12.7 cm ID) to the depth of 280 ft. Coring resumed on April 2, 2025.

Coring with a PQTT-sized bit continued to 738 ft (225 m) BGS, where on April 3, 2025, the drill rods became stuck. Specialized equipment was brought in to cut the drill pipe, leaving the lower 2 ft of pipe and the bit in the hole. MD drilled through the stuck bit and continued coring, but similar issues occurred over the next few days as the hole penetrated several clay-rich intervals associated with a fault zone.

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Between April 15 to 18, 2025, at a depth of 2,217 ft (675.7 m), drilling was paused for DGI Geosciences to complete the first downhole geophysical survey callout. More information on these surveys is provided in Section 3.3, Geophysical Log Data. Drilling resumed after logging and the contact between Cenozoic and Paleozoic rocks was encountered on May 4, 2025, at a depth of 3,881 ft (1,183 m) BGS.

Between May 6 and 11, 2025, at a depth of 4,186.5 ft (1,276 m), DGI returned for the second downhole geophysical survey callout. After geophysical logging was completed, MD reamed the borehole to a 4,189 ft (1,276.8 m) BGS. On May 12 and 13, 2025, the crew installed 4.5 in. OD HWT casing (4.5 in., 11.4 cm OD; 4 in., 10.2 cm ID) to a depth of 4,189 ft to maintain hole stability (Figure 4). The remainder of the hole was cored with an HQTT bit with 3.83 in. (9.7 cm) OD, which produces HQ core (2.5 in. diameter; 6.4 cm).

At 2:28 am on June 18, 2025, UE-27a reached TD at 6,857 ft (2,090 m) BGS. The third and last geophysical logging callout was completed June 18-23, 2025. The drill crew began cleaning and reaming the hole and started derigging operations starting June 19, 2025. The planned and actual drilling schedule showing drilled depth by day is shown in Figure 5.

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Figure 3: Map showing the drill site set-up at the RV/DC testbed, Area 27. The Corehole location is to the right (east) of the drill rig.

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Figure 4: Diagram of UE-27a. Borehole diameters are shown on the left and casing diameters on the right.

Figure 5: Modified drilling subcontractor's whiteboard showing sections of significance to drilling operations. Right: Charted drilling progress showing depth over time, GWS is groundwater sampling.

Table 2: Abridged Drill Hole Statistics for Corehole UE-27a. North American Datum (NAD), Universal Transverse Mercator (UTM), North American Vertical Datum (NAVD), etc.

Location Data

Coordinates Horizontal Datum:

NAD83(2011) UTM

Zone 11N, Meters

EPSG:6340

Vertical Datum:

GEOID18/NAVD88,

Meters EPSG 9229

Horizontal Datum:

NAD27 State Plane Central 2702, US Survey Feet

EPSG:32008

Vertical Datum:

NGVD29, US Survey Feet EPSG:5702

Horizontal Datum:

NAD83 State Plane Central 2702, US Survey Feet

EPSG:3422

Vertical Datum:

NAVD88, US Survey Feet, EPSG: 6360

Northing 4064062.99 717152.57 20402152.58

Easting 578216.43 659257.52 1799415.52

Surface Elevation 1151.56 3777.30 3780.42

Drilling Data

Spud Date 03/27/2025

Total Depth 6,857 ft (2090 m)

Date TD Reached 06/18/2025

Date Well Completed 06/25/2025

Hole Diameter 12.25 in. (31.1 cm) to 57 ft; 5.75 in. (14.6 cm) to 280 ft; 4.9 in.

(12.4 cm) to 4,189 ft; 3.8 in. (9.7 cm) to 6,857 ft (TD).

Drilling Techniques Drill with PQTT bit to 52 ft; ream 12.25 in. hole to 57 ft. Core with PQTT bit to 4,189 ft. Core with HQTT bit to 6,857 ft (TD).

Casing Data 6-5/8 in. carbon steel to 52 ft; 5.5 in. PWT carbon steel casing to 280 ft; 4.5 in. HWT carbon steel casing to 4,189 ft.

Instrumentation Data DFOS cable with DAS and DTS, geophone array from 0 to 6,758 ft (0 to 2060 m).

Drilling Subcontractor: Major Drilling Group International Inc.

Geophysical Logging Subcontractor:

DGI Geoscience US Inc.

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2.2 Borehole Deviation

The borehole path was determined from the Gyrotracer deviation survey log run by DGI Geosciences after drilling was completed. The survey data shows the average dogleg severity in the borehole is 0.59 degrees per 100 ft (30.5 m). The average inclination was 88.7 degrees (planned 90 degrees) over the length of the CH. The maximum displacement of the borehole is

20.2 ft (6.2 m) west and 45.7 ft (13.9 m) south of the collar location, for a total displacement of

48.5 ft (14.8 m). At the bottom of the borehole, the displacements are 20.1 ft (6.1 m) west and

40.9 ft (12.5 m) south of the collar location, for a total displacement of 45.6 ft (13.9 m). Figure 6 shows a cross-sectional and plan view of the borehole profile.

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Figure 6: Deviation survey for UE-27a. The left two columns show the azimuth and inclination in degrees, plotted by depth. The right shows a planar view of the borehole deviation with concentric circles showing feet from the collar location.

2.3 Drilling Problems

Most non-mechanical issues that occurred during drilling were related to the presence of clay and fracturing associated with fault zones as described above. Efforts to free a stuck drill pipe included use of additional drill fluid, and in some instances, drilling through the stuck pipe and bit. The drill crew experienced delays due to a stuck pipe on April 03-06, May 02-03, and May 25-28, 2025.

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On April 14, 2025 a small fire occurred at the mud pump. It was immediately extinguished and MD had a safety stand-down. It was concluded that the fire was caused by improper filling techniques, and a new procedure was put in place to prevent the issue from occurring again.

Excluding coring through fault zones, MD experienced mostly minor delays due to maintenance and bit replacements during drilling of UE-27a; however, the severity of these issues increased in the last 2,500 ft of the hole. Due to equipment issues, a significant delay occurred from May 22-25, 2025, requiring repairs and replacement of various parts on the hydraulic pump and waterline system.

Despite issues encountered during drilling through a known fault zone, MD completed the project 30 days ahead of schedule (Figure 5). An unexpected, non-drilling related issue occurred causing a brief delay in operations on April 1, 2025, when a coyote entered the drill pad and bit a driller. This incident prompted a response from the NNSS protective force and biologists to address the situation.

2.4 Fluid Management

Two sump pits were constructed for the management of drill fluid returns. Because circulation was lost at 120 ft BGS, the sump pits remained dry. An average of 12,050 gallons of fluid per day was lost into the formation, presumably due to the prevalence of faulting.

DGI Geosciences acquired three groundwater samples from the borehole on July 1, 2025. See Section 5 Hydrology for additional information about water in UE-27a.

3 Geologic Data Collection

3.1 Introduction

This section describes the sources of geologic data obtained from the borehole and the methods of data collection. Improving understanding of the subsurface stratigraphy, lithology, and structural characteristics was a primary objective of the drilling program. Proper handling, documentation, and preservation of recovered core was considered fundamental to achieving this objective.

3.2 Coring Processing and Logging

Geologic data collected from the UE-27a consisted of recovered core samples and associated core logging data. Data collection, sampling, and documentation activities were performed using standardized logging procedures (NNSS Organization Procedure OP-2152.204). The geology group’s daily reports on core were uploaded to the RV/DC Data Catalog.

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Core was recovered in approximately 10-ft runs (a run is a sequence of drilling activity that results in the cutting of a measurable interval of hole) and was deposited by the drillers from the drill tube into a core trough. The use of split tubes, which open along the length of the core tube, facilitated more gentle handling of the core than conventional tubes, which require “dumping” of core from the end of the tube. Geologists washed, measured, and documented the core in each run. Measurements included amount drilled, total recovery, Rock Quality Designation (RQD, an industry standard system for estimating rock mass quality), and drilling beginning and end depth. These measurements were recorded on run blocks (wood blocks placed between core runs) and entered into the borehole database. Orientation lines were marked on all samples to preserve core direction (up vs. down); the samples were then boxed in depth order. Drilling, run blocks, and core logging were all done in decimal feet units.

Following initial measurements, marking, and boxing of the samples, the geologists transported the core into the nearby core trailer to collect basic descriptive data. The geologists recorded preliminary lithologic designations and other observable characteristics such as fractures, lithologic changes, and alteration features.

Selected core intervals were preserved close to their natural state for future analyses. In this process a geologist selected representative intervals and applied a beeswax coating to maintain the original state, such as moisture content, of the core. Preserved samples were labeled and stored in their respective boxes.

Core boxes were stacked on pallets and transported twice weekly to the USGS Core Library in Mercury, NV. A total of 1,375 boxes were transported. After NNSS geologists checked the boxes and samples for correct and legible labeling, the USGS took high-resolution photographs of each core box, both dry and wet, with consistent lighting and scale. These photos were uploaded to the RV/DC Data Catalog.

Detailed logging was conducted at the USGS Core Library following completion of drilling operations. Observations were recorded in a centralized database, and were exported as individual logs, including lithology, alteration, fracture, and geotechnical logs.

The lithologic log documents depth intervals, primary and secondary lithologies, color, texture, and grain size. Interpreted alteration characteristics, based on visual examination, include type, intensity, and style. The fracture log includes fracture orientation (alpha angle), morphology, mineral fill, and indicators of displacement, such as slickenlines. Geotechnical logging records rock quality and mechanical characteristics.

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These data collectively provide a detailed record of subsurface conditions and form the basis for stratigraphic interpretation and structural characterization of the borehole. All geologic logs were uploaded as CSV files on the RV/DC Data Catalog. See Section 4, Geology, for more information on the geology of the Corehole. Detailed plots of selected geologic core logs are presented in Appendix A.

3.3 Geophysical Log Data

A suite of downhole geophysical logs was completed for UE-27a by DGI Geosciences. DGI ran geophysical logs on three separate callouts when the CH was at 2,217, 4,186.5, and 6,857 ft BGS. In each case, MD prepared the hole by spinning rods or running a casing shoe up and down the borehole in a ‘wiper trip’ to clear obstructions. The full list of downhole instruments for geophysical logs is provided in Table 3. Detailed plots of selected geophysical logs are presented in Appendix B.

The first logging callout occurred April 15-18, 2025, when the CH was at a depth of 2,217 ft (675.7 m). Tools that required open hole conditions were run at a beginning depth of 280 ft where casing was already installed. Due to borehole instability issues related to faulting, logging was done in two lifts and then the data were combined. This caused a data gap in some datasets between 656 and 736 ft, where faulting was most prevalent.

The second logging callout occurred May 6-11, 2025, when the CH was at a depth of 4,186 ft (1276 m). An obstruction was encountered at the top of a conglomeratic section (~3,378 ft) after the first tool, the dipmeter, was run. It is possible that agitation of the caliper arms on the dipmeter caused this obstruction. Efforts to clear the blocked borehole, including flushing, spinning rods, and reaming, were unsuccessful. The remaining data collection for callout 2 was abandoned, causing a gap in some datasets between 3,378 and 4,200 ft.

The third and last logging callout occurred June 18-23, 2025, after the CH reached TD, 6,857 ft (2,090 m). Due to the increased pressure and temperatures, some logs were run on different tools rated to withstand the borehole conditions. The deviation survey and other logs that could be done within casing (i.e., density, deviation, gamma) were re-run for the entire length of the borehole.

Complete information on each geophysical log, including important metadata, can be found in the associated files (.las, .pdf, .wcl, and .csv) saved on the RV/DC Data Catalog.

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Table 3: Corehole UE-27a Geophysical Log Summary. Intervals are not always continuous and gaps are described in the text.

Tool Model Geophysical Log Type Top of Logged Interval

Bottom of Logged Interval

QL ABI-2G Acoustic Televiewer

Acoustic Borehole Imager 317 ft (96.6 m)

3,385 ft (1031.7 m)

ABI43 (High Temp/High Pressure)

Acoustic Borehole Imager 4,159 ft (1267 m)

6,842 ft (2085.4 m)

QL Spectral Gamma Natural and Spectral Gamma Ray

35.2 ft (10.7 m)

6,827 ft (2080.9 m)

QL40-ELOG Resistivity 322 ft (98.1 m)

6,822 ft (2079.3 m)

QL40-DEN Density 37 ft (11.3 m)

6,806 ft (2074.5 m)

QL40-NEU Neutron Porosity 7.0 ft (2.1 m)

6,803 ft (2073.5 m)

QL FWS-3Rx-1Tx Full Waveform Sonic Velocity

362 ft (110.3 m)

6,837 ft (2083.9 m)

QL40-ELOG Spontaneous Potential 322 ft (98.1 m)

6,818 ft (2078.1 m)

Stockholm Precision Tools Gyrotracer - 2124

Deviation 0.0 ft (0.0 m)

6,791 ft (2069.9 m)

Dipmeter-9412 4-arm Caliper / 4-pad Micro-Resistivity*

250 ft (76.2 m)

3,845 ft (1171.9 m)

2110-4A-XY-Caliper (High Temp/High Pressure)

4-Arm Caliper 3,755 ft (1144.5 m)

6,841 ft (2085.1 m)

QL40 Inline FTC Temperature / Fluid Conductivity

322 ft (98.1 m)

6,826 ft (2080.5 m)

QL40 Induction HI-

456DE

Induction Conductivity 319 ft (97.2 m)

3,385 ft (1031.7 m)

EM Dual Induction Probe

Induction Conductivity 4,250 ft (1295.4 m)

6,758 ft (2059.8 m)

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4 Geology

4.1 Geologic Setting

Borehole UE-27a was drilled within a structurally complex area known as the Rock Valley Fault Zone (RVFZ), located between the Neogene volcanic rocks of the Southwest Nevada Volcanic Field (SWNVF) and the Paleozoic carbonate rocks of the Specter Range (Figure 7; Figure 8).

The RVFZ is a transtensional structure, dominated by long, sinistral strike-slip faults oriented 65° to 80° northeast, as well as short normal, strike-slip, or reverse faults oriented 25° to 50° northeast (Coe et al., 2000; O’Leary, 2000). Faults within Rock Valley have been active since the late Paleogene and produced a sequence of unusually shallow earthquakes in 1993 (Smith et al., 2000).

The oldest rocks in Rock Valley are in a sequence of carbonate and siliciclastic formations ranging in age from Upper Precambrian to Devonian (Hinrichs, 1968; Sargent and Stewart, 1971). This stratigraphic sequence has been deformed by the Specter Range thrust, which placed older Precambrian and Cambrian formations on top of Ordovician carbonate units (Cole and Cashman, 1999). Subsequent Basin and Range extension likely began in the Oligocene.

Normal faulting caused Rock Valley to drop down and form a basin where early basin-fill sediments were deposited. Initiation of valley formation predates volcanic activity from the SWNVF and continued synchronously with eruptions from SWNVF caldera clusters, including the nearby Timber Mountain complex. This produced a sequence of sedimentary rocks with increasing tuffaceous content over time from the Oligocene to the Miocene (Sawyer et al., 1994; Murray et al., 2002). Most of these rocks and structures have since been covered by unconsolidated Quaternary-Tertiary age alluvium, obfuscating important stratigraphic contacts and fault relationships.

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Figure 7: General geologic map of the Rock Valley region showing GFM units. The map differentiates Quaternary and Tertiary Alluvium; however, the GFM groups Quaternary and

Tertiary Alluvium into a single, undifferentiated layer.

A-A’ Cross section shown in Figure 8.

Figure 8: A-A' Cross Section showing geologic interpretation of the subsurface. The approximate location of UE-27a is projected.

Units are same as Figure 7.

Geologic units in this report are described in two ways, reflecting different degrees of detail.

The RV/DC GFM v2.0.1 groups geologic units into four layers for experiment planning, modeling, and simulation. The relationship between the GFM layers and the more detailed list of stratigraphic units described in Slate et al., 1999 is shown in Table 4.

Table 4: Key to Stratigraphic Units and Symbols Encountered in Corehole UE-27a

GFM Layer GFM Map

Symbol (Figure 7)

Stratigraphic Units (Slate et al., 1999) Map Symbol (Slate et al., 1999)

Alluvium AL Surficial Deposits

Qay

Qai

QTa

QTc

Qeo

Tertiary Basin-Fill Sediments Tgy

Cenozoic Volcanic Rocks Tv

Thirsty Canyon Group Tft Tfb

Timber Mountain Group Tma Tmr Tmrf

Paintbrush Group

Tpt Tac Tw Tws

Crater Flat Group Tcg Tcb Tct

Cenozoic Sedimentary

Rocks Ts

Lithic Ridge Tuff Rocks of Pavits Spring Rocks of Winapi Wash

Trl Tgp Tgo

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GFM Layer GFM Map

Symbol (Figure 7)

Stratigraphic Units (Slate et al., 1999) Map Symbol (Slate et al., 1999)

Paleozoic and Precambrian Rocks

Pz

Undivided Dolomite

Ds DSsl DSlm

Ely Springs Dolomite Oes

Eureka Quartzite Oe

Pogonip Group Op

Nopah Group Cn

Bonanza King Formation Cbb Cbp

Carrara Formation Cc

Zabriskie Quartzite Cz

Wood Canyon Formation CZw

4.2 Stratigraphy and Lithology

4.2.1 Introduction

This section provides comprehensive geologic characterization data for the UE-27a Corehole in Rock Valley, Nevada. The primary objective is to establish the detailed lithologic and stratigraphic framework encountered during drilling, which is crucial for understanding depositional history and structural complexities. Data from this borehole will improve the quality and detail of the RV/DC GFM, and provide additional information needed for geophysical structure models (RVDC-1.04-016PLN).

The discussion describes major geological formations, provides specific lithological descriptions from core logging, and consolidates stratigraphic relationships, trends, and key boundaries observed in the UE-27a core, including interpretations from borehole geophysical log data.

4.2.2 Stratigraphic Overview

The UE-27a Corehole reveals a complex geological history for the Rock Valley Area. This section discusses each stratigraphic unit encountered and geologic observations, including lithological composition, contacts, and sedimentary structures, listed from youngest to oldest (Table 4, Figure 9). Core photos of typical sections for each formation are shown in Table 5.

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Alluvium (0-110 ft; 0-33.5 m): The uppermost unit in Rock Valley, consisting of unconsolidated to partly consolidated Quaternary to Tertiary surficial deposits. It is generally coarse-grained, and poorly sorted, reflecting alluvial fan and flood plain origins. The uppermost section of the core comprises unconsolidated orange to pale yellowish-brown sandstone. This unit is notably massive and very poorly sorted, containing occasional light white tuff and basalt cobbles. A clear fining-downward trend is observed: the alluvium becomes progressively finer grained with increasing depth. The contact with the underlying Pavits Springs Formation is a distinct lithological boundary, marked by the last occurrence of mafic cobbles and the first appearance of bedded mudstone.

Pavits Spring Formation (110-2,247 ft; 33.5-684.9 m): Also known as the Rocks of Pavits Spring, this is a Miocene-aged sedimentary rock unit in south-central Nevada. In UE-27a the Pavits Spring comprises sandstone, mudstones, and shales. Sandstones are commonly tuffaceous, fine to coarse grained, and contain significant volcanic material in both the matrix and in gravel-sized clasts of lithic fragments or tuff. They are typically bedded in 10-cm beds and can occur interbedded with mudstone or conglomerate. Tuffaceous material and occasional tuff beds were deposited during regional ignimbrite flare-up volcanism from the SWNVF. Evidence of active depositional processes includes local coarsening upwards sequences, mud cracks, hummocky cross strata, and channel deposits. Mudstone frequently shows signs of soft-sediment deformation and contains bedded to interlaminated sedimentary textures. Shale is friable and has a dark brown to black color, indicating presence of organic material. The contact between the Pavits Springs Formation and the underlying Winapi Wash Formation is identified by a gradual change from tuffaceous-dominated sedimentary rocks to more carbonate-rich clastic rocks and is located where marlstone becomes the dominant lithology.

Winapi Wash Formation (2,247-3,881 ft; 684.9-1,182.9 m): Also known as the Rocks of Winapi Wash, this is an Oligocene to Miocene-aged sedimentary unit found in the subsurface of Rock Valley. This unit is characterized by sandstone, conglomerates, mudstone, and marlstone with occasional tuff beds, and is differentiated from the overlying Pavits Spring by lower amounts of volcanically sourced sediments from the SWNVF. The sandstones are coarse- to medium-grained, massive to bedded, and have some tuffaceous material, but they are often calcareous and have gravel-sized clasts of older Paleozoic carbonates and quartzites. The section also includes thick orange-red marlstone beds showing soft sediment deformation. Below the depth of 3,400 ft the CH penetrated a distinctive interbedded red marlstone and grain-supported conglomerate with 3-6 cm well-rounded cobbles of quartzite and carbonate rock. These interbeds continue, with a general coarsening-downwards trend and increased angularity in the clasts, until the sharp disconformable contact with the underlying Paleozoic carbonate rocks.

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The last ~50 ft of the Cenozoic sediments are interpreted as paleocolluvium, and the contact is marked by a sharp change from red oxidized matrix to grey limestone breccia.

Bonanza King Formation (3,881-6,224.5 ft; 1,182.9-1,897.2 m): The first Paleozoic formation encountered in UE-27a is interpreted as the Bonanza King Formation, a thick, middle to late Cambrian-aged sequence of carbonate rocks. It is distinctive for its mottled texture or alternating light and grey bedded limestone and dolostone and contains minor shale. Limestone and dolostone are identified by differing reactions to dilute hydrochloric acid (HCl) and color, but in many areas, they are difficult to tell apart. This may be due to incomplete dolomitization, which is suggested by the mottled textures. Later calcareous alteration is present, evinced by frequent calcite veins, further obscuring the exact mineralogy of carbonates. Both limestone and dolostone are bedded or laminated with occasional massive sections. The Bonanza King is fine-grained and sparse in fossils. Secondary calcareous shale is interbedded and interlaminated with carbonate rock in local sections. Evidence of fracturing and brecciation is common, with intervals with highly fractured clasts of medium-gray limestone and subordinate light-gray dolomite in a calcite matrix. Silicification is also noted, with sections appearing similar to quartzite. The contact between the massive Bonanza King Formation and the underlying Carrara Formation is gradational and marked by a distinct color change to darker gray and a grain-size transition to fine-grained sand and silt, accompanied by shaley fractures along bedding planes.

Carrara Formation (6,224.5-6,857 ft; 1,897.2-2,090 m): The Carrara Formation is assigned to the middle to early Cambrian era and conformably underlies the Bonanza King Formation. In UE-27a it comprises bedded and laminated shale, siltstone, and limestone. Dark gray shale and bluish gray limestone are interbedded in 1-5 cm beds or thin, millimeter-scale laminations.

Fossiliferous beds are common in limestone and include trilobite clasts and gastropods.

Occasional ooidal or oncoid beds are also observed in the core. This formation is relatively unfractured, with only small shear structures appearing. The CH reached TD at 6,857 ft within the Carrara Formation.

Conclusion

General Trends: The UE-27a Corehole penetrated a distinct unconformity between the Cenozoic and Paleozoic rocks, which represents a gap in time of over 450 million years. The Cenozoic interval is dominated by volcaniclastic and clastic sediments derived from regional ignimbrite flare-up volcanism and subsequent fluvial environments, producing poorly sorted, rapidly deposited units with abundant tuffaceous material. Evidence for later lacustrine environments is also evident from intervals of shale and mudstone. Grain sizes within the Cenozoic section are highly variable; the abundant coarse-grained units within the Pavits

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Springs and Winapi Wash Formations indicate high-energy depositional events. Overall, the Cenozoic section here has a generally defined fining-upwards trend. In contrast, the Paleozoic section is characterized by well-bedded or laminated, typically fractured and partially dolomitized carbonate rocks likely accumulated on platforms in shallow marine environments.

Within the Paleozoic section, interbedding of fine-grained shales with limestones and dolomites indicates cyclic changes in sea level and depositional energy. The presence of various sedimentary structures, such as crossbedding, mud cracks, and channel fills, further elucidates the dynamic interplay of depositional forces over geological time. This comprehensive understanding of the UE-27a subsurface geology serves as an essential foundation for all future analyses and interpretations related to the Rock Valley Venture.

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Figure 9: Geologic data collected from UE-27a from collar (top) to TD (bottom). Detailed core logs are in Appendix A. New abbreviations: fractures (fx), Modified Rock Index (MRI).

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Table 5: Core Photos of Formations in UE-27a

Feature Photo of typical example

Alluvium: Unconsolidated silty sand with cobbles of surrounding lithologies (Photo: 105 ft BGS)

Pavits Spring Formation:

Shales, mudstones, and tuffaceous sandstones (Top photo: 465 ft BGS, Bottom photo: 1,813 ft BGS)

Winapi Wash Formation:

Sandstone, conglomerates, mudstone, and marlstone with occasional tuff beds (Top photo: 2,420 ft BGS, Bottom photo: 3,803 ft BGS)

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Feature Photo of typical example

Bonanza King Formation:

Alternating light and grey bedded limestone and dolostone with minor shale (Top photo: 4,798 ft BGS, Bottom photo: 5,620 ft BGS)

Carrara Formation: Bedded shale and limestone (Top photo: 6,575 ft BGS, Bottom photo: 6,813 ft BGS)

4.3 Alteration

The rocks observed in the UE-27a core show several types of secondary alteration processes critical to characterizing the rocks’ mineralogical and textural fabric, which govern their structural integrity, porosity, and acoustic impedance. The alteration processes inferred from visual core observations are argillic alteration, calcification, opalization, oxidation, and silicification (Figure 9).

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Alluvium (unaltered). Observed above 110 ft (33.5 m) in Corehole UE-27a is unaltered alluvium, presenting as either clast-supported or matrix-supported assemblages of unconsolidated, sub-rounded to well-rounded fragments. The primary porosity is high, as interstitial voids are filled with well-sorted sand or silts. The absence of post-depositional weathering features like bioturbation or redoximorphic features indicates the original texture maturity is preserved.

Argillization. Pervasive argillic alterations are observed from 500 to 2,900 ft (152.4 to 883.9 m) through the Pavits Spring Formation and into the Winapi Wash Formation. Argillized rock is notably friable, indicating a transformation of the primary lithology into a clay-rich matrix. In this zone, the rock is consistently argillized, indicating sustained, high-volume fluid flow under low to moderate temperature conditions that facilitated mineralogical transformation across this interval. Original textures are variably preserved but muted, with evidence of mineral breakdown and replacement by fine-grained clay minerals.

Argillic alteration was also noted starting at the depth of about 3,900 ft (1,188.7 m) in the Bonanza King Formation, to 6,200 ft (1,889.8 m), consisting of patchy to locally pervasive zones of clay enrichment.

Calcification. Observed from 2,900 ft to 6,900 ft (883.9 to 2,103.1 m) within the Winapi Wash Formation and completely through both the Bonanza King Formation and the Carrara Formation. This section is dominated by calcification and typically is more competent due to extensive calcite cementation and matrix replacement. The calcite alteration is expressed through pervasive to locally intense calcite replacement and veining. Veining is prominent, ranging from hair-line fractures to thicker, sparry infills that follow structural pathways. The presence of healed fractures and vein networks suggests fluid movement along structural pathways, contributing to localized increases in alteration intensity.

Silicification. From 3,900 to 6,200 ft (1,188.7 to 1,889.8 m) throughout the Bonanza King Formation, silicification is observed as both disseminated replacement and localized zones of increased hardness and resistance, consistent with silica enrichment and quartz precipitation.

Within these intervals, some original textures are partially preserved but are commonly overprinted by silica cementation or significant quartz crystals.

Opalization. From 3,900 to 6,200 ft (1,188.7 to 1,889.8 m) is observed in a few localized areas along fractures and veins within the Bonanza King Formation.

Oxidation. From 3,900 to 6,200 ft (1,188.7 to 1,889.8 m), pervasive oxidation is evident through variable color transitions (deep rust, orange, and yellow hues). This alteration is concentrated

Document No: RVDC-1.01-026RPT Page 37 of 51 along fractures and permeable zones where oxidizing fluids had greater access. The presence of oxidation contributes to the overall heterogeneity of the alteration assemblage.

Alteration Summary.

The alteration profile across UE-27a records a spectrum of fluid-rock interaction processes ranging from diagenetic stabilization to a dynamic, structurally controlled, hydrothermal system.

These alteration assemblages were the result of episodic tectonic deformation and volcanic activity, which played a critical role in redefining fluid pathways and physiochemical conditions through time. Fracturing and brecciation facilitated episodic fluid ingress and the volcanic activity introduced thermal and chemical perturbations that influenced fluid composition and temperature.

4.4 Core Recovery and Rock Quality

NNSS geologists documented total core recovery (TCR) and rock quality designation (RQD) data for each run in the core trough prior to transferring core into boxes for storage, according to NNSS Organization Procedure OP-2152.204.

The TCR log is a simple percentage value that describes the length of core recovered in each run. The RQD log describes core recovery as a quality percentage where the sum of all pieces of sound core (defined as competent rock samples more than 0.33 ft long) are divided by the length of the core run (Deere and Deere, 1988).

The TCR and RQD logs from UE-27a show that core recovery and rock quality were generally good, with some local exceptions (Figure 9). Over the entire length of UE-27a, total core recovery was 98% and sound core was 80%. In the Cenozoic section of the core (defined here as 0 to 3,877 ft BGS, to correlate with a break at the end of the core run), total core recovery was 97% and sound core was 86%; in the Paleozoic section of the hole (3,877 to 6,857 ft BGS), total core recovery was 99% and sound core was 71%. Areas with low percentages of core recovery and sound core are attributed to poor quality rock, which can be related to damage from faulting or to inherently weak rock. Recovery was poor, with no sound core, in the first 110 ft of UE-27a (alluvium) due to the inherently unconsolidated nature of the material. Notable intervals containing low percentage of sound core due to fractured or faulted rock include 647

– 776 ft, 3,654 – 3,727 ft, 4,827 – 5,631 ft, and 6,336 – 6,446 ft. On average, the Paleozoic section was more faulted and fractured, which contributed to the overall lower sound core

Document No: RVDC-1.01-026RPT Page 38 of 51 percentage. This data can be correlated with other logs for UE-27a core to interpret large fault structures, as discussed in detail in Section 4.6, “Structure.”

4.5 Geotechnical Interval Log

UE-27a core was logged to analyze the frequency of fractures and the relative competency of the rock by grouping these features into geotechnical intervals. These intervals can be smaller than a drill run, which provides more detail than RQD, and include data to characterize the core within the section. Geotechnical intervals are chosen based on:

• Fractures per foot (average number of fractures per foot, fx/ft) o Number of fractures within the interval, divided by the length of the interval to get an average fracture count per foot.

• Rock Strength or Modified Rock Index (MRI) o Broken: In fragments smaller than 2 cubic inches, when delivered from drillers.

o Friable: Competent but can easily be broken with hands.

o Weak: Competent but can be broken with a standard geologic hammer falling under its own weight from 6 inches height.

o Strong: Competent and cannot be broken without applying force on a standard geologic hammer.

• Broken core zones

• Fault zones

• Lost core

A change in any of the above fields starts a new geotechnical interval that captures the new geotechnical properties. For example, if a rock core section remained the same except for an increase in fracture density, geologists identified the footage where fracture density increased and started a new geotechnical interval that captured the change in properties. This new interval continued until another change in any of the above fields was observed.