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Notice

Weir International, Inc. (WEIR) was retained by Arch Resources, Inc. (Arch) to prepare this Technical Report Summary (TRS) related to Arch’s Black Thunder Mine.  This report provides a statement of Arch’s coal reserves and resources at its Black Thunder Mine and has been prepared in accordance with the United States Securities and Exchange Commission (SEC), Regulation S-K 1300 for Mining Property Disclosure (S-K 1300) and 17 Code of Federal Regulations (CFR) § 229.601(b)(96)(iii)(B) reporting requirements. This report was prepared for the sole use of Arch and its affiliates and is effective as of December 31, 2021.

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This report was prepared by full-time WEIR personnel who meet the SEC’s definition of Qualified Persons (QPs) with sufficient experience in the relevant type of mineralization and deposit under consideration in this report.

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In preparing this report, WEIR relied upon data, written reports and statements provided by Arch. WEIR has taken all appropriate steps, in its professional opinion, to ensure information provided by Arch is reasonable and reliable for use in this report.

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The accuracy of reserve and resource estimates are, in part, a function of the quality and quantity of available data at the time this report was prepared.  Estimates presented herein are considered reasonable.  However, they should be accepted with the understanding that with additional data and analysis available subsequent to the date of this report, the estimates may necessitate revision which may be material.  Certain information set forth in this report contains “forward-looking information”, including production, productivity, operating costs, capital costs, sales prices, and other assumptions. These statements are not guarantees of future performance and undue reliance should not be placed on them. The assumptions used to develop the forward-looking information and the risks that could cause the actual results to differ materially are detailed in the body of this report.  

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WEIR and its personnel are not affiliates of Arch or any other entity with ownership, royalty or other interest in the subject property of this report.

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WEIR hereby consents (i) to the use of Arch’s Black Thunder Mine coal reserve and resource estimates as of December 31, 2021, (ii) to the use of WEIR’s name, any quotation from or summarization of this TRS in Arch’s SEC filings, and (iii) to the filing of this TRS as an exhibit to Arch’s SEC filings.

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Qualified Person: /s/ Weir International, Inc​ ​​ ​

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Date:February 10, 2022 ​ ​​ ​

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Address:Weir International, Inc.

1431 Opus Place, Suite 210

Downers Grove, IL 60515

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TABLE OF CONTENTS

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6.1.1 Regional Geology‌25

6.1.2 Local Geology‌27

6.1.3 Property Geology‌27

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8.2.1 Laboratory‌36

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13.1.1 Geotechnical Model‌58

13.1.2 Hydrogeological Model‌59

13.1.3 Other Mine Design and Planning Parameters‌60

13.0 Mining Methods (Cont.)

13.2.1 Production Rates‌60

13.2.2 Expected Mine Life‌61

13.2.3 Mine Design Dimensions‌61

13.2.4 Mining Dilution‌62

13.2.5 Mining Recovery‌62

13.3.1 Surface Development Requirements‌64

13.3.2 Reclamation (Backfilling) Requirements‌64

13.4.1 Mining Equipment‌64

13.4.2 Staffing‌65

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FIGURES

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Figure 1.1-1 General Location Map‌2

Figure 6.3-1 Stratigraphic Column‌28

Figure 6.3-2 Wyodak Seam Cross Section Northwest to Southeast‌29

Figure 7.5-1 Drillhole Collar Locations‌34

Figure 11.4-1 Variogram Model - Wyodak Seam Thickness‌49

Figure 13.5-1 Life of Mine Plan‌68

Figure 14.1-1 Simplified Material Handling Flowsheets‌71

Figure 15.7-1 Mine Infrastructure‌75

Figure 16.1-1 Historical PRB Spot Price‌77

Figure 16.1-3 Historical and Projected Coal Sales Price‌78

Figure 18.1-2 Historical and Projected LOM Plan Capital Expenditures‌87

Figure 18.2-1 Black Thunder Mine Historical and LOM Plan Operating Costs‌88

Figure 19.1-1 Historical and Projected Coal Sales Price‌92

Figure 19.3-1 Net Present Value Sensitivity Analysis‌94

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TABLES

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Table 1.5-1 In-Place Coal Resource Tonnage and Quality Estimate, as of December 31, 2021‌5

Table 1.5-2 Recoverable Coal Reserve Tonnage and Quality Estimate, as of December 31, 2021‌6

Table 1.6-1 Key Operating Statistics‌7

Table 1.7-1 Black Thunder Mining and NPDES Permits‌8

Table 1.7-2 Black Thunder Mine Permitted Area, Reclamation Liability, and Bonds‌8

Table 3.3-1 Property Control‌16

Table 3.4-1 Mineral Control‌16

Table 3.5-1 Permit List‌17

Table 7.2-1 Drilling Programs‌31

Table 11.1-1 Stratigraphic Model Interpolators‌44

Table 11.1-2 Drillhole Statistics‌45

Table 11.4-1 Theoretical Variogram Ranges‌49

Table 12.2-1 Reserve Estimate‌54

Table 12.2-2 Reserve Validation‌55

Table 12.5-1 Average Reserve Quality‌56

Table 13.4.1-1 Mining Equipment‌65

Table 13.4.2-1 Current Staffing‌65

Table 13.4.2-2 LOM Plan Staffing‌66

Table 13.4.2-3 Black Thunder Mine Manhours Worked, NFDL Injuries and NFDL Incidence Rate‌67

Table 17.3-1 Black Thunder Mining and NPDES Permits‌84

Table 17.3-2 Black Thunder Mine Permitted Area, Reclamation Liability, and Bonds‌85

Table 17.6-1 Environmental Achievements‌86

Table 19.2-1 Annual Cash Flow Forecast‌92

Table 19.2-2 After-Tax NPV, IRR, Cumulative Cash Flow, and ROI‌93

Table 19.2-3 Key Operating Statistics‌93

Table 22.2-1 Black Thunder Mine Risk Assessment Summary‌98

Table 25.1 Information Relied Upon from Registrant‌101

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WEIR was retained by Arch Resources, Inc. (Arch) to prepare a Technical Report Summary (TRS) related to Arch’s currently operating Black Thunder Mine. This report has been prepared in accordance with the United States Securities and Exchange Commission (SEC), Regulation S-K 1300 for Mining Property Disclosure (S-K 1300) and Title 17 Code of Federal Regulations (CFR) §229.601(b)(96)(iii)(B) reporting requirements.

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The Black Thunder Mine is located approximately 50 miles south of Gillette, Wyoming, in Campbell County, within the Powder River Basin (PRB) coal producing region of the United States (see Figure 1.1-1).

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The Black Thunder Mine permit and reserve boundary area includes approximately 62,066 acres of controlled mineral property.  Within that boundary, Arch controls the Upper and Main splits of the Wyodak Seam through 18 coal leases covering approximately 62,066 acres.  

Figure 1.1-1 General Location Map 

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The Powder River Coal Basin, of northeastern Wyoming, lies entirely within the boundaries of the Powder River structural and topographic basin.  Coal-bearing strata range in age from Upper Cretaceous in the Mesa Verde Formation to Eocene in the Wasatch Formation.  The Powder River Coal Basin covers parts of Campbell, Converse, Crook, Natrona, Niobrara, Johnson, and Weston Counties and is the largest coal basin in Wyoming.  

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The economically mineable coal in Campbell County occurs within the Tongue River Member of the Fort Union Formation. The Wyodak coal seam occurs at the top of the Fort Union Formation and is overlain by the Wasatch Formation. The coal is low sulfur, low ash, and is subbituminous C in rank.  Surface mineable coal deposits occur along the north-north­westerly striking subcrop of the Wyodak coal seam. The coal seam subcrops on the eastern edge of the lease and dips approximately two to three degrees to the west, with some slight rolling. This seam contains multiple benches or plys of coal of variable thicknesses, although in some local areas, it becomes one seam that reaches a thickness in excess of 100 feet.  Across the permit area, the Wyodak Seam ranges in thickness from 10 feet to 100 feet, averaging approximately 70 feet.

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Arch’s exploration activities exclusively involve drilling performed by competent contract drilling companies. Exploration drilling at Black Thunder Mine has been a two-stage approach.  Initial spot core drilling is conducted on a widely spaced, one-half mile, pattern in order to delineate potential lease areas.  Once the area has been leased, exploration drilling is conducted three to five years in advance of pit development.  Development drilling is generally conducted on a 500 feet north/south grid, with alternating rotary and spot core holes, in conjunction with dewatering endeavors. This arrangement results in a seam geometry data spacing of 500 feet and a coal quality data spacing of 1,000 feet. Drilling is conducted with rotary table drill rigs capable of drilling to depths of 1,000 feet.

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All holes are geophysically logged with a standard coal suite tool consisting of gamma, density, caliper, and resistivity.

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Coal sampling for the Upper split of the Wyodak Seam is in 1.0 foot increments for the top and bottom of the seam, and evenly proportioned samples of 5 to 10 feet for the remainder of the seam. Sampling for the Main split of the Wyodak Seam is in 1.5 feet increments for the top

and bottom of the seam and 10 feet increments for the remainder of the seam.  All partings encountered, down to a thickness of 0.4 foot, are sampled separately.

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A hole with significant lost core or crushed core can result in misleading data. Drillholes with core recovery of less than 90 percent are noted and subsequently reviewed and potentially excluded from geological and coal quality modeling.  WEIR did not exclude any holes for poor core recovery, as all of the drillholes within the Black Thunder Mine permit area attained core recovery of at least 90 percent.

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WEIR finds the planning, implementation and supervision of Arch’s drilling programs, with all data derived from the drilling programs, to be consistent with industry standards, and sufficient and relevant for use in the estimation of reserves and resources.

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The mining method at the Black Thunder Mine is surface mining utilizing draglines and truck/shovel mining equipment. The surface mining method has been successfully utilized in the Powder River Basin since the 1970s, and in other coal producing regions of the United States.

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The Black Thunder Mine is mining the Upper and Main splits of the Wyodak Seam, and parting interval within the seam, utilizing draglines, shovels, front-end loaders, trucks, dozers or scrapers in three long pits.

Historical coal production from the Black Thunder Mine is summarized as follows:

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The Black Thunder Mine LOM Plan projects mining through December 2036, at an expected mine life of 16 years. The LOM Plan projects mining from three pits at Black Thunder Mine; the North, West and South pits.

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Black Thunder currently operates a fleet of four draglines and nine shovels for overburden removal and four shovels for coal removal from the three pits. The pits will typically be 200 to 230 feet wide, with pit lengths ranging from 4,310 feet to 16,906 feet in the LOM Plan.  The typical pit configuration is an initial truck/shovel pass(s) for prestrip, since the draglines cannot

effectively handle the total burden. Cast blasting is normally implemented in the next pass prior to the dragline pass, and this pass sequence can require significant dozer material handling, utilizing Black Thunder Mine’s remote control dozer fleet. Subsequently, the dragline handles the quantity of material for which it was designed, in the next pass. The dragline performs multiple passes typically using a modified extended bench, which results in a spoilside pass before the Main Seam coal is mined.  

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Mining progresses in an orderly and sequential fashion to meet the required sales tonnage and coal quality. The current mining sequence south of State Highway 450, progresses in an east to west manner. North of State Highway 450, mining advances from south to north. Recovery of coal beneath the existing rail spurs, mine facilities, and State Highway 450 is deferred to the later years of the LOM Plan in order to utilize the existing surface facilities as long as possible.

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The Black Thunder Mine coal resources, as of December 31, 2021, are reported as in-place resources and are exclusive of reported coal reserve tons.  Resources are reported in categories of Measured, Indicated and Inferred tonnage in accordance with Regulation S-K Item 1302(d), and summarized in Table 1.5-1as follows:

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Table 1.5-1 In-Place Coal Resource Tonnage and Quality Estimate,

as of December 31, 2021

Notes:

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The conversion of resources to reserves at the Black Thunder Mine considers the effects of projected dilution and loss of product coal quality, projected mineral prices and operating costs, regulatory compliance requirements, and mineral control to determine if the saleable coal product will be economically mineable. The design of an executable mine plan that

accommodates the planned mining equipment and provides a safe work environment is also considered.  

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The coal reserve tonnage representing the economically viable tonnage controlled by Arch, and estimated in accordance with Regulation S-K Item 1302(e), is summarized in Table 1.5-2 as follows:

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Table 1.5-2 Recoverable Coal Reserve Tonnage and Quality Estimate,

as of December 31, 2021

Notes:

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WEIR depleted LOM Plan reserve tonnage using actual mine workings through September 30, 2021 and subtracted actual production, reported by Arch, for the remainder of the year to arrive at reserves as of December 31, 2021.

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WEIR prepared a Preliminary Feasibility Study financial model in order to assess the economic viability of the Black Thunder Mine LOM Plan. Specifically, plans were evaluated using discounted cash flow analysis, which consists of annual revenue projections for the Black Thunder Mine LOM Plan. Cash outflows such as capital, including preproduction costs, sustaining capital costs, operating costs, transportation costs, royalties, and taxes are subtracted from the inflows to produce the annual cash flow projections. No adjustments are made for inflation and all cash flows are in 2021 United States dollars. WEIR’s study was conducted on an un-levered basis, excluding costs associated with any debt servicing requirements. In its assessment of Net Present Value (NPV), WEIR utilized a discount rate of 10 percent.

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The Preliminary Feasibility Study financial model developed for use in this TRS was meant to evaluate the prospects of economic extraction of coal within the Black Thunder Mine resource area.  This economic evaluation is not meant to represent a project valuation. Furthermore, optimization of the LOM Plan was outside of the scope of this engagement.

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The results of WEIR’s Preliminary Feasibility Study demonstrated an after-tax NPV of $512.0 million for the Black Thunder Mine LOM Plan. Key operational statistics for the LOM Plan, on an after-tax basis, are summarized in Table 1.6-1 as follows:

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Table 1.6-1 Key Operating Statistics

A sensitivity analysis was undertaken to examine the influence of changes to assumptions for coal sales price, operating cost, capital expenditures, and discount rate on the base case after-tax NPV. The sensitivity analysis range (+/- 25 percent) was designed to capture the bounds of reasonable variability for each element analyzed.  

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The Black Thunder Mine NPV is most sensitive to changes in coal sales price and operating cost. It is least sensitive to changes in discount rate and capital expenditures.  

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As part of the permitting process required by the Wyoming Department of Environmental Quality (DEQ), numerous baseline studies and impact assessments were undertaken by Arch.  

These baseline studies and impact assessments included in the permit are summarized as follows:

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The Black Thunder Mine has been issued mining permits and associated NPDES permits by the DEQ as shown in Table 1.7-1 as follows:

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Table 1.7-1 Black Thunder Mining and NPDES Permits

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The permitted area, bond amounts and reclamation liability for Permit 233 is shown in Table 1.7-2 as follows:

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Table 1.7-2 Black Thunder Mine Permitted Area, Reclamation Liability, and Bonds

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Arch currently employs approximately 1,024 personnel at the Black Thunder Mine and is projected to have a maximum employment of 1,078 personnel in 2022 and decreasing in subsequent years over the Black Thunder Mine LOM Plan.  The mine also creates substantial economic value with its third-party service and supply providers, utilities and through payment of taxes and fees to governmental agencies.

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Permit No. 233 has not been cited for any permit violations since 2014, which is exceptional for a coal mining operation the size of the Black Thunder Mine.

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Based on WEIR’s review of Arch’s plans for environmental compliance, permit compliance and conditions, and dealings with local individuals and groups, Arch’s efforts are adequate and reasonable in order to obtain approvals necessary relative to the execution of the Black Thunder Mine LOM Plan.

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Arch has a long operating history of resource exploration, mine development, and mining operations at the Black Thunder Mine, with extensive exploration data utilizing drillholes, supporting the determination of mineral resource and reserve estimates, and projected economic viability.  The data has been reviewed and analyzed by WEIR and determined to be adequate in quantity and reliability to support the coal resource and coal reserve estimates in this TRS.

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The coal resource and coal reserve estimates and supporting Preliminary Feasibility Study were prepared in accordance with Regulation S-K 1300 requirements. There are 205.0 million in-place tons of measured and indicated coal resources, exclusive of reserves, and 545.0 million clean recoverable tons of underground mineable reserves within the Black Thunder Mine as of December 31, 2021. Reasonable prospects for economic extraction were established through the development of a Preliminary Feasibility Study relative to the Black Thunder Mine LOM Plan, considering historical mining performance, historical and projected thermal coal sales prices, historical and projected mine operating costs, and recognizing reasonable and sufficient capital expenditures.

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The ability of Arch, or any coal company, to achieve production and financial projections is dependent on numerous factors. These factors primarily include site-specific geological conditions, the capabilities of management and mine personnel, level of success in acquiring reserves and surface properties, coal sales prices and market conditions, environmental issues, securing permits and bonds, and developing and operating mines in a safe and efficient manner.  Unforeseen changes in legislation and new industry developments could substantially alter the performance of any mining company.

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Coal mining is carried out in an environment where not all events are predictable.  While an effective management team can identify known risks and take measures to manage and/or mitigate these risks, there is still the possibility of unexpected and unpredictable events occurring.  It is not possible therefore to totally remove all risks or state with certainty that an event that may have a material impact on the operation of a coal mine will not occur.

WEIR assessed risks associated with the economic mineability of the Black Thunder Mine to be low to moderate and adds that the majority of the risks can be kept low and/or mitigated with proper planning and monitoring of the mining operations.

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WEIR recommends that any future exploration work and mineral property acquisition should include what has been historically implemented related to the following:

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Geology

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Mine Plan

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WEIR was retained by Arch (NYSE: ARCH) to prepare a TRS related to Arch’s currently operating Black Thunder Mine. The Black Thunder Mine is located approximately 50 miles south of Gillette, Wyoming, in Campbell County within the PRB coal producing region of the United States (see Figure 1.1-1).

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This TRS was prepared specifically for Arch’s Black Thunder Mine. The Upper and Main splits of the Wyodak Seam resources at the Black Thunder Mine have been classified in accordance with SEC mining property disclosure rules under Subpart 1300 and Item 601 (96)(B)(iii) of Regulation S-K.  Unless otherwise stated, all volumes, grades, distances, and currencies are expressed in United States customary units.

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The accuracy of reserve and resource estimates are, in part, a function of the quality and quantity of available data at the time this report was prepared.  Estimates presented herein are considered reasonable. However, they should be accepted with the understanding that with additional data and analysis available subsequent to the date of this report, the estimates may necessitate revision which may be material.  Certain information set forth in this report contains “forward-looking information”, including production, productivity, operating costs, capital costs, sales prices, and other assumptions. These statements are not guarantees of future performance and undue reliance should not be placed on them. The assumptions used to develop the forward-looking information and the risks that could cause the actual results to differ materially are detailed in the body of this report.  

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The Black Thunder Mine is a permitted surface mine that commenced production of thermal coal in the fourth quarter of 1977.

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For the Black Thunder Mine, as an established producing mine, this TRS reports both mineral reserves and resources (exclusive of reserves). Supporting the assessment of the economic mineability of reported reserves and prospects of economically feasible extraction of reported resources, this report includes summary detail of a Preliminary Feasibility Study conducted relative to the Black Thunder Mine.  

WEIR’s evaluation of Arch’s coal reserves and resources was conducted in accordance with SEC S-K 1300 definitions for Mineral Resource, Mineral Reserve and Preliminary Feasibility Study as follows:

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The primary information evaluated for this study, including but not limited to maps, plans, schematics, drawings, and discussions, was as follows:

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A detailed list of all data received and reviewed for this study is provided in Sections 24.0 and 25.0 of this TRS.

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WEIR personnel previously visited the Black Thunder Mine on July 24, 2019.  WEIR has also performed numerous annual audits of the Black Thunder Mine reserves for Arch’s annual SEC 10-K filings.  

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WEIR initially held discussions with mine management on July 21, 2021, to review questions relative to the Black Thunder Mine’s geology, mine plans and operations.  The management discussions included key topics as follows:

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Subsequently. WEIR personnel visited the Black Thunder Mine on August 19, 2021. Areas of the mine visited included the following:

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In addition to observance of mine infrastructure, surface facilities and mining conditions, WEIR discussed the Black Thunder Mine LOM Plan with mine management personnel.

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This TRS is the initial TRS to be filed related to the Black Thunder Mine.

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The Black Thunder Mine is located approximately 50 miles south of Gillette, Wyoming in Campbell County, within the PRB coal producing region of the United States (see Figure 1.1-1).  The United States Geological Survey (USGS) 7.5-minute quadrangle map sheets, upon which the Black Thunder Mine can be found, are Hilight, Open A Ranch, Reno Reservoir, Piney Canyon NW, Teckla and Piney Canyon SW.

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The Black Thunder Mine permit area includes approximately 62,066 acres of controlled mineral property.  

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The Black Thunder Mine surface facilities are located within the Black Thunder permit area, near the central area of the mid-north boundary of the permit.  The surface facilities include mine administration, engineering, and operations offices, mine roads, laydown areas, ponds, crushers, rail loadouts, mine maintenance facilities, warehouse facilities, parking lots.  The total disturbed area for the Black Thunder Mine surface facilities is approximately 3,230 acres. The coal, backfill, and topsoil stockpiles represent approximately 5,300 additional acres of disturbed area.

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The Black Thunder Mine reserve boundary comprises approximately 62,066 acres. Within that boundary, Arch controls the Upper and Main splits of the Wyodak Seam through 18 coal leases covering approximately 62,066 acres.  Table 3.3-1 describes the various property control contracts.  

Table 3.3-1 Property Control

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Each individual contract shown above may include more than one type of property control.  

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Coal seam mineral rights are controlled by nine coal leases, six federal leases and seven state leases. All but two leases have minimum annual rental payments ranging from $480 to $18,225. All of the leases have a production royalty rate of 12.5 percent of  the Gross Sales Price (GSP). The leases have a minimum royalty that must be paid annually in order to maintain the lease, with the exception of one lease, which has a one-time minimum royalty payment. Three leases have additional annual rental agreements. The details of the mineral control contracts are listed in Table 3.4-1.

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Table 3.4-1 Mineral Control

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The Black Thunder Mine LOM Plan area is permitted with the Wyoming DEQ, Land Quality Division (LQD).

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A list of Arch’s permits is shown in Table 3.5-1, with a more detailed description of the permits discussed in Section 17.3.

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Table 3.5-1 Permit List

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Since 2014, the Black Thunder Mine has not had a regulatory fine or violation from the Wyoming LQD.

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Given Arch’s controlled interests at the Black Thunder Mine, which relate to property that is held, by and large, by Arch and the BLM, WEIR finds there are no significant issues affecting access to the coal interests, or the ability of Arch to execute the Black Thunder LOM Plan.

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Arch, at the Black Thunder Mine, holds no royalty or similar interest in property that is owned or operated by another party.  

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The Black Thunder property is located within the eastern flank of the Powder River Basin on the Missouri Plateau of the Northern Great Plains Province. The Powder River Basin is a topographic depression between the Big Horn Mountains on the west and the Black Hills on the east.  The topography of the property is comprised of rolling and rugged hills, with 500 to 1,000 feet of vertical relief, in the northern part of the basin, and gentle plains, with up to 500 feet of relief, in the southern part.  Surface elevations range from 4,000 feet above sea level in the north to 5,000 feet above sea level in the south.

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Within the mine permit area, the terrain is gently rolling, except along the eastern edge of the property and to the south of Little Thunder Creek.  In these two areas, the property is transected by steep-sided, irregular gullies and washes, which drain into Little Thunder Creek, forming breaks in the plateau. Elevations within the mine permit area range from approximately 4,570 feet to 5,030 feet. The surface of the mine permit area is made up of eroded shale slopes, alluvial terraces and small playas, minor sheet wash, and floodplains. Scoria (clinker) ledges occur near “burn” lines.

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The Black Thunder property consists mostly of two major vegetation types. These major vegetation types are Mixed Grass Prairie (Upland Grassland) and the Big Sagebrush Shrubland.

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The Mixed Grass Prairie vegetation is generally found on moderately deep to deep soils on gently rolling to flat topography.  Perennial grasses are the dominant vegetative type here.  Western wheatgrass (Agropyron smithii), needleandthread (Stipa comata), blue grama (Bouteloua gracilis), prairie Junegrass (Koeleria macrantha), Sandberg bluegrass (Poa secunda) and threadleaf sedge (Carex filifolia) are generally the most common species encountered.  Cheatgrass (Bromus tectorum) is a common annual invasive species of grass that may be found during years favoring growth of this species.

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The Big Sagebrush Shrubland plant community is found on a variety of soils on the area ranging from very poor and shallow to loamy and deep.  This vegetation type is also found on a wide range of topographies from very steep and rolling to relatively flat.  Perennial grasses

also dominate the Big Sagebrush Shrubland vegetation, but big sagebrush (Artemisia tridentata) is the single most common individual species.  Other common perennial species on the Big Sagebrush Shrubland include needle and thread, western wheatgrass, prairie junegrass and blue grama.  Annual species such as cheatgrass may also be common on the Big Sagebrush Shrubland in certain years.

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The Black Thunder Mine is accessed from Interstate 90 from Gillette, Wyoming by traveling south on Wyoming State Highway 59 for 41.0 miles. Then turning east and traveling on Wyoming State Highway 450 for 9.7 miles. The mine entrance is located on the south side of the highway. The nearest town is Wright, Wyoming, which is located 1.9 miles north of Wyoming State Highway 450.

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Rail transportation is provided by both the Union Pacific (UP) and Burlington Northern Santa Fe (BNSF) railroads with the main line running directly along the west side of the mine and spurs connecting to all three load outs. There is no river transportation available near the Black Thunder Mine.

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The nearest airport is the Northeast Regional Airport located on Wyoming State Highway 59/ US Highway 14 on the north side of Gillette, Wyoming. Connecting flights are available from Rapid City Regional Airport and Denver International Airport.

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The Black Thunder Mine property lies on the rolling high plains of northeastern Wyoming.  The property is approximately 200 miles east-northeast of the low-level Continental Divide of southeastern Wyoming, approximately 60 miles west of the Black Hills, and approximately 70 miles east of the Big Horn Mountains.  Climate in the high plains of northeastern Wyoming is influenced primarily by cold, dense air masses that flow across the Continental Divide from the west and northwest.  Since there are no mountains north of the high plains region, the plains are subjected to periodic outbreaks of Arctic air masses during the autumn, winter, and spring.  Each outbreak causes abrupt changes in weather such as northerly winds, dropping temperatures, and snow.  During the winter, cold, dense air masses originating from the Great Basin (a large basin that lies between the Sierra Nevada and Rocky Mountain ranges) frequently drain across the low-level Continental Divide through southern Wyoming and down into the North Platte Valley. The

air accelerates to higher velocities and spreads over eastern Wyoming.  Some of this air moves northeastward toward the Black Thunder Mine, however, the prevailing northwesterly, westerly, and southeasterly surface wind flows observed at the mine property are due to the channeling of drainage winds between the Big Horn Mountains and the Black Hills.

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The summer climate is typical for the high plains with light to moderate surface winds and occasional violent thunderstorms.  The thunderstorms generate most of the annual precipitation.  Wind gusts, from the occasionally severe thunderstorms, sometimes reach 60 to 80 miles per hour (mph) and may be followed by hail.  The climate of northeastern Wyoming can be classified as semi-arid since mean annual precipitation is approximately 13 inches and relative humidity is rather low, being less than 50 percent on an annual average.

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The mean monthly temperature recorded at Gillette 2E, a meteorological station in the vicinity of the Black Thunder Mine, ranges from 72 degrees Fahrenheit (°F) in July to 22°F in January.  The average frost-free growing season is 127 days.  The average last spring freeze date, recorded at Gillette 2E is May 21, and the average first freeze date is September 25.

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Although extreme weather is experienced at the Black Thunder Mine during all seasons, there is no seasonal limitation to operations at the Black Thunder Mine.

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Power

Electrical power for the Black Thunder Mine is provided by Powder River Energy Corporation (PREC), through a 69 kV transmission line.  PREC’s average industrial price is 6.77 cents per KWH.

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Water

The water used for dust suppression is obtained from the mine’s own highwall dewatering program.  This dewatering program is able to produce 500,000 to 800,000 gallons of water per year. Potable water for the facilities is obtained from two onsite deep-water wells. This water is treated at a flat rate of $2,485 per month. In 2021, the Black Thunder Mine used on average approximately 197,000 gallons of water per month.

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Personnel

The northern Wyoming area surrounding the Black Thunder Mine has a long history of surface coal mining and attracting and hiring mining personnel with qualified skills has not been an issue.  The Black Thunder Mine employed approximately 1,024 personnel, as of August 1, 2021.  The hourly labor force at the Black Thunder Mine remains non-union and no change in this labor arrangement is anticipated.

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Supplies

Supplies for the Black Thunder Mine are available from multiple vendors that service the coal mining industry in the PRB Region. The nearest Caterpillar mining equipment dealerships are located in Gillette and Casper, Wyoming, and there is a Komatsu mining equipment dealership, located in Gillette.

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Prior to the development of the Black Thunder Mine, there was no mining that occurred on the property. The Black Thunder Mine is a surface coal mine utilizing draglines and truck/shovel mining equipment for overburden removal. The mine was opened by Atlantic Richfield Company (ARCO) in 1977 and has been operated under Thunder Basin Coal Company, LLC since that time. In 1998, Arch Coal, Inc. purchased all of ARCO’s domestic coal operations, which included the Thunder Basin Coal Company, Black Thunder Mine.

In 2004, Arch purchased the adjacent North Rochelle Mine from Triton Coal Company and merged it into Black Thunder Mine. The former North Rochelle Mine facilities and reserves were subsequently sold to Peabody Coal Company in 2006.

In 2009, Arch purchased the adjacent Jacobs Ranch Mine from Rio Tinto Coal and merged it into the Black Thunder Mine, which created a mining complex that produced 116.2 million tons of coal in 2010.

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Exploration work conducted by ARCO included both pre-lease, Federal Exploration License drilling and post-lease development drilling.  Pre-lease drilling was generally done on a one-half mile spacing, or one hole per quarter section, which corresponded with the requirements of the BLM for leasing Federal coal. Most development drilling was done two to three years ahead of mining on a nominal 600 feet spacing with alternating rows offset one-half the spacing resulting in a 45-degree rotated grid interval of 424 feet and included over 1,500 drillholes.

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Exploration work conducted by the Jacobs Ranch Mine also included both pre-lease, Federal Exploration License drilling and post-lease development drilling. Pre-lease drilling was also generally done on a one-half mile spacing, or one hole per quarter section, which corresponded with the requirements of the BLM for leasing Federal coal. Most development drilling was done three to five years in advance of mining and was mostly done on a nominal 800 feet spacing, with alternating rows offset one-half of the spacing resulting in a 45-degree rotated grid interval of 565 feet and included over 2,600 drillholes.

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Other exploration work conducted prior to Arch’s acquisition included regional USGS reconnaissance drilling of unleased Federal coal, prior to ARCO’s, at a density of one to two

holes per section and coalbed methane production drilling that was done on a density of anywhere between one and sixteen holes per section.  The coalbed methane drillholes and most of the USGS drillholes were rotary drilled.

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Mine facilities built by ARCO included a rail spur and loadout loop, a loadout with two 12,500-ton silos, a 100,000-ton slot storage barn, two crusher locations, a coal analysis lab, maintenance shop, warehouse, bathhouse, reclamation shop, and an administrative building.

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Initial pit development was conducted with truck/shovel mining equipment, but ARCO subsequently added three draglines by the time the mine was acquired by Arch, including a Bucyrus-Erie 1300W with a 45 cubic yard bucket, a Bucyrus-Erie 1570W with a 90 cubic yard bucket, and a Bucyrus-Erie 2570WS with a 160 cubic yard bucket.

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The Jacobs Ranch Mine also constructed mine facilities similar to those constructed by ARCO, however, as time progressed and mining moved farther west, these facilities, including the loadout, have been idled.  

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The Jacobs Ranch Mine was historically one of the larger truck/shovel mines until a Bucyrus-Erie 2570W dragline with a 121 cubic yard bucket was brought on-line in 2006.

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6.1.1Regional Geology

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The Powder River Coal Basin, of northeastern Wyoming, lies entirely within the boundaries of the Powder River structural and topographic basin.  Coal-bearing strata range in age from Upper Cretaceous in the Mesa Verde Formation to Eocene in the Wasatch Formation.  The Powder River Coal Basin covers parts of Campbell, Converse, Crook, Natrona, Niobrara, Johnson, and Weston Counties and is the largest coal basin in Wyoming.

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The basin is a broad asymmetric syncline bounded on the west by the Big Horn Mountains, on the east by the Black Hills, and to the south by the Casper Arch, Laramie Mountains, and the Hartville Uplift.  The basin continues north into Montana where the Miles City Arch separates it from the Williston Basin.  

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The axis of the syncline is slightly west of the center of the basin.  Flanking dips are gentle on the eastern limb (two to three degrees) but dip more steeply on the western limb.  Faulting occurs in many localities, especially around the basin edge and is in association with folding.  Vertical displacements can be several hundred feet.  Faulting is more common on the western limb of the syncline than on the eastern limb.

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Stratigraphic units of interest in the permit area, from youngest to oldest, include recent alluvial deposits, the Eocene Wasatch Formation, and the Paleocene Fort Union Formation.  Locally, the strata dip two degrees to the west-southwest.  There is no evidence of major faulting, or folding, within the permit area; although, localized warps and minor faults, probably compactional in nature, in the main coal seams have been indicated by exploration work and during the mining process.

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An alluvial covering is present in the drainage patterns and in the slope wash areas adjacent to the drainages.  The alluvial deposits are of recent age and consist of primarily unconsolidated, discontinuous lenses of clays, silts, and sands.  Locally, recent stream channeling has removed portions of the coal seam with subsequent channel infilling of sediment. Varying amounts of oxidized coal are present when alluvium is in contact with the seam.

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The lower Eocene Wasatch Formation consists of interbedded, lenticular clays, silty clays, sandy clays, thin, discontinuous coals, mudstones, and dirty sandstones.  Correlation of individual strata is difficult due to the discontinuous and lens-like nature of the units which is inherent in fluvial deposition, e.g., channel sand deposits.  

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The Upper Paleocene Fort Union Formation underlies the Wasatch Formation.  The Fort Union Formation consists of non-carbonaceous to highly-carbonaceous clays, mudstones, sandstones, and coal.  The top of the Fort Union Formation is designated as the top of the Wyodak coal seam. The Wyodak Seam is the main coal seam, and it lies atop lensoidal clay, silt, and sand beds.  The seam base is variable due to changes in the environment of deposition, from the non-coal forming environment of the sands, clays, and silts, to the fringes of the coal forming, swampy conditions in which the Wyodak coal seam was deposited.

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Clinker (locally known as scoria), a baked or fused rock, is present along the coal outcrop on the eastern edge of the permit area.  This fused material was formed by prehistoric burning of the Main Wyodak coal seam.  Both the Wasatch and the Fort Union formations have been affected by this prehistoric burning and have contributed to the volume of baked material present. Mining conditions often deteriorate in proximity of these clinker deposits.

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The mudstone is a uniformly textured material composed of 40 to 80 percent clay, and generally 5 to 40 percent silt; the remainder being sand.  It is generally medium to dark gray with occasional brown and tan oxidized zones.  The mudstone is basically soft to medium stiff with some extremely stiff waxy mudstone throughout much of the area. The mudstone contains some carbonaceous material and thin coal partings.  

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Sandstone is a major lithologic component of the overburden in the mine Area.  It is generally weakly cemented with clay, but occasionally well-cemented resistant beds are encountered.  Sandstone occurs in discontinuous zones interbedded with similarly discontinuous mudstone and siltstone.  It is very fine to medium grained, gray to dark gray in color, with brown and tan oxidized zones. The sandstone ranges from well-graded, poorly-sorted silty sand to clean, uniform, poorly-graded material, consisting of over 80 percent sand in some instances. Sandstone overlies the coal in some areas.

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Siltstone constitutes approximately 15 percent of the overburden by volume.  Like the mudstone, it is uniformly textured with 20 to 55 percent silt, and generally 20 to 60 percent clay, the remainder being sand. It is light to medium gray in color and slightly more consolidated than mudstone. Like other overburden lithologic units, the siltstone is

discontinuous and occurs interbedded with mudstone and sandstone.  In some areas, it overlies coal.

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6.1.2Local Geology

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Regionally, the most economical coal seams are contained in the Paleocene Fort Union Formation and the Eocene Wasatch Formation.  Individual seams range to greater than 100 feet in thickness. Large quantities of potentially surface-mineable coal are contained in these formations.

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6.1.3Property Geology

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The economically mineable coal in Campbell County occurs within the Tongue River Member of the Fort Union Formation. The Wyodak coal seam occurs at the top of the Fort Union Formation and is overlain by the Wasatch Formation. The coal is low sulfur, low ash, and is subbituminous C in rank.  Surface mineable coal deposits occur along the north-north­westerly striking subcrop of the Wyodak coal seam. The coal seam subcrops on the eastern edge of the lease and dips about two to three degrees to the west, with some slight rolling. This seam contains multiple benches or plys of coal of variable thicknesses, although in some local areas, it becomes one seam that reaches a thickness in excess of 100 feet.  Across the mine permit area, the Wyodak Seam ranges in thickness from 10 feet to 100 feet, averaging approximately 70 feet.

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The Black Thunder Mine reserve area is a relatively flat lying sedimentary deposit of Paleocene Age.   The Black Thunder Mine is actively mining a single coal seam, the Wyodak, that can be divided into multiple splits, the Upper and Main splits of the Wyodak Seam.  Exploration consists of core drilling for the Upper and Main splits carried out each year in advance of mining, to refine the reserve boundary and to define limits of the mine plan. For internal purposes, Arch models the reserve using the Geovia Minex^® mine planning software package, completing model updates subsequent to each phase of exploration drilling. WEIR modeled the reserves and resources using Datamine MineScape^® Stratmodel geological modeling software.  The WEIR model is discussed in more detail in Section 9.1.  

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Figure 6.3-1 and Figure 6.3-2 show the stratigraphic column and the Upper and Main Wyodak splits cross section related to the Black Thunder Mine.

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Figure 6.3-1 Stratigraphic Column

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Source: U.S. Geological Survey Open-File Report 98-0789B-B (1998)

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Figure 6.3-2 Wyodak Seam Cross Section Northwest to Southeast

Source: Arch Land Company

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Drilling has served as the sole form of exploration carried out on the Black Thunder Mine property.  

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Arch’s exploration activities exclusively involve drilling performed by competent contract drilling companies. Exploration drilling at Black Thunder Mine has been a two-stage approach.  Initial spot core drilling is conducted on a widely spaced, one-half mile, pattern in order to delineate potential lease areas. Once the area has been leased, development drilling is conducted three to five years in advance of pit development. This drilling is generally on a 500 feet north/south grid with alternating rotary and spot core holes and is done in conjunction with dewatering endeavors. This arrangement results in a seam geometry data spacing of 500 feet and a coal quality data spacing of 1,000 feet. Drilling is conducted with rotary drilling rigs capable of 1,000 feet depths.

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All holes are geophysically logged with a standard coal suite tool consisting of gamma, density, caliper, and resistivity. Geophyical logging contractors provide paper copies, .TIF files, and .LAS files.

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Spot core holes are rotary drilled to a core point which is projected from the geologic computer model and may be adjusted in the field as drilling progresses. A 3-inch diameter core is then extracted in roughly 20 feet core runs by tripping pipe out of the hole for each core run.  

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Upon reaching the surface, the split-tube core barrel is opened, core is washed down if necessary, and the driller’s reported length of core that was actually cut is compared to the measured length of core actually recovered. Total core loss for the entire seam is generally less than 2 feet.  If a section of core greater than 10 feet, or less if in a critical zone, is lost, then the hole is re-drilled to recover the lost interval.  

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Coal sampling for the Upper split of the Wyodak Seam is in 1.0 foot increments for the top and bottom of the seam and evenly proportioned samples of 5 to 10 feet for the remainder of the seam.  Sampling for the Main split of the Wyodak Seam is in 1.5 feet increments for the

top and bottom of the seam, and 10 feet increments for the remainder of the seam.  All partings encountered, down to a thickness of 0.4 foot, are sampled separately.

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A hole with significant lost core or crushed core can result in misleading data. Drillholes with core recovery of less than 90 percent are noted and subsequently reviewed and potentially excluded from geological and coal quality modeling. WEIR did not exclude any drillholes for poor core recovery, as all of the holes within the Black Thunder Mine area attained core recovery of at least 90 percent. During core drilling, all core samples are boxed, photographed, and stored.  Roof and floor strata core samples are sent to laboratories for geotechnical strength tests. Coal seam core samples are sent to laboratories for quality analyses. Caliper, density, gamma, resistivity, and sonic downhole geophysical logs are completed as drill site and hole conditions allow. Each drillhole collar location is surveyed for accurate map coordinate and elevation data.

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All original drillhole, survey, geological, geophysical, and quality data is scanned and stored on an Arch server, which is backed up nightly, and can be accessed by select Arch personnel and quickly checked against the database, the geological model, or mine mappings. The original copies are stored in an offsite warehouse.

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Table 7.2-1 summarizes the database of Arch’s core drilling data that is within the Black Thunder Mine LOM Plan.

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Table 7.2-1 Drilling Programs

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WEIR did not have direct involvement with the planning, implementation or supervision of Arch’s drilling programs. However, having reviewed the details of each drilling program, WEIR finds the planning, implementation and supervision of Arch’s drilling programs, with all data derived from the drilling programs, to be consistent with industry standards, and sufficient and relevant for use in the estimation of reserves and resources.

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The Black Thunder Mine is situated in the southern portion of the Powder River Basin, within the Cheyenne River watershed and Upper Powder and Antelope sub-basins. The Black Thunder permit area is located on the east limb of the Powder River Structural Basin in northeastern Wyoming.  The east limb of the basin dips two to three degrees to the west.  The primary drainage in the Black Thunder Mine permit area is Little Thunder Creek, fed by several tributaries within the permit boundary.  

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Principal aquifers within the Black Thunder Mine permit area include the Fort Union and overlying Wasatch Formations.  These Tertiary Age sand and mudstones occur in the upper portion of the Wasatch-Fox Hills hydrographic sequence (see Figure 6.3-2).  The Wasatch-Fox Hills sequence is 1,350 feet thick in the northern part of the Powder River Basin and thickens to almost 7,000 feet in Converse County, Wyoming.  On a regional basis, flow moves from peripheral recharge areas (along scoria outcrops) toward the center of the basin, primarily controlled by stratigraphy and surface water streamflow.  Within the Black Thunder Mine permit area, the gradient dips gently to the west, with head elevations ranging from 4,590 to 4,680 feet.

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Arch has engaged in extensive surveying to characterize site hydrogeology and to determine groundwater inventories, water quality, and potential impacts to local usage as part of its National Pollutant Discharge Elimination System (NPDES) permitting process with the Wyoming DEQ. Baseline flow and quality parameters for surface and groundwater inventory have been established and monitored as required by the Wyoming DEQ.

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Water sampling methods for the Black Thunder Mine are outlined and maintained by Arch in a site-specific work practice document. Reviewed annually, this operating procedure document details sampling locations, frequency, and collection protocols, including storage, transport, delivery and required chain of custody documentation. Approved methods for field data

collection and instrument calibration are described, along with methods for creating sample splits, duplicates, and blind standards.  

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Samples are analyzed by independent laboratories that follow the most recent approved Environmental Protection Agency (EPA) sampling methodology and procedures. The laboratories employ internal quality control and quality assurance protocols before reporting results to Arch.  Arch personnel then review the results again, as a second check for quality control and assurance, before the results are published.  

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Groundwater inventories, water quality data, water balance, recharge and seepage rates have been reviewed in the approved permit and current permit revisions, including hydrologic impact assessments outlining risks, monitoring program detail, and mitigation obligations.  Arch’s approach to obtaining and managing its surface and groundwater data for the Black Thunder Mine has been demonstrated to be adequate and aligned with regulatory requirements and standard industry practices.  WEIR finds no material barriers to the continued success of the Black Thunder Mine regarding hydrologic impact or compliance.

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A geotechnical study of highwall stability for the North, West, and South Pits at Black Thunder Mine was completed by Barr Engineering (Minneapolis, Minnesota) in 2021 as part of a review of the mine’s ground control plan (GCP). Previous geotechnical study and GCP review at Black Thunder Mine were completed in 1973, 2002, 2004, and 2009.  Coring, logging, and geophysical logging of 10 boreholes were performed to characterize the lithology of the site and obtain drill core samples of the overlying sand and mudstones. Resulting samples were transported to Soil Engineering Testing (SET), located in Richfield, Minnesota, for geotechnical analysis. Analysis performed include index and soil properties, permeability, and shear strength under the appropriate American Society for Testing and Materials (ASTM) specifications.  

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Each cored drillhole included a companion, offset hole that was logged with downhole geophysics (e-logged) by Goodwell, Inc. located in Gillette, Wyoming, and reviewed by Pronghorn Geologic Services located in Gillette, Wyoming. Lithology for each hole was determined using gamma and density downhole data.  Slope stability and seepage modeling for both drained and undrained mining conditions were completed to assess the stability of the highwall cuts in each pit.  

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A map showing the location of all drillholes used to estimate tonnage on the Black Thunder Mine Property is shown on Figure 7.5-1.

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Figure 7.5-1 Drillhole Collar Locations

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