Estimation of Coal Resources in Texas Gulf Coast, Ohio Northern Appalachian, and Wyoming Powder River Basins

RI0136. Estimation of Coal Resources in Texas Gulf Coast, Ohio Northern Appalachian, and Wyoming Powder River Basins: A Comparison of Statistical Approaches, by S. J. Tewalt, M. A. Bauer, D. Mathew, M. P. Roberts, W. B. Ayers, Jr., J. W. Barnes, and W. R. Kaiser.  137 p., 124 figs., 29 tables, 1983. ISSN: 0082335X: Print.

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ABSTRACT
Official estimates of United States coal resources published during the past 15 years vary from less than 1.5 to 3.5 trillion metric tons (1.7 to 3.9 trillion short tons). These differences imply that a high degree of uncertainty exists in resource assessment. This report identifies sources of uncertainty in coal resource estimation.

Our report focuses on the comparison of variability in coal resource estimates in areas of different ancient depositional environments. The Texas Gulf Coast Basin was chosen for this study because it exhibits a full range of ancient depositional environments: (1) upper alluvial plain, (2) lower alluvial/upper delta plain, (3) delta plain, and (4) strandplain/lagoonal. Four lignite deposits, each representing one of these depositional environments, were evaluated. Important sources of uncertainty in resource estimation include variability of seam thickness, areal distribution, and the number of seams. To test the degree of uncertainty caused by variations in seam thickness, the numbers of boreholes considered in each lignite deposit are reduced and resources calculated for each reduction in data. Various techniques of resource calculation (manual, computer, and geostatistical) are used to investigate the uncertainties associated with each method. Classical statistics is the method used to determine the number of boreholes required to obtain resource estimates of individual seams within a given confidence interval under specified conditions; geostatistical methods (variograms and kriging) are used to measure variability in resource estimates.

Classical statistical methods show that the minimum number of evenly distributed boreholes required to characterize resources of a lignite seam to within a precision of 20 percent is substantially less than might be expected intuitively, and depends on the coefficient of variation of seam thickness. Geostatistical methods indicate that a substantial further reduction in the minimum number of boreholes is possible when a spatial dependency structure can be established by means of a variogram. Resource figures for seams calculated by manual, computer, and geostatistical methods at various levels of data density are well within those predicted by classical statistical theory. These studies demonstrate that the maximum seam thickness variation occurs at the margins of lignite seams and that variations in thickness of individual lignite seams are not a major source of uncertainty in resource estimation, given the level of data usually available. However, determination of the areal extent and seam boundaries of coal beds is a major source of uncertainty.

Data availability for regional-scale resource analysis nullifies seam-by-seam (deposit) methodologies. Our regional test area was the Wilcox Group outcrop in east-central Texas. Geostatistics did not yield a dependency structure for the entire area, therefore alternate methods were used: (1) equal weighting over the entire area: (2) equal weighting within grid cells; and (3) equal weighting within internally homogeneous blocks chosen using statistical or geologic parameters. Our methodology was successfully transferred to the Tongue River Member, Wyoming, and the Allegheny Formation, Ohio. Tonnages calculated for Wyoming and Ohio exceeded official estimates because we included deep-basin, thick continuous coals.

Depositional models were used to calculate resources for the entire Gulf Coast. Calculated resources indicate the magnitude of total resources but do not quantitatively measure the associated uncertainty.


Keywords: Allegheny Formation, Appalachian Basin, coal resources, geostatistics, lignite, resource estimation, Gulf Coast, Ohio, Powder River Basin, Texas, Tongue River Member, Wyoming

CONTENTS

ABSTRACT

INTRODUCTION

DATA USED IN RESOURCE ESTIMATIONS AND THEIR UNCERTAINTIES

GEOLOGIC INVESTIGATIONS

Geologic Setting

   Wilcox Group

   Jackson Group

Depositional Models

   Wilcox Group

   Jackson Group

TECHNICAL APPROACH TO EVALUATION OF LIGNITE DEPOSITS

Introduction

Depositional Setting-Alluvial Plain

   Introduction

   Geologic Evaluation

   Lignite Qualify Evaluation

   Summary of Geologic Features

Depositional Setting-Alluvial/ Delta Plain Transition

   Introduction

   Geologic Evaluation

   Summary of Geologic Features

Depositional Setting- Lower Delta Plain

   lntroduction

   Geologic Evaluation

   Lignite Quality Evaluation

   Summary of Geologic Features

Depositional Setting--Strandplain/Lagoonal

lntroduction

   Geologic Evaluation

   Summary of Geologic Features

GEOLOGICAL UNCERTAINTIES

QUANTITATIVE INVESTIGATION

Hand- and Computer-Calculated Resources

   Introduction

   Effect of Data Reduction on Hand-Calculated Resources

   Effect of Data Reduction on Computer-Calculated Resources

Geostatistically Calculated Resources

   Introduction

   Determination of Class Size and Subzones

   Investigation of Geometric Anisotropy

   Verification of Spherical Variogram Parameters

   Kriging

   Estimation of Variance

Geostatistical Evaluation of Additional Seams

   Alluvial Plain Deposit

   Lower Delta Plain Deposit

   Strandplain/Lagoonal Deposit

   Lower Alluvial Plain/Delta Plain Transitional Deposit

Summary

Number of Holes Required to Characterize Resources of a Seam

REGIONAL INVESTIGATION IN TEXAS

Introduction

Geologic Setting

Geostatistical Methods

Alternate Statistical Methods

   Analysis of Borehole Data

Resource Evaluation Methodology

   Equal Weighting

   Grid Method

   Homogeneous Block Method

   Computer Mapping Method

   Depositional-Model Method

   U. S. Geological Survey Method

Summary of the Methodology Developed in Texas

POWDER RIVER BASIN

Geologic Evaluation

   Introduction

   Previous Work

   Methods of Study

   Sand Maps

   Coal Maps

   Proposed Depositional Model

Quantitative Investigation

   Introduction

   Geostatistical Methods

   Alternate Statistical Methods

   Summary of Classical Statistical Methods

THE ALLEGHENY OF EASTERN OHIO

Geologic Evaluation

   Introduction

   Types of Data and Previous Work

Quantitative Investigation

   Introduction

   Technical Approach

   Geostatistical Methods

   Alternate Statistical Methods

U.S. COAL RESOURCES

Introduction

USGS Resource Estimates

Basin Resource Estimation

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

124 Figures, 29 tables


Citation
Tewalt, S. J., Bauer, M. A., Mathew, D., Roberts, M. P., Ayers, W. B., Jr., Barnes, J. W., and Kaiser, W. R., 1983, Estimation of Coal Resources in Texas Gulf Coast, Ohio Northern Appalachian, and Wyoming Powder River Basins: A Comparison of Statistical Approaches: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 136, 137 p.