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Chapter AOverview of Studies Related to Energy Resources, Northern Front Range of ColoradoBy Neil S. Fishman
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The infrastructure of a populated area, including roads, airports, water and energy transmission and distribution facilities, and sewage treatment plants, is critical to the vitality and sustainability of the area. Construction of new and ongoing maintenance of existing infrastructure both require large volumes of natural resources such as energy (oil, natural gas, and coal), construction aggregate (stone, sand, and gravel), and water. However, sufficient natural resources may not always be available for ready use due to (1) scarcity of local sources, (2) inaccessibility (for example, gravel cannot be mined from under a housing subdivision), (3) unsuitability of the resource (for example, polluted ground water may be unfit for domestic use), or (4) land use or legal restrictions limiting access to local sources. Should local sources of these natural resources either be unavailable or not used, then costs incurred to construct and maintain an area’s infrastructure through use of more distant supplies will be greater than they would be if local sources were used. Thus, the ability to explore for and develop local accumulations of natural resources for use in infrastructure construction and maintenance, in large part a function of accessibility to the resources, is of particular interest and benefit to areas of significant population growth or where growth is expected. The challenge for communities is to adequately factor maintenance and growth of the area’s infrastructure into comprehensive land-use planning efforts and to consider how changes in land-use designation can influence the availability of these vital natural resources.
The U.S. Geological Survey’s Front Range Infrastructure Resources Project (FRIRP) was designed, with direct input from stakeholders (see following section), to advance our understanding of the location and characteristics of accumulations of energy, construction aggregate, and water—herein termed infrastructure resources—in the plains immediately east of the northern part of the Front Range in Colorado (fig. 1). The project study area, approximately 2,200 square miles (mi2), extends from the metropolitan Denver area north to Fort Collins, and from the mountain front eastward for about 40 miles (fig. 1). The study area is in the western part of the Denver Basin (defined by Matuszczak, 1973), a large (about 70,000 mi2) sedimentary basin in northeastern Colorado, southeastern Wyoming, and western Nebraska (fig. 2). The basin is a structurally asymmetric foreland basin with a steeply dipping western flank and gently dipping eastern flank (fig. 3), and the basin axis lies beneath part of Denver. The basin formed adjacent to the Front Range during the Laramide orogeny, approximately 71 to 50 Ma, although most downwarping probably occurred between 64 and 50 Ma (Weimer, 1996).
The FRIRP study area was selected for two primary reasons. First, this area has undergone substantial population growth over the last 30 years, with the attendant need for infrastructure resources. The population is expected to increase by as much as 1 million people within the next 25 years (Denver Regional Council of Governments, 1999). Not surprisingly, the need for infrastructure resources will correspondingly grow as the existing infrastructure requires maintenance and new construction is undertaken. Second, the northern part of the Front Range of Colorado was chosen for this study because urban and commercial development has encroached upon some areas that are supplying or have historically supplied infrastructure resources. Furthermore, urban growth is projected for areas that currently produce infrastructure resources or have the potential to do so. Thus, the project study area was a natural laboratory to consider the interplay between population growth and its effects on the availability of infrastructure resources.
An important project goal is to deliver to decisionmakers and other stakeholders the scientific findings of the project in a form that is both understandable and readily usable, assuming that integration of project findings could lead to more informed decisionmaking by planners, developers, and other interested groups and institutions. To that end, direct stakeholder participation was solicited by project managers, which led to stakeholder involvement from initial project planning through its completion. Stakeholders, including governmental (city, county, and State) planning officials and representatives from both industry and academia, were invited to evaluate and critique the original project design before its formal acceptance by U.S. Geological Survey management. Direct feedback from stakeholders led to modifications of the project design so as to better address expressed needs. Ongoing communication with stakeholders was achieved through several mechanisms, including (1) the project Web site, (2) periodic publication and distribution of project highlights, (3) a stakeholder’s meeting (fig. 4) in November 1998 to present preliminary findings (U.S. Geological Survey, 1998), and (4) publication of the results of various studies conducted by FRIRP scientists on infrastructure resources in the Front Range (Knepper, 2001). A field trip to the project study area to discuss project findings and a formal USGS publication (Knepper and others, 2001) marked the final direct interaction between stakeholders and project members, although additional publications, such as this one, continue to be published to convey the detailed results of project work.
This volume contains five chapters that address various aspects of energy resources in the region (oil, natural gas, coal, coal-bed methane), as well as some of the environmental and land-use implications resulting from the exploration, development, and production of such resources. Specifically, efforts were focused on (1) reviewing the oil, natural gas, coal, and potential coal-bed methane resources in the study area and surrounding region (Higley and Cox, this volume; Roberts, this volume); (2) evaluating the potential for future exploration and production of oil and natural gas in the region (Cook, this volume); (3) assessing the relation between elevated soil salinities and waters produced along with oil and natural gas (Otton and others, this volume); and (4) determining the effects of energy resource production on the availability of aggregate resources and other land uses (Fishman and others, this volume).
The following two sections provide a brief overview of the geology and history of energy resource production in the northern part of the Front Range region of Colorado. This overview is intended as a general orientation and not an exhaustive description of the geology and energy resources of the region. For more details, the reader is referred to the technical chapters in this volume.
Although petroleum was first produced along the Front Range more than 130 years ago (Higley and Cox, this volume), and significant drilling and production occurred in the 1950s and 1960s, most of the discoveries and production in the region have occurred since about 1970. Post-1970 production has been concentrated in what has become known as the greater Wattenberg area (GWA), an area defined by the Colorado Oil and Gas Conservation Commission (an agency of the State of Colorado) for regulatory purposes to ensure the responsible development of petroleum resources. The FRIRP study area encompasses approximately the western one-half of the GWA (fig. 1). There are more than 7,000 wells that produce oil and (or) gas in the study area, and permitting continues for drilling new wells, particularly in Weld County and to a lesser degree in Adams County. Production through 1999 from wells within the study area has exceeded 2.15 trillion cubic feet of gas and more than 245 million barrels of oil (D.K. Higley, U.S. Geological Survey, unpub. data, 2003). Accessibility to local markets in the Front Range region, coupled with the sheer volume of contained petroleum, has made the GWA an important energy-producing province in Colorado. Indeed, of the 96 individual oil and (or) gas fields that exist within the project study area, four oil and two gas fields are among the 25 largest oil and gas fields in Colorado in terms of cumulative production (Higley and Cox, this volume).
Rocks of Cretaceous age serve as both reservoir and source rocks for much of the petroleum produced in the GWA, although there is some production also from Permian rocks. The dominant Cretaceous reservoir rocks include (1) Muddy (“J”) Sandstone of the Dakota Group; (2) other sandstones in the Dakota Group, including the Plainview Sandstone Member of the South Platte Formation (“Dakota” of drillers) and Lytle Formation (“Lakota” of drillers); (3) “D” sandstone of the Graneros Shale; (4) Codell Sandstone Member of the Carlile Shale; (5) Niobrara Formation; and (6) Terry “Sussex” Sandstone, Hygiene “Shannon” Sandstone, and Sharon Springs Members of the Pierre Shale (see fig. 5). Within the study area, oil is the principal resource produced from the Cretaceous Terry Sandstone Member and the Permian Lyons Sandstone, whereas either oil or natural gas, or both, have been produced from the other formations of Cretaceous age (Higley and Cox, this volume). Source rocks for most of the petroleum in the GWA include the Cretaceous Mowry, Graneros, and Carlile Shales and the Greenhorn Limestone (fig. 5) (Weimer, 1996; Higley and Cox, this volume). The Skull Creek Shale (fig. 5) also may have served as a source rock, although to a lesser degree than the others (Higley and Cox, this volume).
Because much of the western part of the GWA overlaps an area in the northern Front Range region that has recently undergone substantial urban expansion and is expected to experience much additional urban growth in the next few decades, a more complete understanding of the volume and distribution of undeveloped petroleum resources was an important project goal. Determining the remaining oil and gas resources, number of wells (existing and new) that might be required to extract these resources, geographical areas of potential discoveries, and possible timeframe for resource depletion were emphasized. In the chapter titled “Oil and Gas Exploration and Development Along the Front Range in the Denver Basin of Colorado, Nebraska, and Wyoming,” Higley and Cox (this volume) review the history of petroleum exploration and production in the Front Range region back to 1862 and provide an overview of its petroleum geology. Recently, as an extension of work done on this project, a geologically based assessment of undiscovered petroleum resources, which represents an estimation of the endowment of petroleum within the entire Denver Basin, was performed for all of the major petroleum systems in the Denver Basin, as well as for hypothetical accumulations of oil in Pennsylvanian rocks and coal-bed methane in Cretaceous and Tertiary rocks (Higley, 2003; Higley and others, 2003). An understanding of the regional geologic framework, also an outgrowth of the FRIRP studies, proved critical in assessing the undiscovered energy resources in the region as well as in evaluating the region for areas that are potential targets for new exploration and production.
Although the assessment of undiscovered resources in a region is important, it is performed at a scale that may be too coarse to adequately address the needs and interests of such disparate stakeholders as petroleum production companies, municipalities, and developers. These stakeholders are keenly interested in knowing more closely where prospective oil and gas development could take place within the study area because this information bears on possible future land-use conflicts. Building on the work of Higley and Cox (this volume), Cook (this volume), in the chapter titled “A Model for Determining Potential Areas of Future Oil and Gas Development, Wattenberg Field, Front Range of Colorado,” presents the results of a study undertaken to determine the potential (high, moderate, or low) for future oil and gas development, particularly from Cretaceous rocks. Several steps were required in formulation of the model, the first of which was to calculate the initial volume of oil and gas likely to be present in the various Cretaceous petroleum reservoirs. The estimated volume of ultimately recoverable oil and gas was determined from production data and then subtracted from the initial volume of petroleum in the rocks to estimate the volume of petroleum that might be accessible from new wells or by undertaking a program of “recompleting” existing wells into additional reservoir rocks. Any future drilling or recompletion of existing wells assumes there is economically producible petroleum remaining in the rocks. Finally, Cretaceous formations within the region were ranked from low to high for their potential for future petroleum development. Plots showing the areal distribution of potential for future petroleum development reveal that most areas of moderate to high potential are concentrated between Denver and Greeley, in much of the same area for which future urban growth is expected.
The typically saline water produced along with petroleum can be voluminous and requires proper handling and disposal to ensure minimal adverse environmental effects. The juxtaposition of saline soils with oil-production facilities at some sites in the study area, identified through reconnaissance field investigations at the start of the project, led to questions regarding the significance of produced waters in the formation of saline soils around or near some petroleum production sites. Otton and others (this volume) report on detailed mapping and mineralogic studies that address the origin of the saline soils as well as the nature and composition of contained salts. Their research indicates that saline soils containing largely sulfate-bearing minerals are in specific geomorphic and hydrologic settings, particularly where ground water enriched in dissolved solids is close to the soil surface for part of the year (Otton and others, this volume). The distribution of the saline soils and the isotopic composition of saline minerals indicate that salts may have originated from outcrops of the Pierre Shale (fig. 5) in the region. Although Otton and others (this volume) identified some sites where saline soils surround or are near oil-production facilities, including large tanks used to store produced waters, at only two sites did their studies reveal that a portion of the saline minerals (chloride-dominant minerals) in the soils may have been derived from produced waters. Thus, much of the observed saline soil mineralization in the study area appears to have originated by natural processes, locally enhanced by irrigation practices, rather than by leakage of saline waters from oil-production facilities.
The effects of petroleum production are not limited to environmental concerns, but such production may well preclude use of the land for other purposes, including urban and commercial development, farming, and notably, extraction of aggregate resources (sand, gravel, and crushed stone). With more than 7,000 oil and (or) gas wells currently producing within the study area, and with large deposits of aggregate also being mined within the same area, it is not surprising that numerous examples exist where oil and (or) gas wells have been established over economically viable aggregate deposits. In the chapter titled “Effects of the Oil, Natural Gas, and Coal Production Infrastructure on the Availability of Aggregate Resources and Other Land Uses, Northern Front Range of Colorado,” Fishman and others (this volume) evaluate the effects of oil and (or) gas production on the availability of aggregate resources. At two sites that were studied in detail, the presence of energy-production equipment precludes production of a large portion of the available aggregate resources. As the need for more resources (energy and aggregate) grows to keep pace with future demands, conflicts between aggregate production and oil and gas operations may become more common (Fishman and others, this volume). However, petroleum-production companies, aggregate-production companies, developers, and farmers are increasing efforts to mitigate conflict and promote good relations.
Coal was first produced along the Front Range in the early 1860s in the southwestern part of the Boulder-Weld coal field (BWCF) in Boulder and Weld Counties (fig. 1). In the northern part of the Front Range, within the project study area, coal was mined continuously for more than 100 years until closure of the last mine in 1979 (Kirkham and Ladwig, 1980). In addition to the BWCF, coal was also produced in Jefferson and Douglas Counties in the northern part of the Front Range (Kirkham and Ladwig, 1980; Roberts, this volume). Focus in this volume, however, has been on the coal mines in the BWCF because of the wealth of data concerning the extent and depth of mining available for the more than 130 mines in it. Furthermore, coal produced from the BWCF totaled approximately 107 million short tons, which represents more than 82 percent of all of the coal mined throughout the Front Range region (Kirkham and Ladwig, 1980; Tremain and others, 1996). Although no longer mined, interest in coal for this study stemmed from the effects of past coal mining, including mining-related subsidence and coal-mine fires, and, notably, the potential for coal-bed methane resources (Roberts, this volume).
The Cretaceous Laramie Formation (fig. 5) is the dominant coal-producing unit in the study area (Kirkham and Ladwig, 1979; Roberts and Kirschbaum, 1995; Weimer, 1996), with only minor production of lignite from the overlying Tertiary Denver Formation (fig. 5). Laramie Formation coal ranges from subbituminous B to subbituminous C in rank, has a sulfur content of generally less than 1 percent (Kirkham and Ladwig, 1979), and was used locally for both domestic and industrial purposes, as well as in the numerous mining towns and camps in the nearby mountains (Tremain and others, 1996).
Most of the abandoned mines in the BWCF are in those parts of Boulder and Weld Counties that have either recently undergone urban growth or are slated for development during the next few decades. Rather than focusing research efforts on understanding the volume and distribution of remaining coal resources and the potential for future mining in the BWCF, emphasis in the FRIRP was on post-mining land-use effects and on the potential for extraction of methane from coal remaining in the subsurface in the region. Roberts (this volume), in the chapter titled “Coal in the Front Range Urban Corridor—An Overview of Coal Geology, Coal Production, and Coal-Bed Methane Potential in Selected Areas of the Denver Basin, Colorado, and the Potential Effects of Historical Coal Mining on Development and Land-Use Planning,” presents a structural and geologic framework for the Denver Basin and the Laramie Formation, respectively, that bears directly on coal resources in the region. The geologic framework provides the foundation for evaluation of the coal-bed methane potential of the Laramie Formation, which is also discussed in Roberts’ (this volume) chapter. The geologic framework is also important when considering the post-mining effects on land use in the region because depth to coal and the nature of the rocks overlying mined areas figure prominently in the potential for subsidence and coal-mine fires (Roberts, this volume). Study results of coal-bed methane potential of the Laramie Formation and post-mining environmental effects provide planners, developers, and natural gas operators with information needed for future land-use considerations.
I am grateful for the support and encouragement extended by Gene Whitney for this project and volume, and for the project’s financial backing provided by the U.S. Geological Survey’s Energy Resources Program. This paper benefited greatly from constructive reviews by James Otton, Daniel Knepper, Jr., William Keefer, and Mary Kidd. I greatly appreciate the lengthy but fruitful discussions about stratigraphic nomenclature with Thomas Judkins of the U.S. Geological Survey and John Ladd with Kerr-McGee Rocky Mountain Corporation, which materially aided construction of the stratigraphic column presented in this paper, as well as for other chapters in this volume. Finally, I thank Steve Cazenave for his assistance in drafting the figures for all chapters in this volume.
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