Exploration for oil shale involves the initial “discovery,” as well as the geologic work required to adequately characterize a deposit for engineering planning. Once a viable deposit is confirmed and operation starts, development geology begins. The main thrust in development geology is geologic control for exploitation.
Exploration geology should not be approached as a cook book exercise. Every oil shale property, deposit, or region has its own particular geologic, physical, and economic characteristics that require one or more modifications to standard exploration practice. The exploration techniques presented here are most applicable to the oil shale deposits of the Green River formation located in Colorado, Utah, and Wyoming. These techniques, with some possible modification, are applicable to oil shale deposits of the eastern United States, as well as those throughout the world.
The most important consideration in exploration is to identify and characterize a deposit at a minimum cost and in the shortest possible time. To accomplish this, a well organized and phased approach is necessary. This phased geological exploration effort, integrated with engineering studies and intermediate decision points, results in an efficient, well organized program. A flow diagram showing the three main phases of exploration is given in Fig. 2.9.2.
Figure 2.9.2.
Reconnaissance Geology—Phase I
The reconnaissance geology phase represents the initial examination of a property or properties to identify tracts or areas having the best potential for acquisition and development. During this phase of work, existing data is collected and analyzed, aerial and ground reconnaissance geology mapping completed, and reconnaissance drilling performed. All information derived from this work is evaluated to determine the best potential areas for more detailed exploration.
The engineering effort which normally follows this phase of work is a conceptual engineering feasibility study. The quality of this study depends to a large extent upon the quality and completeness of the geologic data obtained during the reconnaissance phase. Following the initial exploration and engineering work, a decision can be made either to reject the area or property, or to move on to the next phase of exploration. The time usually required for this phase of work is one year, but less time may be needed if more initial data is available.
Exploration Geology—Phase II
During the exploration geology phase, more detailed and site specific data is acquired to better define the geology for feasibility engineering. This phase involves additional drilling, preliminary geotechnical testing, geologic mapping, preliminary hydrology testing, and geologic evaluation. This phase may last one or two years, depending upon the size of the property under investigation and the amount of drilling required.
The data obtained during this phase may be used in performing a feasibility study that would more accurately define the mining potential of the project. The exploration geology phase and feasibility engineering usually provide sufficient information to evaluate the feasibility of the project so that a decision can be made whether or not to proceed.
Definitive Geology—Phase III
The definitive-geology phase is directed at better defining the immediate area of the mine and to gather very site specific data for detailed and final mine engineering. This phase includes drilling, geotechnical testing, hydrological testing, and a geologic evaluation, and is normally completed in one year.
Following this phase of work, the geologic data base should be sufficient for detailed final mine design engineering and construction, which may take several years to complete.
Development Geology—Phase IV
During the operating life of a mine, continued geological work is needed for grade control, structural control and to better define, in advance of mining, the hydrological and geotechnical characteristics of the strata.
EXPLORATION TECHNIQUES
The techniques of oil shale exploration may vary in particular cases, but will often include many things in common. The actual techniques used will depend upon the circumstances of the case. The techniques of oil shale exploration described below are commonly used in the Piceance Creek and Uinta basins of Colorado and Utah. Most of these techniques are also applicable to other oil shale deposits of the world.
Information Review
An important initial step in an exploration program is to compile and review all available and existing information. This information may be in the form of raw data, such as assay reports, or it may be reduced data which has been synthesized from the raw data. Reduced data, such as oil yield maps, are by far the easiest to work with, but for a reliable evaluation, all available information must be considered. It must be remembered, however, that with the passing of time, procedures and techniques are improved and popular theories are modified to reflect new technology. Existing information, therefore, must be viewed subjectively.
A bibliography is included in the reference section of this chapter that lists some of the many published sources of information on oil shale in the United States. This list is not all inclusive but is representative of the most commonly used sources. Since 1917, the U.S. Geological Survey, U.S. Bureau of Mines, Energy Research and Development Administration, Department of Energy, and several state geological survey offices have added to the available oil shale data base. Most information developed by private industry is confidential, and is not commonly available.
Information on international oil shale deposits usually can be obtained from the government geological agency in the country of interest. In former colonial countries, additional information may be available from government agencies in the mother country. Considerable geologic work is also performed in overseas countries by the U.S. Geological Survey, the United Nations, and other world agencies.
Mapping
Good maps are necessary for performing any detailed geological or engineering work on a property. Topographic base maps are required for recording and presenting geologic information, and for engineering planning and design. Photogeologic maps should be prepared early in the reconnaissance phase. More detailed maps are required in the later phases of exploration.
Topographic Mapping: For reconnaissance exploration, the standard 1:24,000 scale
USGS topographic maps are usually satisfactory. These maps can be effectively photoenlarged up to four times to a scale of 1:6,000. The smaller scale 1:62,500 USGS topographic maps can be enlarged effectively about five times to 1:12,000 scale. These maps, however, are not of the quality and accuracy needed for feasibility engineering, exploration, and definitive geology.
For more accurate maps, the area of interest should be mapped using photogrammetric techniques. Maps, at a scale of 1:12,000 or 1:6,000, with contour intervals of 6 m (20 ft), can be prepared from aerial photographs and are satisfactory for most geological work. Maps that are generally used for engineering studies, however, are usually of larger scales, such as 1:6,000 to 1:1,200 for particular areas of concern. When mapping is carried out, a base datum elevation should be specified, which is an average elevation within the mining area, so that the greatest accuracy in data location control and volumetric calculations can be achieved.
Photogeologic Mapping: Aerial photography is a valuable tool for examining and
evaluating large areas of land rapidly. The amount of detail varies with the scale at which the photographs are taken. Most government aerial photographs are developed at a scale of 1:50,000 that renders them useful only for reconnaissance work. High altitude U-2 photographs, developed at a scale of about 1:120,000, are also available, as are enhanced satellite images. However, since they cover extremely large areas and are not detailed, they are useful only for regional studies of geologic structures.
Good quality, low altitude aerial photographs are commonly taken to cover the area of interest for photogeologic mapping. Photographs should be taken in the late fall or early spring when the deciduous trees are devoid of leaves and there is no snow on the ground. Overlapping vertical photographs should be taken for stereo viewing. A series of oblique photos should also be taken when there are cliff faces on the property since they do not appear clearly on vertical photographs. For geologic mapping, vertical photographs should be taken at a 1:12,000, or smaller, scale.
Government aerial photographs are produced primarily in black and white, which is adequate for most photogeologic mapping, since gray tonal contrasts and textures can be interpreted. Superior geologic mapping can be produced using color photography
because color photographs show the actual color of the ground surface, making interpretations easier and more reliable.
Multispectral Imaging: Multispectral imaging, or remote sensing, from satellites, for
example, Landsat, and aircraft has become a very valuable tool in recent years for regional studies as well as evaluations of large properties. The information derived from remote sensing is developed as an image, not as a photograph. An image of the earth’s surface is developed by computer processing of digital sensor data. The sensors detect electromagnetic radiation in selected wavelength ranges, most of which are not visible to the human eye. The image is then enhanced by computer and presented in a variety of ways visible to humans. Satellite imagery, especially that from the new Thermatic Mapper (TM), has proven to be very useful in the evaluation of structures in oil shale exploration. Computer enhancement of the digital data can be used to show linear structural features. Radar imagery can be used to produce an image of the earth’s surface that enhances linear features and is useful for performing structural studies.
Geologic Mapping: A preliminary photogeologic mapping study should be performed
prior to any geologic mapping in the field. This will familiarize the field geologist with the project area and provide valuable geologic information that will help in field mapping. When thorough photogeologic mapping is available, the geologist’s field time can be significantly reduced and used primarily for confirming the interpretations made from the photographs.
The most reliable method for compiling field data is to use frosted mylar overlays on aerial photographs so that data will not obscure features that may be important keys to underlying structures. Several overlays may be used to differentiate between various data such as structure and lithology. If the photographs are at a scale of about 1:12,000, the data recorded on them can be easily transferred to 1:12,000 scale base maps. Another common technique is to use aerial photographs, enlarged twice, on which mapping data can be directly recorded. Normal stereo pairs of the aerial photos can be used for reference.
Geotechnical Mapping: Another important objective of an exploration geology
program is to determine the geotechnical characteristics of the strata. The feasibility of mining an oil shale deposit, by either underground or surface methods, is dependent upon the competency of the rock. Therefore, it is important to start gathering geotechnical data early on in the exploration geology phase.
Much of the information recorded during geologic mapping is also useful for geotechnical evaluation. Normally, however, geotechnical mapping records a great deal more detail about the characteristics of discontinuities, i.e., joints, partings, and faults. Geotechnical mapping consists principally of area-wide geotechnical data gathering and site specific line mapping. The area-wide effort consists of developing a representation of the geotechnical characteristics of an area. The area is first surveyed to identify major structural features and regional joint patterns. Extensive data are collected concerning the strike and dip of the strata and of discontinuities such as joints, fractures, and faults. A detailed description is made of the beds, discontinuities, and bedding planes or partings. Data on strike and dip are normally analyzed using rose diagrams, histograms, and stereographic projections. Detailed line mapping presents site specific information
on discontinuities. This mapping includes detailed observations of all discontinuities along a traverse section of about 30 m (100 ft) or more in length. Several sections are measured along several orientations to assure a complete investigation of all discontinuities. Detailed line mapping requires good unweathered rock exposures, which can be provided by driving a test edit or making an open cut face.
Drilling Program Planning
A detailed plan for the drilling program is made before any drilling is carried out on a property. Proper planning will ensure a coordinated program so that needed data is gathered in an efficient manner. The objective of the program must be agreed to by all interested parties. Once the program is approved, a geologist can begin program planning, which is closely coordinated with the project’s engineering, geotechnical, and hydrological personnel. A well planned program will yield maximum information from each drill hole at a minimum cost.
The exploration drilling program should be supervised by an experienced geologist with a sound background in sedimentary and structural geology. The geologist responsible for the overall program will coordinate all geologic work and logistics. Large programs require several drill rigs, each of which has a geologist assigned to it. Each rig geologist is responsible for logging all the core recovered.
Drill Hole Design: One of the first steps in planning a drilling program is to determine
the number of holes required and to show the location of each hole on a map. Only a few holes are required in a reconnaissance exploration geology program (Phase I), that is directed at identifying a potential property for more detailed exploration. However, in a definitive exploration geology program (Phase III) the objective is to accurately define the geology for final mine design. This may require many drill holes. The trend during the complete cycle of exploration geology (Phases I through III) and operations geology (Phase IV), is to increase definition of the geology through tighter drilling control, by increasing the number of drill holes and decreasing the spacing between them.
Drilling during an oil shale reconnaissance program (Phase I) is carried out to verify previous drilling data or gather information on the total stratigraphic oil shale section. During the reconnaissance phase, a minimum number of holes is required, generally with a spacing of up to many kilometers (Fig. 2.9.3a). During exploration drilling (Phase II), however, holes are usually spaced several kilometers apart (Fig. 2.9.3b), which should provide sufficient data for performing a mining feasibility study to determine the optimum mining area. During a Phase III definitive drilling program, holes are concentrated in the initial mine area, where the hole spacing varies between 1 and 2 km (0.6 to 1.2 miles). This spacing will usually provide the necessary detail for performing final mine design and grade control. Development drilling (Phase IV) is performed ahead of mining (Fig. 2.9.3d), during the mining operation where the holes are spaced at much smaller intervals.
Drilling and Coring Methods: Two drilling and coring methods are basically
applicable to oil shale. In one, a cylindrical rock sample is recovered from the drill hole during the drilling operation (coring), and in the other, cutting samples are recovered during rotary drilling. Usually, oil shale holes are cored from top to bottom, or at least through the main zone of interest. Rotary drilling is not satisfactory when samples are required for grade analysis, and should be used only as a cost efficient, rapid method for penetrating through rock overlying the target zone to be cored.
Diamond drilling is most commonly used for coring. Core drilling is expensive but it is the only method by which good, undisturbed samples can be obtained for assaying, structural geology studies, stratigraphical analysis, and geotechnical testing. Oil shale usually cores very well. Very good core samples of oil shale are normally recovered except through rubblized (due to burning or collapse) and fractured zones.
Two general types of coring tool are available: conventional and wireline. Conventional coring normally comprises a 6.1-m (20-ft) long core barrel that is placed at the bottom of the drill string. Once the barrel is full, the whole drill string is removed from the hole to recover the core from the barrel. The drill string is again lowered into the hole to core the next section of rock. This coring technique is simple but slow, particularly at depths greater than 150 m (500 ft), Progress is particularly slow when conventionally coring through rubblized zones, because the core barrel can become plugged by pieces of oil shale.
Wireline coring is most commonly used in oil shale, whereby core is recovered without pulling the drill string out of the hole after each run. The core is retrieved through the center of the drill string using a cable hoist with a special overshot latching device to attach to the inner core barrel. Usually, two core barrels are used during coring so that while one is being emptied of core, the other is being filled. The use of two barrels saves time between runs and allows the core to be carefully extracted and prepared for logging.
The size of core to be recovered during a drilling program depends upon the purpose of the sampling program. Most coring operations in oil shale produce conventional NX or wireline NXWL core 5.4 cm (2 1/8 in.) and 4.76 cm (1 7/8 in.) in diameter, respectively. Core samples of either size are large enough for geotechnical testing and for assaying and are also fairly easy to handle and store neatly in core boxes designed to hold 3.05 m (10 ft) of core.
Conventional 7.35-cm (3-in.) or larger core is normally recovered when larger samples are required for bench-scale testing. Large diameter wireline core drilling, designated PQWL and 8.21 cm (3.35 in.) in diameter, has also been successful in oil shale.
Where a deep hole is to be drilled and continuously cored, and in all reconnaissance drilling, wireline core rigs should be used. These rigs are self-contained with a mud pump and all necessary equipment. Coring rates of over 75 m (250 ft) per 12-hr shift with a good driller can be attained. However, wireline rigs are slow and inefficient for rotary drilling.
Rotary drill rigs are used when short sections are to be cored at depth or when air drilling is desired. These rigs (Fig. 2.9.4) are also self-contained with mud pump, water
injection pump, and air compressor to meet most requirements. In very deep holes, auxiliary air compressors and boosters are commonly used. Rotary drilling is accomplished using a tricone roller bit. A good driller can rotary drill over 152 m (500 ft) in 12 hours. Once the core point is reached, the tricone bit is replaced with a core barrel and core bit to recover the desired section of rock. Conventional coring rates with a rotary rig are usually about 35 to 50 m (100 to 150 ft) per 12-hr shift at hole depths of about 328 m (1,000 ft). Conventional coring often requires drill collars to add weight for increased penetration.
Figure 2.9.4.
Core drilling is usually conducted using water as the drilling fluid, but drilling muds are added when needed to retain circulation. In many cases, however, the addition of drilling muds is insufficient to restore circulation when lost, and consequently drilling is carried out blind. Blind drilling can be safely accomplished providing sufficient water is pumped down the hole and penetration is slowed down to assure a clean, cool bit. This can be performed by a good driller without sacrificing core recovery or creating bad hole conditions.
Drilling with air is necessary if any drill holes are to be used later for hydrological monitoring or testing. The air compressor units found on most rotary drill rigs provide