Mapping chapter
- 1. 99 • • 8 Mapping Surface and subsurface maps are important tools that geologists use to find gas and oil. All maps are oriented with north to the top, south to the bottom, east to the right, and west to the left. Topographic Maps A topographic map shows the elevation of the earth’s surface (fig. 8–1). To illustrate the third dimension (elevation) on a flat, two-dimensional map, contour lines are used. A contour line is a line of equal value on a map, and a contour line on a topographic map is a line of equal elevation. A contour line is always labeled with an elevation that is above or below sea level. All along that contour line, the elevation is exactly the same. For example, anywhere along the +400 ft contour line on a topographic map, the elevation is exactly 400 ft above sea level. The contour interval of a topographic map is the difference in elevation between two adjacent contour lines. The contour interval of the topographic map in figure 8–1 is 100 ft. If the elevations on contour lines increase in a direction, the slope is rising (fig. 8–2). If the contours are spaced relatively close together, the elevation is changing rapidly, and the slope is steep. If the contours are relatively far apart, the slope is gentle. 08_Hyne.indd 99 1/25/12 10:52 AM
- 2. Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production 3rd Edition 100 • • Fig. 8–1. Land and a topographic map of the land Fig. 8–2. Contoured topographic map showing steep and gentle slopes There are some important characteristics of contours on a topographic map. Contour lines never cross. Contour lines are single lines; they never branch. Contour lines are continuous; they always close or run off the map and never end on the map. Elevations can be accurately estimated from a topographic map. If a point is on the +300 ft contour, it must be, by definition, exactly 300 ft above sea level. If the point is about halfway between the +300 and +400 ft contour, an elevation of +350 ft is a good estimate. The shape of the contours is characteristic for many topographic features such as hills, ridges, and canyons. A topographic map (or any contoured map) cannot be drawn without some accurately surveyed points. After the elevations or values are located on a map (spotted), contours can be drawn between the points. Contouring 08_Hyne.indd 100 1/25/12 10:52 AM
- 3. Chapter 8 Mapping 101 • • of any map can be done either by hand or computer. The position of a contour line between two data points can be accurately located by using proportions. For example, the 400 contour line must run between data points of 402 and 399 (fig. 8–3). A straight line is drawn between the two data points. Because there is a difference of 3 between the data points (402 and 399), the line is divided into three equal segments. The 400 contour line is located one segment from the 399 point and two segments from the 402 point. Anything that can be expressed by mathematics can be programmed into a computer, and computer-generated contour maps can be made. Fig. 8–3. Locating a contour using proportions Geologic Maps A geologic map (fig. 8–4) shows where each rock layer crops out on the surface of the earth. Each rock layer is given a different pattern, color, and symbol on the map. The basic sedimentary rock layer used for geologic mapping is called a formation. A formation is a mappable rock layer with a definite top and bottom. Geologists have divided all sedimentary rocks into formations. Each formation has a two-part name. The first part is a town where the layer crops out on the surface. The second part is the dominant rock type, such as sandstone or limestone. San Andreas Limestone, Bartlesville Sandstone, and Barnett Shale are formation names. If the sedimentary rock layer is a mixture of rock types, such as alternating thin sandstones and shales, the word formation is used, for example, the Coffeyville Formation. Formations can be subdivided into smaller units 08_Hyne.indd 101 1/25/12 10:52 AM
- 4. Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production 3rd Edition 102 • • called members. A member is a distinctive but local bed in a formation (fig. 8–5). It is also given a formal, two-part name. For example, the Layton Sandstone Member is part of the Coffeyville Formation. Adjacent formations of similar rocks can be joined to form a group and given a geographic name (i.e., the Chase Group). If a rock layer occurs deep in the subsurface and does not appear to crop out on the surface or if it is located offshore, it is given a letter and number designation such as the H5 sands. Fig. 8–4. Geologic map Fig. 8–5. Stratigraphic column showing formations and members 08_Hyne.indd 102 1/25/12 10:52 AM
- 5. Chapter 8 Mapping 103 • • A geologic map is a flat, two-dimensional representation of the earth’s surface. The orientation of rock layers, the third dimension, is shown with a strike-and-dip symbol. Strike is the horizontal orientation of a plane (fig. 8–6a), such as a sedimentary rock layer or a fault. It is measured with a compass orientation, such as north 30˚ east. Strike is shown as a short line on the geological map (fig. 8–6b) that is oriented in the measured compass direction. Dip is the direction and vertical angle of the plane. It is measured perpendicular (90˚) to the strike (fig. 8–6a). The dip symbol on the map is a small bar attached to the middle of the strike line (fig. 8–6b). It points in the direction that the plane goes down into the earth. The angle in degrees is often on the dip symbol. The dip of a rock layer is the angle and direction it goes into the subsurface. Drilling updip means that the drillsite will be up the angle (dip) of the rock layer from the last drillsite. Updip in a reservoir is usually a favorable position from a dry hole (fig. 8–7). You may assume that any reservoir rock is filled with water. Gas and oil are lighter than water and will flow (migrate) updip in the reservoir rock to a high area. One would almost never want to drill downdip from a dry hole; one would want to drill updip. a b Fig. 8–6. (a) Strike and dip of a sedimentary rock layer and (b) strike-and-dip symbol on a geologic map 08_Hyne.indd 103 1/25/12 10:52 AM
- 6. Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production 3rd Edition 104 • • Updip Updip Fig. 8–7. Updip from a dry hole A stratigraphic column (fig. 8–5) is a convenient method for presenting the vertical sequence of rocks on a geologic map or in a basin. Any deformation of the rocks, such as faulting or tilting, has been removed. The youngest formation is at the top of the column, and the oldest is located at the bottom. The column is drawn as a cliff of weathered rocks with the weaker rock types (e.g., shales) indented. Stronger rock types (e.g., sandstones) protrude outward as they would weather in nature. Common geological symbols (fig. 8–8) are used for rocks, structures, and wells on a geological map. Base Maps A basemap is a map that shows the location of all the wells that have been drilled in an area. Spotting a well involves locating a wellsite and placing the well symbol (fig. 8–8) on a base map. Base maps can also include seismic lines and other data. 08_Hyne.indd 104 1/25/12 10:52 AM
- 7. Chapter 8 Mapping 105 • • Fig. 8–8. Common geological symbols Global Positioning System Accurate positioning is very important to geologists, geophysicists, and petroleum engineers. They need to know the exact location of proposed drillsites, existing wells, and seismic lines. These sites used to be located with considerable time and expense using surveying tools. Since the 1980s, accurate location in all weather and anywhere on the earth with no cost has been determined by the Global Positioning System (GPS). GPS involves the use of satellites and a receiver. There are 24 solar-powered satellites very precisely orbiting the earth twice a day at an altitude of 12,550 miles (20,200 km) in six planes with four satellites each. Each satellite transmits extremely accurate time signals and the satellite’s orbital information. The receiver at the location has an antenna tuned to each satellite’s frequency, a processor, and a very stable clock. It compares the satellite time signal 08_Hyne.indd 105 1/25/12 10:52 AM
- 8. Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production 3rd Edition 106 • • with the same time on the receiver to determine how much time it took the satellite signal to reach the receiver. It then uses that information to compute the distance from the receiver to the satellite. By using the computed distance from three, or more accurately from four satellites, the location, usually in latitude and longitude, and the altitude of the receiver are calculated and displayed on the receiver. Very precise receivers can calculate positions to less than 10 ft (3 m) on average. Subsurface Maps Three important types of subsurface maps are structural, isopach, and percentage. All three maps use contour lines to describe a subsurface rock layer. Structural map A structural map uses contour lines to show the elevation of the top of a subsurface sedimentary rock layer (fig. 8–9). The contour lines are usually in minus feet below sea level, as most rocks are located below sea level. An important structural map would be one contoured on the top of a potential reservoir rock or drilling target. Fig. 8–9. Structural map 08_Hyne.indd 106 1/25/12 10:52 AM
- 9. Chapter 8 Mapping 107 • • Domes, anticlines, and faults can be identified on structural maps. Both a hill on a topographic map and a dome on a structural map have a bull’s eye pattern (fig. 8–10) with the highest elevation in the center. Both a ridge on a topographic map and an anticline on a structural map have a concentric but oblong pattern (fig. 8–11) with the highest elevation in the center. Dip-slip faults are characterized by a rapid change in elevation along a relatively straight line (fig. 8–12). A normal dip-slip fault that causes a lost section in the rock layer being mapped (see fig. 5–18 in chapter 5) is seen on the map as two lines separating the contour lines (fig. 8–13). Fig. 8–10. Topographic map of a hill and structural map of a dome 08_Hyne.indd 107 1/25/12 10:52 AM
- 10. Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production 3rd Edition 108 • • Fig. 8–11. Topographic map of a ridge and structural map of an anticline Fig. 8–12. Fault on a structural map 08_Hyne.indd 108 1/25/12 10:52 AM
- 11. Chapter 8 Mapping 109 • • Fig. 8–13. Normal dip-slip fault on a structural map Isopach map An isopach map (fig. 8–14) uses contour lines to show the thickness of a subsurface layer. If an oil or gas field has been drilled, an isopach map can be made of the reservoir rock pay zone. The pay zone is the vertical distance in a well that produces gas and/or oil. Grosspay contours the entire reservoir thickness including nonproductive water-bearing and shaly zones. Net pay contours only the productive thickness of the reservoir. A net pay isopach map of a reservoir is used to calculate the oil and gas volume and reserves. Fig. 8–14. Isopach map 08_Hyne.indd 109 1/25/12 10:52 AM
- 12. Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production 3rd Edition 110 • • An isopach map can be used in exploration to delineate a sandstone pinch-out (fig. 8–15a) where the isopach contour line becomes zero. The aerial patterns of beach and river channel sandstones are seen on an isopach map (fig. 8–15b). Fig. 8–15. Isopach map of (a) a sandstone pinch-out and (b) a beach or river channel sandstone An isopach map of a limestone layer can also be used to locate a reef. A reef is a mound and is shown by thick contour lines (fig. 8–16). Barrier reefs that are long (fig. 8–16a) can be distinguished from pinnacle reefs that are circular (fig. 8–16b). Fig. 8–16. Isopach map of (a) a barrier reef and (b) a pinnacle reef 08_Hyne.indd 110 1/25/12 10:52 AM
- 13. Chapter 8 Mapping 111 • • Percentage map A percentage map (fig. 8–17) plots the percentage of a specific rock type such as sandstone in a formation. Higher percentages of reservoir quality rocks, such as sandstones and carbonates, imply a better reservoir quality. Fig. 8–17. Sandstone percentage map of a formation composed of sandstone and shale 08_Hyne.indd 111 1/25/12 10:52 AM