Princeton Natural Gas
The Search for Natural Gas
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hen people first began to search out gas and petroleum, the only way they knew to find it was to look for surface evidence of a formation. This usually took the form of surface oil seeps in the ground. Of course, in the early days, these people had little idea of what the formations actually were, much less how or why they formed where they did. Today, geologists have given the industry much more information about petroleum formations and their history. This information, along with new technology that allows us to 'see' into the ground provides gas exploration companies with a much better chance of finding gas and other petroleum resources when they drill wells.
- In the late 1800's, prospectors noticed that oil seeps often occurred on anticlinal
(diagram..)
slopes, which was one of the first hints that geology could aid in the
search for oil and gas. By the turn of the century, some domestic oil
companies began forming geological departments that aided them in their
efforts to find gas and oil formations. Since this time, geology has played
an increasing role in helping gas companies find their mark. Gone are the
days when wells were dug by intuition alone.
How Does Natural Gas Form?
Source: NGSAT oday, most believe that petroleum products, and natural gas come from ancient plants and animals that have died and their bodies have decomposed. The erosion process carried these biological remains down rivers and streams onto shorelines, where they were deposited along with mud and silt. Over time, they were covered by increasing amounts of this sediment, and gradually they were compressed by the weight of the sedimentary layers. With time, the material that originally contained the biological remains became sedimentary rock. Today, these sedimentary rocks, sandstone, shale and dolomite are often where deposits of petroleum are found.
- Eventually, the organic materials transform into petroleum products due to the intense pressure and heat present in the rock formations. The oil and gas migrate through the pores in the sedimentary rock, upwards to the earth's surface. If the gas reaches the surface, it is dispersed into the atmosphere. Light oils eventually evaporate also. However, most often, the petroleum products never make it as far as the surface. Many times they are trapped beneath the surface by layers of rock that have formed above the sedimentary rock layer that produced them.
- The layers of rock that trap the deposits are impermeable layers that are usually shaped into domes by folding or faults. The layer that traps the gas and oil is called a cap rock, and the resulting formation is called a trap. As the gas and oil move upwards in through the permeable layers of rock, the oil and gas displace sea water that was also trapped in layers of sedimentary rock. When the oil and gas reach the trap and cease upward movement, they separate from one another. If you placed equal amounts of gas, oil and water into a glass, you would see that they naturally separate themselves according to their varying densities. It's easy to see the same sort of thing by putting vegetable oil and water in a glass. Not all of the water is separated from the petroleum, however. Between ten and fifty percent of the oil and gas accumulation contains salt water, which must be removed before the gas or oil can be used by people.
- There are two main categories of gas and oil traps. They are structural traps and stratigraphic (picture) traps. Structural traps are caused by the folding or deformation of layers of rock. Two common varieties of structural traps are anticlines and faults. An anticline trap is an upward folding in the layers of rock, like an arch. Oil and gas migrate to the highest point in the anticline's dome, where they come to rest when they encounter a layer of cap rock. The second common variety of structural traps are fault traps. These form when faults fracture layers of rock, and layers containing natural gas and oil are enclosed by layers of cap rock that are adjacent to them.
- The stratigraphic category of traps is characterized by a change in the reservoir layer itself trapping gas and oil. This may occur when a layer of sediment began with larger pores, which allowed the petroleum to move through it, but then the grain of the sediment became smaller, or more tightly compacted, and this gradually forms an impermeable layer of rock. Picture.
Geologists study geological maps and data to determine the areas where gas and oil can most likely be found.
Source: NGSAThe Geologist's Role in Exploration
B y knowing about how traps form, and where they are most likely to be found, the geologist makes the search for oil easier. The search for gas begins by having geologists find an area of the country where gas and oil are most likely to be found. This involves the evaluation of the earth's history, and comparison with other areas where gas and oil are known to be present.
- Once such an area is designated for exploration, the geologist begins to conduct more specific tests to determine the likelihood that gas and oil are indeed present in the area. He or she may study above ground rock formations to determine the precise area where the folding of layers may have caused traps to form. In areas where wells are already present, the geologist may study samples of rock that were taken from previous drillings, as well as information acquired from logs, or instruments lowered into wellbores to measure the properties of rock layers.
Technology's Role in Exploration
0 ne of the most exciting and effective technological advances that has aided the geologist in finding gas deposits is seismology. Seismology is the study of how sound or seismic waves move through the earth's crust. It was first used to study earthquakes, and it still is today. However, it has also proved an invaluable tool for the geologist in studying the lower layers of the earth's crust without actually drilling into them. The idea behind seismology is that earthquakes, or other sources of vibrations in the crust interact with different kinds of rock differently so that by recording how a wave of vibration is reflected by a certain layer of rock, the geologist can make an educated guess regarding what kind of rock is present, and approximately how deep in the crust it can be found. A good analogy is the waves of energy from the source of the vibration react with different layers of rock much like bouncing a ball on different surfaces produces different results. If you bounce the same ball on concrete, and then on soft dirt or sand, the ball will bounce much higher after hitting the concrete than it will after hitting the dirt or sand. By judging the distance that a rubber ball bounces on an unknown material, one could judge the relative hardness of that material.
This diagram shows an explosion causing seismic waves that bounce off of the different layers of rock. The vibrations are then recorded at the surface by geophones.
Source: API- In the earlier uses of seismology, dynamite was used to create predictable vibrations from known locations. These vibrations were then recorded by seismographs, and by placing these electronic devices over a large area, geologists are able to create a model of the rock layers beneath the earth's crust. Today, exploration companies often use specialized trucks that produce vibrations on the surface, rather than explosions of dynamite. The trucks produce more reliable, and less obtrusive vibrations than dynamite explosions.
- In off-shore exploration, ships often pull arrays of sensors, along with an 'airgun' that shoots out highly pressurized air into the water, creating vibrations that can be measured by the array of sensors, and then used to create the same profiles of the rock layers beneath the surface. Picture.
- Eventually, the organic materials transform into petroleum products due to the intense pressure and heat present in the rock formations. The oil and gas migrate through the pores in the sedimentary rock, upwards to the earth's surface. If the gas reaches the surface, it is dispersed into the atmosphere. Light oils eventually evaporate also. However, most often, the petroleum products never make it as far as the surface. Many times they are trapped beneath the surface by layers of rock that have formed above the sedimentary rock layer that produced them.
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This is a 3-D seismic computer image of a section of the Earth's surface that can be analized by a geologist to reveal possible traps. Source: NGSA
3-D Seismology
R ecently, computer technology has increased the value of seismic data by allowing geologists to construct what is known as a 3-D Seismic, or a three dimensional map of the rock layers beneath the surface. In order to create this type of detailed map, thousands of seismic measurements need to be taken. All of the data from these measurements is entered into a computer, which analyzes the data, and constructs its 3-D model using the information. Although this technology is costly, both for the acquisition of the seismic data and the computer hardware, the benefits of this kind of information are impressive. Using traditional techniques, the chances of finding gas and oil when drilling a well range from between one in five and one in ten. 3-D seismology can increase the odds considerably for an exploration company.
In addition to seismology, geologists sometimes rely on information regarding the magnetic properties of rocks to find gas and oil. Geologists get measurements of magnetic characteristics of the crust by using a device known as the magnetometer. This device is able to measure small changes in the earth's magnetic field at the surface, which indicates what kind of rock formations might be present underground. Originally, this technology was only mildly useful because the magnetometers were bulky and only small areas could be surveyed. However, with increasing technology, magnetometers can now be placed in helicopters, airplanes, and in 1981 NASA launched a magnetometer satellite, named Magsat.
- Even with all this advanced technology, the only way to be sure whether or not gas and oil are to be found at a certain site is to drill. However, these days, exploration companies are conducting as much research as possible prior to drilling because the costs associated with drilling are high, and they lose money when wells come up dry.
A field worker places geophones to retrieve data about the layers of rock beneath the surface of the Earth.
Source: API
SOURCES
Bill Gerger and Kenneth Anderson's Modern Petroleum: A Basic Primer of the Industry, 3rd Edition
Copyright 1992 by PennWell Publishing and,
Arlon R. Tussing and Bob Tippee's The Natural Gas Industry: Evolution, Structure, and Economics, 2nd Edition
Copyright 1995 by PennWell Publishing.Publications of the Natural Gas Council, Natural Gas Supply Association and Independent Petroleum Association of America.
- Even with all this advanced technology, the only way to be sure whether or not gas and oil are to be found at a certain site is to drill. However, these days, exploration companies are conducting as much research as possible prior to drilling because the costs associated with drilling are high, and they lose money when wells come up dry.