Where shall we drill?
Nonetheless, piles of highly processed seis-mic data, satellite images, high-tech well logs and computer-aided mapping programs can’t create oil or gas where none exists. The ex-plorer still must hunt for some basic clues. One of the most tried-and-true axioms of pet-roleum exploration is that the best place to look
for oil and gas is near where it’s already been found. Working in a known hydrocarbon basin eliminates many of the uncertainties of source, reservoir and seal, and the hunt can focus on lo-cation of possible traps.
The first phase of exploration is a search for traps similar to those that are already produc-ing. Explorers are also ever alert for possibili-ties of new trap types that have not yet been known to produce in an area, such as updip pin-chouts of permeability or fault traps. Oil and gas “shows” in previously drilled wells nearby are of supreme importance, be-cause they are direct indications that a trap ex-ists. Other important clues include changes in the rate of dip, dip reversal or flattening of dip of the rock layers. The location and position of faults is another key. These can indicate an un-usual structural condition. Seismic attributes offer other clues. Ampli-tudes of seismic signals contain tremendous ge-ological detail, and much effort is invested in interpreting these subtleties. One seismic signa-ture may show a promising gas-charged sand;
another may indicate a tight limestone bed with no commercial potential. Geologists and geophysicists formerly worked with pencils and paper maps and sec-tions; today they build complex, 3-D interpreta-tions
on their desktop workstations. The fundamentals of prospecting remain un-changed, but phenomenal trides have been made in an individual’s ability to view and inte-grate data from many sources.
The decision to drill
If, after carefully weighing all the data—and being fully aware of its varying degrees of reli-ability— an explorer still believes that an un-drilled trap does exist, a prospect has been born.exploration tools Maps have always been key tools of petro-leum exploration. Since the early 1900s, ex-plorers have found many il and gas fields by drilling domes and anticlines that could be identified from surface mapping. The size, po-sition, dips and strikes of surface beds are all recorded with instruments such as plane tables and runton compasses. Today, the map of an explorer includes this “traditional” information, as well as that gleaned from aerial photographs and satellite pictures. Early geologists made common use of sam-ple and drilling information from wells. As technology advanced, well log data and core data added a great deal of knowledge. Today, information from such instruments as formation sampling tools can assist in the evalu- ation of both rocks and fluids in a wellbore. Among wireline tools that provide valuable in-sights into the subsurface are electric, radioac-tive and acoustic logs, as well as dipmeters, borehole imaging logs and magnetic resonance imaging logs. Data from drillstem tests are also incorporated into a subsurface picture. Even geochemical techniques, which seek to corre-late surface measurements of various chemical compounds with the underground occurrence of hydrocarbons, are sometimes called into play.
Geophysicists also bring some tremen-dous tools to the trade of petroleum ex-ploration. Three common geophysical methods used to look for oil are magnetic, grav-ity and seismic exploration. Magnetic methods
measure the strength of the Earth’s magnetic field at a specific point on the surface, while gravity techniques seek to determine the strength of the Earth’s gravity at a location. Both methods are useful in reconnaissance mapping and are usually employed in the early stages of basin evaluation. Seismic is the real workhorse of the industry, however. In seismic prospecting, acoustic sources such as dynamite, vibrations or sonic impulses from compressed air transmit sound into the ground. As acoustic signals pass into the subsurface, they are reflected and refracted off the various sedimentary layers. Signals that
bounce back to the surface are recorded and processed to form an image of the subsurface. Two-dimensional (2-D) seismic yields a cross-sectional view of the subsurface in two planes, length and height, while a 3-D survey delivers a complete volume of data that allows the explorer to image the subsurface in fine de-tail. A further enhancement is 4-D seismic, which adds the dimension of time to the geo-physical
process. In this technique, successive
for oil and gas is near where it’s already been found. Working in a known hydrocarbon basin eliminates many of the uncertainties of source, reservoir and seal, and the hunt can focus on lo-cation of possible traps.
The first phase of exploration is a search for traps similar to those that are already produc-ing. Explorers are also ever alert for possibili-ties of new trap types that have not yet been known to produce in an area, such as updip pin-chouts of permeability or fault traps. Oil and gas “shows” in previously drilled wells nearby are of supreme importance, be-cause they are direct indications that a trap ex-ists. Other important clues include changes in the rate of dip, dip reversal or flattening of dip of the rock layers. The location and position of faults is another key. These can indicate an un-usual structural condition. Seismic attributes offer other clues. Ampli-tudes of seismic signals contain tremendous ge-ological detail, and much effort is invested in interpreting these subtleties. One seismic signa-ture may show a promising gas-charged sand;
another may indicate a tight limestone bed with no commercial potential. Geologists and geophysicists formerly worked with pencils and paper maps and sec-tions; today they build complex, 3-D interpreta-tions
on their desktop workstations. The fundamentals of prospecting remain un-changed, but phenomenal trides have been made in an individual’s ability to view and inte-grate data from many sources.
The decision to drill
If, after carefully weighing all the data—and being fully aware of its varying degrees of reli-ability— an explorer still believes that an un-drilled trap does exist, a prospect has been born.exploration tools Maps have always been key tools of petro-leum exploration. Since the early 1900s, ex-plorers have found many il and gas fields by drilling domes and anticlines that could be identified from surface mapping. The size, po-sition, dips and strikes of surface beds are all recorded with instruments such as plane tables and runton compasses. Today, the map of an explorer includes this “traditional” information, as well as that gleaned from aerial photographs and satellite pictures. Early geologists made common use of sam-ple and drilling information from wells. As technology advanced, well log data and core data added a great deal of knowledge. Today, information from such instruments as formation sampling tools can assist in the evalu- ation of both rocks and fluids in a wellbore. Among wireline tools that provide valuable in-sights into the subsurface are electric, radioac-tive and acoustic logs, as well as dipmeters, borehole imaging logs and magnetic resonance imaging logs. Data from drillstem tests are also incorporated into a subsurface picture. Even geochemical techniques, which seek to corre-late surface measurements of various chemical compounds with the underground occurrence of hydrocarbons, are sometimes called into play.
Geophysicists also bring some tremen-dous tools to the trade of petroleum ex-ploration. Three common geophysical methods used to look for oil are magnetic, grav-ity and seismic exploration. Magnetic methods
measure the strength of the Earth’s magnetic field at a specific point on the surface, while gravity techniques seek to determine the strength of the Earth’s gravity at a location. Both methods are useful in reconnaissance mapping and are usually employed in the early stages of basin evaluation. Seismic is the real workhorse of the industry, however. In seismic prospecting, acoustic sources such as dynamite, vibrations or sonic impulses from compressed air transmit sound into the ground. As acoustic signals pass into the subsurface, they are reflected and refracted off the various sedimentary layers. Signals that
bounce back to the surface are recorded and processed to form an image of the subsurface. Two-dimensional (2-D) seismic yields a cross-sectional view of the subsurface in two planes, length and height, while a 3-D survey delivers a complete volume of data that allows the explorer to image the subsurface in fine de-tail. A further enhancement is 4-D seismic, which adds the dimension of time to the geo-physical
process. In this technique, successive
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