Changes

A large amount of data is available to the production geologist for reservoir evaluation. Much of the data will have been expensive to acquire, particularly if obtained from wells offshore. For instance, core taken from a drilling operation on an offshore drilling rig may have cost more than a million dollars to recover. There is an obligation to take good care of the data and to make sure that the information is accessible, either as well-organized paper data files or as data on a computer shared drive. Data files stored on a computer should be labeled with the originator's initials, a date, and some idea of the significance of the data, e.g., "MS August 31, 2008, final top reservoir depth map." Well files should be compiled with all the available data collected on a well-by-well basis. Good data management can make all the difference between a project that is well organized and effective, and one that is disorganized and inefficient.

A large amount of data is available to the production geologist for reservoir evaluation. Much of the data will have been expensive to acquire, particularly if obtained from wells offshore. For instance, core taken from a drilling operation on an offshore drilling rig may have cost more than a million dollars to recover. There is an obligation to take good care of the data and to make sure that the information is accessible, either as well-organized paper data files or as data on a computer shared drive. Data files stored on a computer should be labeled with the originator's initials, a date, and some idea of the significance of the data, e.g., "MS August 31, 2008, final top reservoir depth map." Well files should be compiled with all the available data collected on a well-by-well basis. Good data management can make all the difference between a project that is well organized and effective, and one that is disorganized and inefficient.

Obtaining data in an oil field environment is expensive; therefore, it is necessary to justify the economics of gathering the information. In the early stage of field life, the value of information is enormous; the data are essential for reservoir evaluation. Later on in field life, it becomes more important to justify the expense of the data. The new information should be gathered on the basis that it significantly improves the project value and reduces the company's investment risk.<ref name=Gerhardtandhaldorsen_1989>Gerhardt, J. H., and H. H. Haldorsen, 1989, On the value of information: Presented at Offshore Europe, Society of Petroleum Engineers, September 5–8, Aberdeen, United Kingdom, [https://www.onepetro.org/conference-paper/SPE-19291-MS SPE Paper 19291], 11 p.</ref>

Obtaining data in an oil field environment is expensive; therefore, it is necessary to justify the [[economics]] of gathering the information. In the early stage of field life, the value of information is enormous; the data are essential for reservoir evaluation. Later on in field life, it becomes more important to justify the expense of the data. The new information should be gathered on the basis that it significantly improves the project value and reduces the company's investment risk.<ref name=Gerhardtandhaldorsen_1989>Gerhardt, J. H., and H. H. Haldorsen, 1989, On the value of information: Presented at Offshore Europe, Society of Petroleum Engineers, September 5–8, Aberdeen, United Kingdom, [https://www.onepetro.org/conference-paper/SPE-19291-MS SPE Paper 19291], 11 p.</ref>

==Types of data==

==Types of data==

* grain sorting

* grain sorting

* induration

* induration

* lithofacies

* [[lithofacies]]

* fractures, faults, and other structural features

* fractures, faults, and other structural features

[[File:M91Ch6FG48.JPG|thumb|300px|{{figure number|9}}Seismic boat and streamers (courtesy of Woodside Petroleum, Web site: www.woodside.com.au, whose permission is required for further use).]]

[[File:M91Ch6FG48.JPG|thumb|300px|{{figure number|9}}Seismic boat and streamers (courtesy of Woodside Petroleum, Web site: www.woodside.com.au, whose permission is required for further use).]]

Recording devices on land consist of arrays of connected geophones laid out in long lines. At sea, hydrophones are strung together within a long plastic sheath known as a streamer. The streamer can be several kilometers long. At the end of the 20th century, a streamer was typically 3500–4000 m (11,500–13,000 ft) long. The trend today is for increasingly longer cables to allow a greater distance between the source and the furthest hydrophone on the streamer (known as the far offset, the distance between the source and the nearest hydrophone being known as the near offset). This greater distance allows for better discrimination of the variation in the recorded amplitudes for a given reflector with increasing offset, a technique known as amplitude versus offset or AVO. This can be helpful in determining whether hydrocarbons are present at a given location (Russell, 2002). Several sources and several streamers can be towed behind the seismic boat at one time ([[:file:M91Ch6FG48.JPG|Figure 9]]).

Recording devices on land consist of arrays of connected geophones laid out in long lines. At sea, hydrophones are strung together within a long plastic sheath known as a streamer. The streamer can be several kilometers long. At the end of the 20th century, a streamer was typically 3500–4000 m (11,500–13,000 ft) long. The trend today is for increasingly longer cables to allow a greater distance between the source and the furthest hydrophone on the streamer (known as the far [[offset]], the distance between the source and the nearest hydrophone being known as the near offset). This greater distance allows for better discrimination of the variation in the recorded amplitudes for a given reflector with increasing offset, a technique known as [[amplitude versus offset]] or AVO. This can be helpful in determining whether hydrocarbons are present at a given location (Russell, 2002). Several sources and several streamers can be towed behind the seismic boat at one time ([[:file:M91Ch6FG48.JPG|Figure 9]]).

Land and marine acquisition techniques differ slightly but in principle are mostly the same. The following describes the marine case. A seismic boat acquires data by sailing as carefully as it can along a predetermined line over the area of interest. When it reaches the end of this line, it turns around and acquires data along a parallel line in the opposite direction. The boat will steam back and forth line after line acquiring the seismic survey for up to months at a time depending on how large an area is to be acquired and on the weather conditions. The boat travels slowly along the predetermined line, and periodically (every 12.5 m [41 ft] or perhaps every 25 m [82 ft]) discharges the airgun. The point on the line where this occurs is known as a shotpoint. The hydrophones then record the reflection echoes from the subsurface. Simultaneously, compressors will recharge the airgun ready for the next discharge, and the process repeats over and over again. The result is a record of a large number of shot and receiver pairs for each reflection point in the subsurface. The data are recorded digitally and will include the time it takes for the seismic pulse to return to the surface, the waveform of the seismic signal, and the sound and source location. The time that the seismic energy takes to travel from the source to the reflection and back to the surface again is called the two-way traveltime (TWT). This can take 2–3 s or more. Because of the rapid velocity of seismic waves through the subsurface, seismic intervals are measured in milliseconds; 1000 ms equals 1 s.

Land and marine acquisition techniques differ slightly but in principle are mostly the same. The following describes the marine case. A seismic boat acquires data by sailing as carefully as it can along a predetermined line over the area of interest. When it reaches the end of this line, it turns around and acquires data along a parallel line in the opposite direction. The boat will steam back and forth line after line acquiring the seismic survey for up to months at a time depending on how large an area is to be acquired and on the weather conditions. The boat travels slowly along the predetermined line, and periodically (every 12.5 m [41 ft] or perhaps every 25 m [82 ft]) discharges the airgun. The point on the line where this occurs is known as a shotpoint. The hydrophones then record the reflection echoes from the subsurface. Simultaneously, compressors will recharge the airgun ready for the next discharge, and the process repeats over and over again. The result is a record of a large number of shot and receiver pairs for each reflection point in the subsurface. The data are recorded digitally and will include the time it takes for the seismic pulse to return to the surface, the waveform of the seismic signal, and the sound and source location. The time that the seismic energy takes to travel from the source to the reflection and back to the surface again is called the two-way traveltime (TWT). This can take 2–3 s or more. Because of the rapid velocity of seismic waves through the subsurface, seismic intervals are measured in milliseconds; 1000 ms equals 1 s.