Introduction:
In response to the many readers' requests for more technical information on oil and gas discovery, 'Shell in the Middle East' focuses on the different methods of seismic surveying. We talk to Roar Bekker, Geophysicist with Schlumberger's Seismic Reservoir Evaluation Division - one of Shell's long-standing contractors - who gives an overview of the use of seismic surveys to locate and identify oil and gas reserves on land and at sea.
We then focus on three current surveys being undertaken for Shell - in the NEMED (North East Mediterranean Deepwater) Concession in Egypt, on land for PDO in Oman and in the relatively shallow offshore fields of Nowrooz and Soroosh in Iran.
"Seismic survey is a process which is used to map the sub-surface geology of an area. Its purpose is to produce an image of the structures and strata’s which lie either beneath the sea or the ground," says Roar Bekker, Geophysicist with Schlumberger's Seismic Reservoir Evaluation Division.
The company is also known as GECO-Prakla, and it is this company which carried out the 3D seismic survey - the largest 3D seismic survey ever undertaken for Shell - for Shell Egypt NV's NEMED (North East Mediterranean Deepwater) Concession, using a towed streaming array method of survey.
"The information collected from seismic survey provides us with a topographical map, which is then used, in the oil and gas industry to identify structures which may contain hydrocarbon deposits of either oil or gas, or both.
"The way seismic data is collected, or to use the industry term 'acquired', is by the transmission of sound waves from an acoustic source into the ground," he says. "The acoustic source produces sound waves, which travel down into the earth's sub-surface. They are then reflected back up to the surface from each geological layer or boundary they come into contact with.
"At the surface, these reflected sound waves are recorded either by geophones or hydrophones. If the seismic survey is being carried out on land, then geophones are used. If the survey is carried out in the sea, from the sea surface, hydrophones are used.
However, both hydrophones and geophones are used if the marine survey requires the use of ocean bottom cables [OBCs], whereby the cables are laid on the sea floor rather than towed behind a vessel in a streaming array some six to 10 meters below the sea surface.
"In a typical seismic survey for oil or gas, an acoustic source would transmit sound waves to penetrate the earth from the surface down to a depth of some 4,000 to 5,000 meters or more. Depending on the reservoir depths [gas is typically found deeper than oil], the seismic acquisition unit would normally use a more powerful acoustic source to acquire data from deeper targets.
"The reflected sound waves, once recorded, are then processed and converted. With 2D seismic, they are processed and converted into sub- surface profiles, or vertical slices, of the sub-surface geology. With 3D seismic, the results produce a 3D image in the form of a cube, which produces a volumetric representation of the sub-surface geology."
He goes on to explain, "A 2D seismic survey is more commonly used in early oil and gas exploration programmers to facilitate the early identification of prospects in new or immature exploration areas.
It is also used to identify areas suitable for the more detailed, and more expensive, 3D seismic survey, which is used to improve the accuracy of the information acquired by 2D and to provide a much more detailed profile.
"Whilst 3D seismic was first introduced in the 1970s, it was not until the 1980s that it became widely used in the marine environment. The acquisition of seismic data at sea requires the use of a streamer, which is towed behind a vessel.
It was only in the 1980s that a handling system was developed to facilitate the acquisition of 3D seismic data at sea using several towed streamers simultaneously rather than a single streamer, and this obviously improved quality and reduced costs.
"On land, 3D seismic acquisition still remains both more difficult and expensive than 2D seismic acquisition. However, the great advantage of using 3D seismic surveying is that it saves time and money in the exploratory drilling stages of oil and gas field development.
Global industry figures show that, in fact, the success rate from drilling wells using 3D seismic data is more than twice that of 2D seismic and can be as high as 50 per cent or more in some hydrocarbon basins.
"Today, 3D seismic survey is also being used extensively in oil and gas field development where companies are looking for smaller finds and where greater drilling accuracy is required.
The improved quality of the data acquired from a 3D seismic survey can be used to map structures, faults and individual geological sequence boundaries, and this enables interpreters to map and estimate the volume of the reservoirs in more detail.
"So, basically, we first use the data required to map various structures, layers and faults to build up an overall picture. From this, we can go on to identify potential traps of oil or gas, and then further identify the presence of hydrocarbons.
"Many of the old and major oil fields in the world were basically discovered because there were leakages of oil from the reservoirs below the surface, which manifested themselves in the form of tar sands.
However, in the case of the UAE's Zakum Oil Field - one of the largest offshore oil fields in the world, with reserves of some 50 billion barrels of oil - hydrocarbons seeped into the sea. Whilst this oil field, and other major fields, was discovered without the use of seismic surveying, many are now undergoing detailed seismic 3D survey to map the sub-surface geology in order to optimize production.
"Seismic data can also assist geologists in identifying rocks and their properties to enable a more accurate model of the reservoir to be developed. The porosity of different rocks varies, as does their ability to absorb or inhibit the flow of oil.
By studying the sub-surface geology, geologists can help to optimize recovery. When seismic data is used along with the logging data, which is acquired when a well is drilled, the integration of seismic data and borehole log information can greatly enhance the overall picture of an oil or gas reservoir.
"Seismic survey has its limitation," he concludes, "and the only way to confirm the presence of oil and gas is to actually drill for it. What seismic survey does do, however, is greatly increase the success rate - and this, after all, is surely what it's all about."
Deepwater in Egypt:
"The Egyptian Government awarded the NEMED [North East Mediterranean Deepwater] Concession to Shell Egypt in 1998," says Joost Soethout, Geophysics Manager for Shell Egypt. "It covers 41,500 square kilometers of the southeast Mediterranean in an area offshore the Nile Delta, and by any standards, it is a large concession in the oil and gas industry.
"We began with a regional 2D seismic survey of the whole concession in early 1999. This took three months and was acquired using a towed seismic array method.
It uses a single mother ship, which lays out behind it a single streamer of six kilometers, towed seven meters below the sea surface. A series of hydrophones are built into the streamer at regular intervals.
The ship then travels up and down the survey area along pre-determined lines, shooting compressed air guns five meters below the sea surface. The high pressure air released by the guns causes sound waves to travel down from the surface of the sea to the sea bed and on into the sediment layers below.
"The depths of water in which the 2D survey took place ranged from 800 to 2,800 meters but the sound waves from the air guns penetrated to as much as five kilometers down into the earth's surface.
"The sound waves were continuously reflected from both the sea bed and the different layers below the surface of the sea bed. They were recorded by hydrophones in the seismic streamer and the data was transmitted to the vessel. It was then recorded on magnetic tapes for further processing on board the vessel before being transmitted by satellite directly to the offices of Shell in Cairo.
"We finished the 2D survey in May 1999, then spent three months interpreting the data. The purpose of the survey was to map the depth of different layers and define prospects in order to pinpoint the best sector of the concession in which to carry out a 3D seismic survey.
"The 2D survey enabled us to get a vertical slice through the sub- surface of the ocean floor. It is much faster and cheaper to obtain than 3D seismic information, and you get an overview of a very large area.
However, 2D is less detailed and you can miss large prospects. In the central part of the concession, we acquired a grid of 2D seismic lines with a spacing of eight by eight kilometers. This meant that we had no information on what was happening in the sub-surface between those lines.
"Following the interpretation of the 2D seismic data, we selected an area of 7,000 square kilometers. That area is in the southern part of the concession due north of the Nile Delta. It measures 160 kilometers in an east-west direction and 50 kilometers north south.
"The 3D seismic survey [carried out by GECO-Prakla] used a vessel which towed an array of six streamers, each six kilometers long. They were towed 100 meters apart and two sets of compressed air guns were used instead of one.
The vessel then bisected the survey area in closely spaced parallel lines. Using system, we were able to obtain a cube of the sub-sea sediment layers, which gave us a very detailed picture of the geology down there.
"One of the problems we had was the presence of sharks. They were chewing our streamers, which had to be repaired constantly. Another problem with these long streamers is that they are not visible to other vessels, so we had to use a guard vessel to warn other vessels not to cross them.
"We began the 3D seismic survey in July 1999 and continued until March 2000. The interpretation of that data is under way and has confirmed the likely presence of hydrocarbons. The data quality has been good and we believe we can see the difference between potential oil and gas fields.
However, only the drilling of these prospects will uncover the truth. The next stage will be the preparation of the first few well proposals, and drilling should begin by early 2001," he concludes.
Source:Shell.com/me
© 2000 Mena Report (www.menareport.com)
