Oil and Gas Subsurface Geology

In Oil and Gas Subsurface Geology lesson, we’ll discuss the following:

Understanding Subsurface Geology
Onshore Seismic Surveys
Thumper Trucks
Seismic Shot Point Base Map
Offshore Seismic Survey

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Understanding Subsurface Geology

Once the aerial survey work finds a play with geological promise, the next step in an exploration program is to develop a more detailed understanding of the subsurface geology.

Seismology began with the desire to understand the destructive nature of large earthquakes. Seismologists soon learned that the seismic waves produced by an earthquake also contained valuable information about the large-scale structure of the earth’s interior.

Today, much of our understanding of the earth’s mantle, crust, and core is based on the analysis of the seismic waves produced by earthquakes. Thus, seismology became an important branch of geophysics, the physics of the earth.

Onshore Seismic Surveys

The underlying concept of seismology applied to oil and gas exploration is simple. Man-made seismic waves are just sound, or acoustic, waves.

These sound waves leave the seismic source and travel downward into the earth. As they encounter changes in the earth’s geological layering, echoes (or reflections) travel upward to the surface.

Electromechanical transducers called geophones or hydrophones detect the echoes arriving at the surface and convert them into electrical signals, which are then amplified, filtered, digitized, and recorded.

Where there is no reflecting stratum, seismic is useless. The sound waves keep on going into the earth until they deteriorate.

The recorded seismic data then undergo elaborate computer processing to produce images of the earth’s shallow structures, similar to the ultra-sound used to create pictures of unborn babies.

An experienced geologist or geophysicist can interpret these images to determine what type of rocks are represented and whether or not the rocks might contain valuable resources.

Thumper Trucks

Thumper trucks are 30,000 pound vehicles. They generate vibrations under the ground by elevating themselves above the ground on a short pole, thus concentrating their entire weight on a platter and “shaking” for several seconds per location.

This process is very precise as it uses controlled vibrations that are spread over period of time as opposed to the explosion vibration that is just a giant burst of energy.

Vibrator trucks are also safer than explosives and are able to operate inside populated areas.

Seismic Shot Point Base Map

To conduct an onshore survey, a seismic crewfirst sets out a number of shot holes and a carefully patterned array of geophones. The geophones detect the reflected sound waves and transmit the data to recorders.

Land seismic crews often contend with rough, mountainous terrain, jungles and desert conditions, population centers and permafrost when “shooting seismic.” Because of these difficulties, land seismic acquisition costs are generally higher per kilometer than the cost of acquiring marine data.

Raw seismic data can be reused many times and over a long period. Thanks to modern reprocessing with computer interpretation software that “sharpens” the data, seismic data recorded in the 1960s is still being used for exploration.

Offshore Seismic Survey

In offshore seismic surveying, the choice of equipment depends upon the accuracy required and distance offshore.

Explosives are not used offshore as an energy pulse. Various types of air guns are used and the signature of the source is known before a survey begins.

Offshore surveys’ costs and data quality are all affected by the number of cables or streamers towed. For both 2-D and 3-D lines, the length and group interval of the streamers are important. Long streamers at wide intervals improves interpretation accuracy by reducing the amount of data/location distortion due to the ship itself and any wave action.

Marine crews must constantly pay attention to their latitude and longitude locations to ensure accurate data readings.

Satellite navigation or onshore and offshore range equipment must be used to carefully position the ship. The GPS satellites that are common today have greatly aided with this.

The cost of both marine and land acquisition is influenced by the mobilization and demobilization costs of the seismic crew to the survey area and the cost of the capital equipment. For large surveys, marine acquisition costs-per-kilometer tend to be lower because, once the vessel is in the survey area, a larger amount of data can be recorded per day.