Petroleum Exploration

By Tedy Muslim Haq

Geophysical Engineering, Bandung Institute of Technology

Petroleum is the name for all hydrocarbon related, naturally occurring materials, including oil, natural gas, and tar. It is made up of hydrocarbon molecules (hydrogen combined with carbon). Oil is generally formed in young sedimentary rocks, especially the ones which have not undergone by metamorphic activities. Oil needs heat to be formed, but too much heat will make the oil get driven out.

There are four requirements to form an oil deposit; source rock, trapping mechanism, reservoir rock, and heating. The fluid in the deposit initially is a mixture of oil and water. During a time, because the oil is less dense than water, it separates and goes up to higher position in the reservoir rock. The gas also separates from the oil and takes the highest position available.

Recently, most petroleum resources are contained deep underground, so exploration relies heavily on drilling methods. In support for drilling, there are a variety of geophysical methods, including seismic and gravity.

The duty of petroleum explorer is to find the source rock sediment, reservoir rock sediment, and oil trap. The first method to start the exploration is earth surface mapping and interpreting. From this data, the possible locations of oil deposit can be guessed. The next step for exploration is gravity survey. Gravity surveys are performed to examine bedrock topography under the earth’s surface, map large metallic mineral deposits, and locate subsurface caverns and also contacts between geologic units of differing mass and density. It is a fairly complicated system but it is based on the premise that a target, oil reserve in this case, has a different density from the surrounding geology. Computer models are generated to depict the general area where such gravity changes occur. Once the gravity survey indicates an area where there are density differences, it is time to perform a seismic survey.

When the gravity survey has done and indicates an area that has differences density, seismic survey is performed. The seismic methods are the most widely used of all geophysical methods used in petroleum exploration. The main advantage is that it provides the most accurate rendition of the geometry of subsurface layers. Unfortunately the cost of seismic surveys is much greater than the cost of other types of geophysical surveys. Seismic methods measure seismic velocity of rock layers to detect both lateral and depth variations. The objective is to determine the lithology and geometry of the layers.

A seismic wave can be thought of as shock wave (elastic wave) or vibration traveling through the ground. The rate of travel, or velocity, of the wave is related to the density of the rock. There are two types of elastic waves produced: P-waves, which are primary or compression waves, and S-waves, or shear waves. The procedure used is to lay out a survey line with geophones set at equal spacing along the line. A shock wave is produced at one of the stations by dropping a heavy weight (seismic truck) or detonating an explosion (dynamite) at a shot point. The shot point is the point on the surface directly above the zone of interest. Ground motions caused by the explosion or impact are transmitted in the form of P-waves and S-waves. A seismic timer is used to measure the travel time of the wave from the instant it is generated until the time the wave reflection is detected back at the surface. Times are measured for each of the successive stations along the line. The return waves are measured by sophisticated computers traveling in front of the seismic truck or explosion site. The computer will calculate the timing of the returns and will then be able to draw graphs called, seismographs, of the topography and layers of the crust.

By examining these charts, expertise would be able to identify the traps. By identifying the traps one would be likely to identify an oil reservoir. Many seismographs can be manipulated by the computer to produce 3-D images of the rock formations. Extensive modeling can also be built, all by computer, based on the data that is returned from the seismic waves.

Geologic and geophysical clues are enticing, but drilling is the only way to learn if an oil or gas field really exists. Once a well is drilled, well logs yield data on the types of rock present and what fluids these rocks contain. The information interpreted from the logs is used to decision whether a well should be completed and used to produce oil and gas, or filled with cement and abandoned. The logs are also used to update the geologic models originally used to locate the well.

Petroleum exploration needs a wide earth science discipline such as geological surface mapping, structures, stratigraphy, and geophysical subsurface mapping including gravity, seismic interpretations, and well logging.