Oil and Gas Exploration

Once Company has obtained requires the right over a certain field, exploration activities begin as per Minimum Work Program as outlined in the agreement at time of bidding. This generally included seismic surveys, remote sensing, few wild cat wells etc.

Usually, there are these few techniques are applied for exploration-

1.   Gravity and Magnetic Surveys

2.   Remote sensing

3.   Seismic

4.    Wild cat wells

1.   Gravity and Magnetic Surveys

Gravity surveys are based on difference in the gravity of the material. Based on the concept that different material will have different density and thus will have a different gravitational pull. For example gravitational pull from igneous rocks will be more than that of sedimentary rocks.

In magnetic survey magnetic properties of rocks are exploited. In general basement and igneous rocks are relatively highly magnetic. This helps in distinguishing subsurface sedimentary rocks from others.

2.   Remote Sensing

Remote sensing is done through aerial satellites. This looks for any seepage on the surface, any out crop of reservoir rocks, faults etc. Even these days Gravity and magnetic surveys can be done satellites.

3.   Seismic

This includes 2D, 3D seismic survey. Seismic surveys are based on the concept of sound waves generated from source on the surface and reflected back from sub surface layers. The two main things measured in seismic is velocity of the waves and density of the rocks. Geophysicts these days look for Acoustic Impedance (AI) which is the product of density of rock and velocity of wave passing through it.

Seismic data is collected by blasting in the ground to create sound waves. The reflected sound waves are collected by devices known as Geophones. In sea Air gun is used to create sound waves and Hydrophones collect the reflected sound waves form the sub surface. It is worthwhile to note that cost of seismic survey on land is 4-5 times than that on the open sea.

3D surveys are more expensive than 2D surveys. But 2D surveys cover large area compared to 3D surveys. So generally on a virgin land first 2D survey is done and then 3D is done where there is strong potential for hydrocarbon.

4D seismic surveys are done mainly on mature fields. A fourth factor i.e. time is introduced. 4D seismic survey is 3D survey taken over a period of time (generally 1 year). 4D gives information about the movement of fluid in the reservoir and how reservoir is behaving over a period of time.

4.   Wild Cat well

A wild cat well is drilled in an area where there is no or little previous knowledge of sub surface. Geologist, based on initial seismic, magnetic and gravity analysis, release the location of 3-4 drilling wells in the area. With drilling exact sub surface information is obtained through coring and analyzing the cuttings from the bottom.

Seismic Interpretation

Now days geophysict and geologist prefer reading the seismic data in terms of acoustic impendence (AI). The geo scientist stich the data obtained from all the above mentioned sources with the help of softwares such as PETRAL, LANDMARK etc to draw the first picture of the oil field. This picture is further refined as more information pours in.