In order to locate geological features that are advantageous for oil and natural gas accumulations, different types of evaluations may be conducted and the most common of these, during the early stages of exploration, is geophysical seismic.
Sound waves are transported at different speed in different materials and, at the transition between different materials, they partly bend and reflect back to the surface. Since rocks have different compositions, it is possible based on variations in the speed of the sound wave and angle, to estimate the location of features that could hold oil and/or natural gas reserves. The two-dimensional, or 2D seismic provides data along two axes, length and depth, while the three-dimensional, or 3D seismic is done across multiple lines simultaneously and the third dimension of width is gained. 3D-seismic offers much greater den sity of information about the subsurface but is much more costly and covers a smaller area.
Features which are deemed to have the potential to contain oil and natural gas are called prospects and the presence of oil and natural gas must be confirmed by drilling. During the drilling, rock and drilling fluid are recovered at the surface and evaluated during the drilling of the well. At the completion of drilling, the hole is logged.
Logging can provide information with respect to the physical properties of the rocks penetrated by the drilling operation. Once interpreted by experienced analysts, a determination can be made about rock porosity and about the content of any substances (oil, natural gas or water) contained within the porosity.
If the analysis of the drilled rocks and the logging shows positive indications, a production test of the drilled hole is executed, whereby potential oil and natural gas zones are allowed to flow into the hole and up to the surface for measurement and analysis. Both the production rate and the amount of reserves can be calculated through logging and testing.