Oil and gas explorers begin by examining the local geology. They assess if it is likely to have the kinds of rocks that can produce oil and gas and can form reservoirs that can hold oil and gas.
They then use survey technology, such as seismic surveys, to detect whether the rocks are likely to contain oil and gas deposits and how large these deposits are likely to be.
Explorers generate seismic (sound) waves and measure the time taken for the waves to travel from the source, reflect off subsurface features and be detected by receivers at the surface.
The time taken to travel from the source to the receivers can indicate features such as rock density and the likely presence of fluids or gases. This can help build an image of the subsurface.
In onshore operations, seismic is often followed by the drilling of coreholes.
Coreholes between 10cm and 30cm across are used to extract core samples of rock strata. These samples enable measurements of gas content, rock permeability, thickness of the reservoirs and other information. Core holes simply acquire rock samples; they are not wells and they cannot extract oil or gas.
If interpretation of survey results shows it is likely that oil and gas deposits exist in a particular area, an exploration well can be drilled. But even positive survey results do not guarantee success.
During and after the drilling of an exploration well information is acquired in various ways, including:
- acquiring core (rock) samples
- examining rock cuttings brought to the surface in the circulating drilling fluid
- lowering specialised logging tools into the wellbore.
These tests give a clearer picture of whether oil or gas is present and if it can be commercially recovered.