Feature Report: Exploring for Oil and Gas Deposits

6 items of content in this feature report

Going in depth


Observations and Measurements During Drilling

The purpose of drilling is to confirm the presence of oil and/or gas in a prospect. This can be done continuously, by using various qualitative and quantitative methods that range from direct observations of drill cuttings to more elaborate measurements.

Carothèque située à Pau
Total's core sample library in Pau, France conserves geological core samples for future reference. © M.DUFOUR - TOTAL

Direct Methods

While a site is being drilled, geologists use a variety of “direct” methods to detect the presence of preservation (hydrocarbons)The final phase in petroleum system formation, after a deposit has accumulated... :

Indirect Measurements or Logs

Drilling involves an array of observations and measurements before a development decision is made.

But direct methods do not provide enough quantitative data on the hydrocarbons present in the rock or the physical properties of the reservoir, which are essential to determine whether the depositAn accumulation of natural resources, such as oil, natural gas, coal, uranium, metal ore or another commodity... can be developed. So-called indirect exploration methods are used to measure these parameters.

Indirect exploration is known as wireline logging and it involves taking measurements after each drilling phase, before the casing tube is inserted. These measurements provide information on:

  • The type of rock encountered (lithology and speed at which sound travels through the rock).
  • The porosity of the reservoir rock, i.e. the proportion of empty space within the rock. This information is important, because the oil and gas is contained within these spaces.
  • The saturation of the reservoir rock encountered, in other words the relative proportion of water and hydrocarbons present in the reservoir (there is always a certain amount of residual water in a hydrocarbon reservoir).
  • Fluid temperature in the different reservoirs and the natural radioactivity of the rock, which indicates the quality of some reservoirs.

All this data is essential to ascertain whether the deposit can be produced and make the decision to launch development.