How to know if oil or gas has been found?

It is the role of the geologist based permanently at the drilling site. There exist several ways of knowing if a geological layer impregnated with hydrocarbons has been found.
The direct methods allow the presence of hydrocarbons to be confirmed. The gas dissolved in the mud is analysed continuously. When a rock is drilled, small quantities of methane gas are always liberated. It is what is called the gaseous background. If it is stable or it remains proportional to the rate of progress of the drilling, then everything is normal. But if it rises suddenly it is a signal to be on the alert! It is possible that one has entered a hydrocarbon reservoir.
The geologist is also constantly examining the drilling spoils. He washes them to remove the impregnated mud and passes them under an ultraviolet lamp. If the spoils contain hydrocarbons, they will emit an orange fluorescence. If the geologist thinks that the well is going through a reservoir, he can ask for a core-sample. In this case the tube string is lifted and the drilling tool replaced by a core-sampler. The tube string is lowered again and drilling continues but this time without grinding the rock: the sampler gently cuts out and “swallows” a cylinder of the rock that is raised once it is filled. The core-sample is very useful: one can see if it contains hydrocarbons, the reservoir characteristics can be directly measured and it can be used as a support to indirect methods of evaluation.

The direct methods allow us to know if there are hydrocarbons in the rocks being drilled through, but they don’t give sufficient indications of the quantities that are present. For that, indirect methods exist, based on logging tools. These are tools that are lowered to the bottom of the borehole by cable. Then they are lifted slowly back up and on their way record certain data. It is by analysing the recorded data that the characteristics of the reservoirs and the proportions of gas or petrol contained in the pores of the rock can be defined.
A seismograph is also lowered into the borehole, allowing a vertical seismic profile to be made that is then used to support the seismic sections taken around the well being drilled.
Finally, there exists another direct method of hydrocarbon detection, a test that can be carried out: a pressure reduction is applied to a vertical interval where an oil or gas reservoir is thought to exist. The fluids contained in the rock are precipitated into the borehole and sometimes even climb back up to the surface. In all cases, they can be recovered and analysed.