When drilling an oil well?

Question

When first breaking into the mantle when drilling for oil, if no hydrocarbons are detected, does this mean it will probably be a dry well?

Answer 1

Whoa! Back up just a bit. While we usually drill several thousand feet through the crust, oil wells never drill into the mantle. All oil is formed within the crust of the Earth. That said, I assume what you are really asking is whether you can tell if the well is dry when you first encounter the reservoir rock that is expected to contain oil. The answer is no. We don't allow the fluids in the formation (whether oil, gas or saltwater- one or the other will be filling the pore spaces of the rock) to come gushing out of the wellbore during drilling these days. A constant pressure is maintained inside the wellbore (using what we call drilling muds) to balance the pressure of the fluids in the rock and prevent this from happening. To determine if hydrocarbons are present measurements are taken during drilling or during and after drilling using what are called well logging tools.

Most wells now have tools on the drill string while the well is being drilled that give us information about the rock type we are drilling through and whether the fluid in the porous rock is salt water or oil or maybe gas. This is called logging while drilling. We measure various things like, for example, the resistivity of the rock. Oil and gas would show much higher resistivity to current than the salt water that fills the pore space normally. If you don't run the measurements while actually drilling then you get the information afterward by running what are called wireline logs. You actually can get much more information with this type and they are usually run in all exploration wells. These logs are measuring things like resistivity, density, how fast sound travels through the rock, how much gamma radiation they give off, and other things. This information can be used to tell what the rock type is, how porous it is, if the rocks contain hydrocarbon (oil or gas), and how well it is likely to flow. I also run what are called mudlogs on my wells sometimes. These are basically the notes of a geologist that collects samples from the mud and cuttings coming up the drill pipe as the well is drilling and analyzing them for traces of hydrocarbon (as well as describing the rock type). Mudlogs can give an early indication of the presence of hydrocarbons but I would never assume a dry hole based entirely on presence or absence of hydrocarbons in a mudlog.

Hope this helps answer your question