Evaluating Cuttings
To help the operator make his decision, several techniques have been developed. One thing that helps indicate whether hydrocarbons have been trapped is a thorough examination of the cuttings brought up by the bit. The mud logger or geologist (Remember him? He’s been there all along, monitoring downhole conditions at the location.) catches cuttings at the flow ditch and by using a microscope or ultraviolet light can see whether oil is in the cuttings. Or he may use a gas-detection instrument.

Well Logging
Another valuable technique is well logging. A logging company is called to the well while the crew trips out all the drill string. Using a portable laboratory, truck-mounted for land rigs, the well loggers lower devices called logging tools into the well on wireline. The tools are lowered all the way to bottom and then reeled slowly back up. As the tools come back up the hole, they are able to measure the properties of the formations they pass.

Electric logs measure and record natural and induced electricity in formations. Some logs ping formations with sound and measure and record sound reactions. Radioactivity logs measure and record the effects of natural and induced radiation in the formations. These are only a few of many types of logs available. Since all the logging tools make a record, which resembles a graph or an electrocardiogram (EKG), the records, or logs can be studied and interpreted by an experienced geologist or engineer to indicate not only the existence of oil or gas, but also how much may be there. Computers have made the interpretation of logs much easier.

Coring
In addition to these tests, formation core samples are sometimes taken. Two methods of obtaining cores are frequently used. In one, an assembly called a "core barrel" is made up on the drill string and run to the bottom of the hole. As the core barrel is rotated, it cuts a cylindrical core a few inches in diameter that is received in a tube above the core-cutting bit. A complete round trip is required for each core taken. The second is a sidewall sampler in which a small explosive charge is fired to ram a small cylinder into the wall of the hole. When the tool is pulled out of the hole, the small core samples come out with the tool. Up to thirty of the small samples can be taken at any desired depth. Either type of core can be examined in a laboratory and may reveal much about the nature of the reservoir.