6.3.2 Sample Examination Procedure for Hydrocarbon Shows

It is a good practice, to examine both unwashed and the sample for lithological evaluation under the UV box. This will allow you to take note of the background fluorescence. In practice, this is rarely done due to time constraints.

One of the first signs of an entry into a possible hydrocarbon-bearing zone would be a drilling break. This is an increase in ROP due to the porous nature of the sediment drilled. On most wells, the drilling program would call for a flow check and a CBU when two to five meters into the drilling break. CBU is circulating until all the samples are recovered and evaluated.If there is a significant gas peak, a spot sample should be taken and examined under the UV light. The spot sample should include an unwashed sample, lithological sample and a mud sample.

The mud logger must get all the relevant chemicals and tools ready before drilling a potential hydrocarbon zone.

Below is a typical flow of sampling procedures that are done when hydrocarbon showsare encountered.

1. Whenever there are significant gas shows, the mud logger must get a mud sample from either the flowline or the possum belly, aside from the regular sample or bottoms up sample. If the significant gas peak arrives in between sampling intervals, a spot sample is caught along with a mud sample.

2. The mud sample is poured into a shallow dish and placed under UV light to observe the signs of fluorescence in the mud or droplets of immiscible oil popping to the surface. If nothing is seen, water is added to the mud and the mixture is stirred. Again the sample is observed under UV light.

3. The unwashed sample is also observed under UV light. The distribution, color, and intensity of the fluorescence, if any, is noted.

4. For the lithological samples, smell the sample first before observing it under the microscope. If an oil odor is detected record it (see section on Odor). Under the microscope, an estimate is made of the percentage of oil-stained cuttings. The appearance (oily, waxy, dry residue), distribution (even, patchy, spotty, streaks) and color of the oil stains will be described. If oil-stained cuttings are observed these can be separated and placed into the depressions of the spot plate.

5. The sample tray is then observed under UV light. The proportion of the cuttings fluorescing and its distribution (even, spotty, pinpoint, patchy, and streaky), color and intensity of the fluorescence, if any, is noted. Some samples, if none were selected in Step 4, are placed into the depressions of the spot plate.

6. The samples in the spot plate are observed under the microscope for any oil stains. This is also to verify the lithology of the sample that will be subjected to the solvent cut test.

7. If no stains were observed, the spot plate is put inside the UV box and the samples are subjected to a solvent cut test.