When using drill cuttings in order to determine formation lithology, representative sampling from all shale shaker screens, desander and desilter is essential. Procedures for correctly doing this and for correct washing and processing of samples are fully discussed in the Field.
Representative samples of rock cuttings from retrieval point (shale shaker, desander, etc.) and from the washed and sieved cuttings are of prime importance.
The geologist should always be aware of what size and type shale shaker screen is in use, when the screen is changed and when desilters or desanders are being used. By knowledge of the “cut” at each retrieval point and inspection of all samples and sieved samples of barite and drilling clays, it is possible to establish the presence and type of contaminants and drilling solids. This establishes a background against which future cuttings and formation of detrital material may be judged.
Figure 1 indicates standard Wentworth particle and relative mesh sizes for shale shaker screens and sieves. In using the chart for comparison, remember that, unless the formation is unconsolidated or very poorly cemented, the particle size within the rock fabric will be less than the size of the recovered cuttings.
Take at least a pint of cuttings in addition to that needed to fill sample sacks. If the sample contains a large proportion of unconsolidated clay and the oil company wants this washed out of washed and dried samples, a larger quantity of cuttings will be required in order to provide reasonable quantities of washed and dried sample.
If the shaker is the “single-deck” type (Figure 2), take separate samples from all three screens. If it is the “double decker” type (Figure 3), take separate samples from top and bottom screens. After taking the sample, accumulated material on the shaker screen should be scraped off. If a board or bucket is being used to catch cuttings, it should be emptied after the sample is taken. Use of a board or buckets to catch cuttings may be necessary when there are few or poor sample returns. They are not a substitute for regular visits to the shale shaker and should not be required for normal returns.
Take the sample to the logging unit and use the following procedure.
- Place the cuttings in an 8-mesh screen stacked on top of 80-mesh and 170-mesh screen. Rinse lightly to remove drilling mud.
- If the sample consists of unconsolidated clay, remove a small sample for microscopic examination.
- Blend 100cc of the sample with 600cc of water for thirty seconds. Let stand for a further thirty seconds and take a cuttings gas reading.
- Inspect the water surface for oil droplets and petroleum odors.
- Report the amount and color of oil droplets. If sufficiently abundant, skim off a cuttings sample.
- Set aside the blended sample for later examination.
- If oil indications are seen or expected, place about 50 ccs of the unwashed sample in a dish and inspect under ultraviolet light for oil fluorescence.
- If no fluorescence is seen, add 100 ccs of water and stir the sample. Observe for fluorescent oil droplets “popping” to the water surface.
- Report the amount, color, intensity, and distribution of oil and fluorescence.