6.3.2.2 Staining and Bleeding

The amount by which cuttings and cores will be flushed on their way to the surface is largely a function of their permeability. In very permeable rocks only very small amounts of oil are retained in the cuttings. Often bleeding oil and gas may be observed in cores, and sometimes in drill cuttings, from relatively tight formations. The amount of oil staining on cuttings and cores is primarily a function of the distribution of the porosity and the oil distribution within the pores. The color of the stain is related to oil gravity. Heavy oil stains tend to be a dark brown, while light oil stains tend to be colorless. The color of the stain or bleeding oil should be reported. Ferruginous or other mineral stains may be recognized by their lack of odor, fluorescence or cut.