6.3.1 Introduction

Although petrophysical analyses, coupled with well testing, may give a conclusive determination of the presence of commercial quantities of oil, it is still the mud logger’s responsibility to report and log all hydrocarbon shows.

Unfortunately, there are no specific criteria to establish if a show represents a potentially productive interval. The color and intensity of the stain, fluorescence, cut, cut fluorescence, and residual cut fluorescence vary with the specific chemical, physical, and biological properties of each hydrocarbon accumulation. The aging of the shows (highly volatile fractions dissipate quickly) and flushing by drilling fluids or in the course of sample washing tend to mask or eliminate evidence of hydrocarbons. The presence or absence of obvious shows cannot always be taken as conclusive. In many cases, the only suggestion of the presence of hydrocarbon may be a positive cut fluorescence. In other cases, only one or two of the other tests may be positive.