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One of the earliest indications of reservoir quality comes from the mud logging unit, where loggers measure formation gas entrained in the drilling fluid and perform visual descriptions of the cuttings with a low-power microscope. Formation gas is extracted from the recirculated drilling fluid, utilizing a mechanical agitation device commonly known as a gas trap. The liberated gas is then introduced to a gas sensor or gas chromatograph (GC) to provide a gas-in-air composition. The quality of these data is limited, due to the mixing of the liberated gas with air contained in the gas trap, and because substantial amounts of unmeasured formation gas remain in the mud. This characteristic tends to skew the results toward overestimation of methane (C1) and underestimation of the heavier alkanes (C2+), as well as providing a less accurate total gas reading from the reservoir interval. For cuttings, visual inspection is qualitative, and details of the lithological description may vary, based on the experience of the individual mud logger.
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