Most old mud loggers will tell you that there is no official definition of a unit of gas. If that is the case then why do mud loggers, geologists, and other oilfield personnel talk about gas in units. The reason is that units of gas are indeed defined in the oilfield. They are defined by tradition, by mud logging companies, and by oil and gas exploration companies. In my experience, only two standards for calibrating mud logging gas detectors exist in the United States. Both of these standards define gas units that are equal to a known concentration of methane in air. The first standard is: one percent methane in air (or nitrogen) is equal to 50 units of gas. The second standard is: one percent methane in air (or nitrogen) is equal to 100 units of gas. I would also put forward that the 100 unit standard is by far the most common. Also, in my experience, the determination on whether to use the  50 unit or 100 unit standard is almost always determined by one of three factors; geography, preference of the oil company drilling a well, or the preference of the mud logging company contracted to handle the mud logging service for a particular well.  Geography is a big one because in my experience the 50 unit standard is almost exclusively used either off shore in the Gulf of Mexico, or in South Texas. The 100 unit standard is used almost everywhere else, at least in the United States. If the oil company has a preference it is almost always the 100 unit standard. This is because most oil companies prefer higher gas readings to lower gas readings when given the choice. Mud logging companies usually have their own preferences, but most will defer to oil companies on the matter. I would also add that whether the 50 unit or 100 unit standard is used, the gas curve will look exactly the same on a linear scaled gas column on a mud log and only slightly different on a logarithmic scaled mud log. The differences on the logarithmic scaled mud log will be more pronounced with lower gas readings.  All of that being said there is still no official definition of a unit of gas; at least not one that I am aware of.