Mud logging was named during the early days of the oilfield from old-timers that gave some pretty colorful names to the people and equipment they worked with. In order to drill a well, there must be some way to get the drill cuttings out of the hole or they will just accumulate on top of the bit and around the pipe and eventually get stuck. In the early days they simply pumped water without any additives down through the pipe and back up the outside of it to facilitate the removal of the drill cuttings and the water would become muddy due to the clays in the formations. When someone finally decided to look at what was coming out of the hole to determine what they were drilling in, they had to catch some of this mud and wash it off to reveal the particles that didn't dissolve. Eventually someone decided to plot this information on a log and hence a new job was created; mud logging.
Mud logging is the process of examining drill cuttings (formation rock chips), gas hydrocarbon and it's constituents, basic chemical and mechanical parameters of drilling fluid or drilling mud (such as chlorides and temperature), as well as compiling other information about the drilling parameters and then plotting them on a graphic log called a mud log. Example1, Example2
Other real-time drilling parameters that may be compiled include, but not limited to; rate of penetration (ROP) of the bit (sometimes called the drill rate), pump rate (quantity of fluid being pumped), pump pressure, weight on bit, drill string weight, rotary speed, rotary torque, mud volumes, mud weight and mud viscosity. This information is usually obtained by attaching monitoring devices to the drilling rig's equipment with a few exceptions such as the mud weight and mud viscosity which are measured by the derrick hand or the mud engineer.
Mud logging is often written as a single word "mudlogging". The finished product can be called a "mud log" or "mudlog". The occupational description is "mud logger" or "mudlogger". In most cases, the two word usage seems to be more common. The mud log provides a reliable time log of drilled formations.