A mud log is a chemical and visual analysis of the drilling mud and well cuttings for traces of subsurface natural gas and crude oil as the well is being drilled.

Any oil or gas above the normal expected background is called a show. The mud log is made by a service company in a mud-logging trailer at the wellsite (Fig.1). The purpose of the mud log is to identify oil- and/ or gas-bearing rocks in the subsurface. Drilling mud circulating out of the well is sampled from a gas trap in the shale shaker, along with well cuttings from the shaker screens.

Fig. 1. Mud-logging trailer

The mud loggers are usually geologists. Typically, two geologists work 12 hours shifts at the mud-logging trailer, so that the mud log is maintained 24 hours a day.

The American Petroleum Institute has published a standard mud log format. A mud log (fig. 2) has a header at the top with operator, well name, location, elevation, and other information. A depth strip, showing depth in the well, runs down a column near the middle of the log, along with a sample log that was made by the mud loggers.

Fig. 2. Mud log

On the left side is a drilling-time log recorded in ROP. The right side of the mud log shows the amounts of gas and oil detected in the drilling mud and well cuttings. A curve shows the total gas in the drilling mud in gas units measured by a gas detector. A more detailed chemical analysis (show evaluation) can be made of a gas show with a gas chromatograph. It measures the percentages of methane (C1), ethane (C2), propane (C3), butane (C4), and pentane (C5). A black bar shows that the mud logger saw oil staining on the well cuttings in what is called a show of oil.