A Simple Mud Log

	The Mud Log This is a simple example of a "mud log", prepared by the Petroleum Geologist's (PG's) assistant during the drilling of a well (the assistant is called a "Mud Logger"). This is an old-style mud log, drawn by hand. As the well cuttings come up from the bottom of the hole, they are examined and identified. The mud log is updated each day of drilling. The mud log and the electric logs are the two most important items the PG must have to evaluate the drilled well. On the right side of the mud log, the Mud Logger records his interpretation of the rocks drilled. There are two types of interpretation shown, a drawing down the center of the log (called "Litho"), and a "Sample description". Let's talk first about the section called "Litho" (lithology, or rock type). The logger makes a drawing down the middle of the page. He uses standard symbols and colors to illustrate the different kinds of rocks, textures, fossils, and bedding. Limestones are shown as blue blocks on the log, shales are shown as short black horizontal lines, and sandstones/siltstones are shown as a dotted pattern. He looks at the small well cuttings under a microscope to figure out what the rock types are. It takes a *lot* of experience and know-how to figure out what rocks are being drilled just by looking at the tiny well samples. The "sample description" is more detailed summary of what the logger sees. Abbreviations are used because so much detail must be recorded. For example, look at the words inside the blue circle. It's a little hard to read, but it says "shale, black to dark-gray, medium hard, calcareous (containing limestone, or calcium carbonate). Right below the shale is a limestone described as dark-gray to brown, medium-crystalline (medium grained), arenaceous (containing quartz sand), friable (breaks apart like sand), with fair to good inter-crystalline porosity. The mud logger has also picked a couple of formation tops, and placed them on the left side of the log. These are the "L-1 and L-2 Zones," and the Neva Formation. Mud loggers are often the most experienced workers in a given area, and they are often able to pick up obscure tops or markers the geologist is unfamiliar with. Other data the mud logger will post on the log includes all the chemical properties of the mud (viscosity, weight, salinity, etc.), "trips" that are made in or out of the hole, records of gas flares, oil in the mud pit, or any other circumstances he might notice while the well is drilling. It is especially helpful when the mud logger notices some action being taken at the rig that he does not believe to be correct, and notes it on the log (and calls the geologist). On the left side of the log, "Drilling Time" is recorded. The rig keeps track of the drilling time and a lot of other parameters using a clock device known as a "geolograph." This is a precise record of the time it takes the drill bit to drill the rock. Drill-time is recorded in increments of 2 feet. The numbers across the top indicate how long it took to drill that two feet, in "minutes per foot". For instance, the depth from 3200' to 3202' (1 block), drilled at 7 minutes per foot, or a total of 14 minutes for the two feet. Drill time is incredibly useful in picking out zones that may be productive, or correlating the electric logs to the mud log. Making a chart of drill time is very important, because it gives the PG one more hint to help figure out what rocks are being drilled. Remember that the well has not been logged at this point. Notice that in this well, the shales tend to drill "slow" (5-10 minutes per foot), while the limestones drill "fast" (2-6 minutes per foot). This is because the soft shales are ground into a thick, sticky mud by the drill bit, and "ball up" on the end of the bit. This causes the bit to cut through the rock slower. Usually the slow-drilling formations will be extremely hard and non-porous sandstones or limestones. For these types of formations, a special (and expensive) bit called a PDC bit, sometimes used. Porous sandstones and limestones don't ball up, so they usually drill really fast...just as seen on this log. In fact, one of the things a Petroleum Geologist likes to see is a good "drilling break" in his target sand. This means the sand drills very fast (3 minutes per foot or less), indicating it is soft or porous. These days, most mud logs are made with a computer. The mud logger still does all the sample and gas-monitoring work he did before, but he no longer needs to be a draftsman. Here is an example: If the logger sees any oil in the well samples, it will be very clearly noted on the mud log. Oil and gas "shows" are the most important thing the mud logger will find. At the right is a picture of the complex mud-monitoring equipment inside a portable mud-logging trailer. This equipment's main job is to detect oil and gas in the mud. The mud log will be used in combination with the electric logs run on the well to make a decision about whether to complete the hole (try to produce oil or gas from it) or "plug" the well. Mud logs are a very important tool to the petroleum geologist.. Below is a chart of common symbols used on mud logs: