The Mud Log
This is a relatively simple example of a "mud log", prepared by the 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.
On the right side of the mud log, the Mud Logger writes his interpretation of the rocks drilled. There are two types of interpretation shown, a drawing (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 a 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. This reads: "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), with fair to good inter-crystalline porosity.
On the left side of the log, "Drilling Time" is recorded. 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.
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 (click to see PDC bit) is sometimes used. This special bit is named for the Prairie du Chien Formation, which can be extremely tough to drill, and for which the PDC bit was developed.
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.
Finally, the mud logger will note formation tops he has picked. On this log, the "L-1", L-2", and "Neva" formations are picked.
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:
