Page 31 - Well Logging and Formation Evaluation

Basics 21

should be treated as being completely reliable, and there will be many
cases in which it has to be disregarded in favor of a commonsense
approach to all the other sources of information.

1.7 WELLSITE MUD LOGGING

During the drilling of a well there will typically be a mud-logging unit
on the rig. This unit has two main responsibilities:

1. To monitor the drilling of the parameters and gas/liquids/solids returns
from the well to assist the drilling department in the safety and opti-
mization of the drilling process
2. To provide information to the petroleum engineering department that
can be used for evaluation purposes

Typically the mud-logging unit will produce a daily “mud log,” which
is transmitted to the oil company office on a daily basis. Items that will
be included are:

• Gas readings as measured by a gas detector/chromatograph
• A check for absence of poisonous gases (H 2S, SO 2)
• A report of cuttings received over the shale shakers, with full litholog-
ical descriptions and relative percentages
• ROP
• Hydrocarbon indications in samples

The mud log may be of great use to the petrophysicist and geologist in
operational decision making and evaluation. Areas in which the mud log
may be particularly important include:

• Identification of the lithology and formation type being drilled
• Identification of porous/permeable zones
• Picking of coring, casing, or final drilling depths
• Confirmation of hydrocarbons being encountered and whether they are
oil or gas

1.7.1 Cuttings Descriptions

The mud-logging unit will generally take a sample of the cuttings
received over the shale shakers at regular time intervals, calculated to cor-