Page 32 - Well Logging and Formation Evaluation

22 Well Logging and Formation Evaluation

respond to regular changes in formation depth (e.g., every 5m). Some of
these samples are placed into sealed polythene bags as “wet samples” and
retained. Other samples are washed, dried, and retained as “dry samples.”
Washed samples are examined under a microscope in the mud-logging
unit and a description made that may be communicated to the office.
In order for the information received from the rig to be useful, it is
essential that rigid standards for reporting are followed that are agreed
upon between the rig and the office. Standards will typically vary among
companies. Items that should be included are:

• Grain properties
Texture (muddy/composite)
Type (pelletoid/micropelletoid)
Color
Roundness, or sphericity
Sorting
Hardness
Size
Additional trace minerals (e.g., pyrite, calcite, dolomite, siderite)
Carbonate particle types
Skeletal particles (fossils, foraminifera)
Nonskeletal particles (lithoclasts, aggregates, rounded particles)
Coated particles
• Porosity and permeability
Porosity type (intergranular, fracture, vuggy)
Permeability (qualitative as tight, slightly permeable, highly permeable)
• Hydrocarbon detection

Hydrocarbons may be detected with one of the following methods:

Natural fluorescence

Examining the cuttings under UV light may indicate the presence
of oil, since oil will fluoresce. However, fluorescence will not in itself
prove the presence of movable oil, due to other sources of fluorescence
that may be present, such as fluorescent minerals; OBM or lubricants used;
other sources of carbon, such as dead oil or bitumen; and Gilsonite
cement.
The correct procedure is for a portion of the lightly washed and undried
cuttings to be placed on a dish and observed under UV light (other light