BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
In petroleum geology, one is mainly concerned with sedimentology and stratigraphy, as well as
structural geology. Sedimentology is concerned with the origin, transportation, deposition and
diagenesis of sedimentary rocks. It includes their description, classification and interpretation.
Since sedimentary rocks are the fundamental materials of stratigraphy, the stratigrapher is primarily
concerned with sedimentology and palaeontology or the science of life through geological history.
The stratigraphic record is a result of sedimentary processes operating through geologic time. A
fundamental principle in sedimentation is the law of superposition. “If one series of rocks lies
above another, then the upper series was formed after the lower series.” The solution of
stratigraphical and structural problems begins with the systematic observation of the sedimentary
succession exposed in surface outcrops or made available through drilling. It is important to
remember that tectonic activity influences the thickness and character of accumulating sediments.
From all the accumulated subsurface information, a stratigraphical column is prepared and
subdivided according to geological age. Stratigraphy is the study of rock succession and the
interpretation of these successions as sequences of events in the geological history of the earth.
Properties of Sedimentary Rocks
Most sediments are mixtures of two main components, a detrital fraction, brought on site of
deposition from the source area, and a chemical fraction, formed at or near the site of deposition.
Example: a clastic rock is composed mainly of detrital material.
Texture: Detrital rocks have a fragmental texture, whereas chemical rocks have crystalline
textures. Four components give a rock its textural characteristics; the grains, the matrix, the
cement, and the pore space. Cement should be distinguished from the matrix. Most common
cement materials are silica, calcite, dolomite and siderite.
Particle properties: Described are color, size, shape, roundness, surface texture, orientation, and
mineralogical composition. Particle properties describe both clastic and chemical rocks. Large sand
grains imply strong transportation currents in a high energy environment, whereas large calcite
crystals reflect particular physical and chemical conditions at the deposition areas.
• Particle color is affected mainly by the association of different chemicals, such as iron’s
reddish color.
• Particle size is mainly indicative of forces and currents involved in transportation.
• Particle shape is related to the energy and distance to which the particle was subjected
before deposition.
Genesis of Sedimentary Rocks (Particles)
A sedimentary rock is the product of provenance (source area or pre-existing rock) and process.
Weathering processes breaks down rocks at the earth’s surface to form minute particles. Erosion is
the name given to processes which removes the product of weathering. Transport follows, and
finally when the energy of the system is exhausted, deposition takes place.
There are three types of weathering and two categories of weathering products.
Chemical: Selectively oxidizes and dissolves material.
Physical:
Mechanical disaggregation.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-2