2.1.3 Sedimentary rock types
In order to differentiate between the various rock types, several classifications are
structured either on the basis of the grain size or on the fundamental mineralogy. The
sedimentary processes which have formed the rock can as well be invoked into the
classification.
When it comes to classifying rocks seen at the well site, we stick to a descriptive
classification and leave environmental factors alone. This simplifies matters a great deal.
We will now go on to look at the main rock types encountered in the drilling of oil wells
and how these rocks can induce the drilling process.
2.1.4 Terrigenous sediments (clastic)
These are land derived sediments and are represented by the clay minerals that coarser
material formed from the fragmentation of silicate rocks. They have been sub-divided
further on the basis of their grain size.
· Group A - Clays (particles with a diameter less than 0.004 mm)
· Group B - Silts (particle diameter 0.004 to 0.06 mm)
· Group C - Sands (particle diameter 0.06 mm to 2 mm)
· Group D - Rudites (coarser rock fragments).
2.1.4.1 Clays
Clay minerals are hydrous platy aluminosilicates. They form a complex and extensive
series due not only to variations in ordering of the sheet-like crystal lattices, but also to
the presence of different cations between the lattices.
Clay minerals can be subdivided into five important groups with different chemical and
physical characteristics; kaolinite, illites, smectites, chlorites and glauconites.
The term “shale", generally used to name those argillaceous sediments, mostly describes
the tendency of those materials to split, especially when they have been exposed to high
compactions and pressures. Soft clays which are encountered in the topmost sections of
wells usually drill fine unless their affinity to water causes them to form so-called gumbo
formations.
Gumbo is a term used to describe claystone formations which absorb water, hence,
hydrating shales tend to expand. Sticking mechanisms are associated with such
formations:
· Contraction of the wellbore behind the BHA makes it difficult to trip out.
· Large clumps of gumbo will fall into the wellbore and will eventually stick to the
BHA. Chemical inhibitors can be added to the drilling fluid in order to restrict or
avoid such phenomenon.
Kaolinite clays generally form by sub-aerial weathering of granites: rocks with a low
proportion of iron or magnesium rich minerals. The clay particles may be washed out as
colloids or formed in situ. When they come in contact with water rich in potassium ions
(for instance sea water) they slowly alter to illite.
Illites are the dominant clay mineral group. They are formed by the direct weathering of
feldspars or by alteration of kaolinite and montmorillonite under marine or later
post-depositional conditions.
Montmorillonites form by the alteration of minerals rich in iron and magnesium. For
instance, certain types of volcanic ash. They too gradually change to illite when
transported into sea water.
Clays may also be the subsidiary minerals of other sedimentary rocks. Their origin
(petrogenesis) may be primary in that they were deposited at the same time as other
major constituents. Alternatively, they may have formed as a later alteration product of
those less stable minerals in the original sediment, their origin in this case being
secondary or diagenetic.
2.1.4.2 Silts
Silts are clastic sediments, intermediate in size between clays and fine sands (particle
diameter 0.004 to 0.06 mm). They are derived from fragmented rocks or minerals and are
called clastic or detrital sediments. Silt size particles are generally the result of extreme
abrasion (mechanical wearing down), and therefore all the minerals found in that size
range may also be found as sand-grade particles. They consist of quartz, feldspar, heavy
minerals, iron ores and phosphates. While sand may be silt free, most clays and
claystones contain about 35% silt or more (thus named silty claystones or argilaceous
siltstones). Very abrasive siltstones can be encountered in the drilling processes and,
therefore, frequent bit changes become necessary.
2.1.4.3 Sands
Sands, like silts, are defined by their grain size (0.062mm) and not by their mineralogy.
Terrigenous or siliclastic sands are of prime, economic importance because they are
often of wide lateral extent and are frequently porous and permeable, thereby satisfying
three basic requirements for major aquifers and hydrocarbon reservoirs.
Quartz, feldspar, lithic fragments, micas and heavy minerals are the major mineralogical
groups found in detrital sands.
2.1.4.4 Rudites
These are sediments whose grain size exceed 2 mm in diameter. This coarse rock
fraction is not unique to terrigenous deposits, and rudite grade particles are common in
both the carbonate and pyroclastic groups.
The shape of the class is also important. Rounded rock fragments which have undergone
physical abrasion are called conglomerates. Angular ones, physically as well as
chemically immature, are termed breccias.
2.1.5 Pyroclastic sediments
These are derived by volcanic eruption into the air. They may be chemically weathered
or physically reworked to closely resemble terrigenous deposits. This is because they
may have similar mineralogies, and the range of grain sizes are comparable.
Volcanic ash or tuff can fall as clay, silt or sand grade particles whereas the still coarser
agglomerate is the direct equivalent of conglomerate.
While traces of volcanic ash are common in most deep sea sediments, pyroclastic
deposits are generally rare within sedimentary sequences.
2.1.6 Carbonates
This major group of sediments is fundamentally different to the terrigenous (or
siliclastic) and pyroclastic rocks just discussed. In these clastic groups, the mineralogy of
the deposits is largely controlled by the processes of weathering and erosion of the
bedrock in the area of the sediment source; namely outside the basin. In carbonate rocks,
however, it is the depositional environment within the basin which exerts the prime
control on the mineralogy and sediment type. In this respect carbonates have closer
affinities to the evaporite and carbonaceous rocks.
Biological activity around the area of deposition is of prime importance in generating the
basic particles of carbonate sediments.
In order to determine the depositional environment and the genesis of carbonate rocks, a
fairly precise description of the chemical and physical components is required.
2.1.6.1 The Chemical Components of Carbonates
The chemical components are:
· Aragonite
· Magnesian
· Calcite
· Dolomite
All these minerals, know as polymorphs of calcium carbonate (CaC03), present different
degrees of chemical stability depending on the environmental characteristics of the
depositional basin.
2.1.6.2 The physical components of carbonates
Four basic physical components are taken into account for the description of carbonates:
The grain types either mineral or biological.
The matrix which consists of the fine material.
The cement which grows in the pore spaces of the sediment after it deposition.
The pore space remaining after cement has taken place.
2.1.6.3 The Classification of Limestone Rocks
In an essentially monomineralogical calcium carbonate system, there is apparently as
wide a range of particle type as there is in the multi-mineral terrigenous group. The most
successful attempts to solve the nomenclature problem are those of Folk, in which the
basic components of the rock are described, and of Dunham where the basic fabric is
described.
Four more terms are frequently used to describe grain size in carbonate rocks.
· Group A - Calcilutite up to 0.004 mm grain diameter
· Group B - Calcisiltite 0.004 to 0.065 mm
· Group C - Calcarenite 0.065 to 2 mm
· Group D - Calcirudite above 2 mm grain diameter
Certain specific types of limestones like chalk, marl, bituminous limestones and dolomite
can be as well mentioned at this stage.
· Chalk This is a soft white limestone composed of the tests (or skeletons) of once
floating micro-organisms.
· Marl This is a calcareous clay - generally an intermediate mixture of terrigenous
clay and micrite.
· Bituminous Limestones These are micrites which contain much organic or
carbonaceous matter, mostly in the form of tarry hydrocarbons which are usually
described as bitumen.
· Dolomite This term is applied to limestones where the calcium carbonate has
been completely replaced by the mineral dolomite.
2.1.7 Evaporites
These sediments, which include mineral salts such as anhydrite. gypsum and rock salt
(halite), are believed to form by precipitation from brines (waters concentrated in salt by
evaporation processes).
They are important as mineral deposits sometimes occurring in thick, relatively pure
mono-mineralogic sequences. They play an important role in petroleum geology, being
excellent cap rocks for oil or gas reservoirs. They are also very plastic and thick salt
sequences deform and flow to produce salt domes. Salt movements frequently produce
hydrocarbon traps.
The more frequently encountered evaporate minerals are listed Table 2-2.
2.1.8 Carbonaceous rocks
Small traces of organic material are present in most sedimentary deposits with the
notable exception of desert red-beds where it has been entirely destroyed by oxidation.
However, in certain very reducing anaerobic (oxygen free) environments it may form an
appreciable proportion of the sediment.
2.1.8.1 Coal
Coals are formed by the action of fungi and anaerobic (oxygen hating) bacteria on
decaying vegetal or “humic" matter in a reducing environment. Compaction by deep
burial is an important agent in reducing the volatile content of the rock. The series
PEAT, LIGNITE, HUMIC COAL, ANTHRACITE expresses the increase in the carbon
content as oxygen and hydrogen are progressively driven off.
2.1.8.2 Oil Shale
This is more an economic rather than geological term, and refers to argillaceous
sediments with an organic content of at least 5%, but generally meaning considerably
higher (20 to 50%). They must be sufficiently rich in organic matter to yield free oil on
heating.
They generally form in lakes where algae matter decays in a strongly reducing (or
anaerobic) environment, thereby preserving the organic material. This is referred to as
"sapropelic" matter and is a good source for oil. Marine equivalents are also known.
2.1.8.3 Bituminous Limestone
This is again an economic rather than geological concept being the direct carbonate
equivalent of terrigenous oil shales. Such sediments may form in lagoons behind a reef.