Chapter Three
Minerals and Rocks
Minerals:
Minerals are the fundamental building blocks of rock materials in the earth. However, most minerals, as found in rocks, are not pure.
In table 4, the first eight elements are the most.
Mineral Identificati on a) Physical properties:
Are often the only guidelines for field identification of specimens (table 6).
1) How a mineral resists scratching is the measure of its hardness. Mohs' hardness scale (table
8).
2) Color is a physical property of limited value in mineral identification.
3) Minerals have different weights relative to each other.
4) Some minerals have very distinctive taste qualities that facilitate their immediate identification, like rock salt (halite).
5) A few minerals respond to common chemical regents that can be applied to hand specimens. Limestone (calcite) responds readily to hydrochloric acid. b) Chemical analysis
It is used to identify the mineral by measuring its exact chemical composition.
But this method needs long and difficult procedures c) X-Ray diffractio n method
The result of this test is a number of peaks. Each mineral has distinctive peaks (fig.1).
Rocks
Rocks are important to the petroleum industry because they provide source beds and reservoirs for hydrocarbons. They consist of aggregates of minerals (table 9) in various proportions.
Reservoir rocks need porosity, permeability, and access to source beds to be productive
1) Igneous Rocks
Igneous rocks are those that have resulted from the cooling and crystallization of magma.
Igneous rocks have no source potential. Fractured igneous rocks or porous and permeable weathered igneous rocks can and do provide adequate reservoirs in many locations.
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2) Sedimentary rocks
Classified according to environment of deposition, rock type, or by origin. i) Classification by environment of deposition:
Include marine, swamp, lake, fluvial (river), eolian (wind) environments. ii) Classification by rock type or lithology:
There are numerous types of sedimentary rocks, including limestone, feldspathic sandstone, sandy siltstone, chert, etc. If limestone is deposited as a reef, it is a reef limestone. iii) Classification by origin: a) Classic rocks : are composed of fragment or grains of pre-existing rocks, transported, deposited, and hardened, or lithified, into a new rock. The grains may touch each and surrounded and held together by a cement such as clacite (fig. 1.24). Often the spaces between the grains are not completely filled, leaving openings or voids that can contain water or other fluids. Clastic rocks are classified on the basis of the size of the fragments that make up the rocks (Table 11).
Conglomerate composed of gravels cemented together, surrounded by a matrix of much finer sand and mud and are cemented together to form the hard rock. Conglomerates are often good reservoirs for hydrocarbons.
Sandstone is composed of sand-size particles that have been compacted and cemented together. Sorting of sandstone particles ranges from good to poor and the rock can be designated as coarse, medium, or fine based on the size range of the sand grains. Since sandstone fragments can be of virtually any composition, the sandstone rock itself can be designated according to its mineral constituents (Table 12).
Sandstone can be tightly or loosely cemented and therefor can have a wide range of porosity and permeability, which makes it a good petroleum reservoir rock.
A sandstone of limestone grains is not only a possibility but a common reality and is called calcareous sandstone.
Siltstone consists of muddy fine-grained particles, which feel gritty on the teeth. Rocks of this classification have limited porosity and permeability because of their potentially high clay content and small grain sizes.
They have limited potential as reservoir beds.
Shale consists of clay particles of less than 1/256-mm diameter. Shale is smooth and not gritty on the teeth.
It has a very low permeability and a good porosity.
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Shale, with high-preserved organic content, is considered to represent the most favorable of potential source rocks.
After their formation waters and fluids are expelled, shale provide sealers for porous reservoir rocks and are instrumental in forming hydrocarbon traps. b) Nonclastic rocks : formed inorganically an organically precipitated from water or by accumulation of organic materials (Table 13). In Non-clastic or crystalline texture, crystals interlock or touch each other on all faces, which usually resulted in very low porosity.
Limestone is formed by precipitation from solution and deposited as lime mud
Limestone is typically dense with small porosity.
Chert is formed by the precipitation of silica from seawater.
Chert has no porosity or permeability.
Evaporate deposit result from increase in saturation of a solution as water is evaporated. The mineral constituents that are left behind precipitate out of the solution as their concentrations increase. (rock salt, gypsum, potassium, and magnesium salts).
Carbonate rocks : formed by accumulation of skeletal materials.
Often have extensive porosity and permeability and are excellent reservoirs.
Sedimentary Rocks and petroleum :
All petroleum sources rocked are sedimentary. Black shale, rich in pre-served organic materials, is considered to have the best source-bed characteristics.
Petroleum reservoir rocks include many types of sedimentary rocks.
Sedimentary rocks with limited porosity can become good reservoirs if it was fractured rock.
3) Metamorphic rocks
Pressure, heat directed pressure, and catalytic action related to chemical agents are important factors in changing pre-existing rocks into rocks with different textures and compositions.
The process, which involves these agents, is called metamorphism and results in metamorphic rocks.
Metamorphic agents act upon rock in its solid phase.
Pressure : due to the weight of overlying rocks and is called overburden pressure.
Chemical agents : recombination of mineral constituents that are already present and results in more stable materials. Water, which is present in the pore space of most rocks, provide the
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medium for transportation of atoms through existing pore spaces and promote the development of new crystalline materials
Metamorphic rocks and petroleum :
Metamorphic rocks (Fig. 75) have no petroleum source pottential. Fractured and weathered porous and permeable metamorphic rocks are locally significantly productive.
Rock cycle
Figure 79 demonstrates that any rock type can result from alteration of a pre-existing rock type whether by re-melting, metamorphism, or erosion.
Rock cycle and petroleum
Sedimentary rocks are source and reservoir.
The rock cycle forms, changes and destroys rocks. Petroleum experiences a similar kinetic history, it is being formed, changed and destroyed with its host rocks.
Igneous and metamorphic rocks under proper circumstances can provide reservoirs or seals for petroleum accumulations.
Igneous activity and metamorphism are not compatible with petroleum generation and accumulation and these processes can destroy petroleum.
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