Lithological Terminology And Definitions 575g5i

Lithological Terminology & Definitions. KM 002 Julie Pearce, Knowledge Management Department Technical Summary Definitions:

Rock Groups:

Lithology: The lithology of a rock defines the actual composition or rock type, e.g. sandstone or limestone. A comprehensive lithological description will include information about the structure, texture, grain size, colour, mineralogical composition and general rock classification e.g. thin-bedded fine-grained buff sandstone.

There are three fundamental rock groups: Sedimentary Rocks: Sedimentary rocks are sediments that have been compacted or cemented to form solid rock bodies, lithification. The sediment comprises of a combination of rock & mineral fragments, chemical precipitates such as salt or calcium carbonate, and organic matter such as coral, shell fragments and vegetation. This constituent material is generally eroded or weathered, transported and deposited at a different site from where it was formed, known as the sedimentary deposition site, where it is then deposited in strata or layers, producing a distinctive feature of this rock group known as stratification.

Formation name: The formation name is the name given to a particular bed or beds in a specific area. This name may or may not give indications as to the lithology of the rock, e.g. Bunter Sandstone, Travis Peak Formation. The same bed or unit of rock may be given several different formation names as it traverses laterally. The formation name can be likened botanically to a common name of a plant species.

Compaction and cementation form the final process whereby loose unconsolidated material is lithified in to solid rock. Compaction is achieved through the shear weight of overlying sediment compacting and consolidating the grains and fragments below. Cementation occurs through precipitation of mineral matter, circulated by pore fluid, forming cement around the sedimentary grains thus holding them together. Rock that has undergone cementation can be classed on a weakly to strongly cemented scale; the degree of cementation will influence directly the ‘drillability’ of a rock. Examples of common cementation minerals are calcite and quartz. Compaction and cementation both result in a decrease in the porosity of the rock.

The importance of a lithological, or petrographic, rock description in conjunction with a formation name provides a comprehensive of the sample and allows a comparison of like lithologies. Stringer: A stringer is a thin bed of a differing mineralogical or lithological composition from the surrounding unit. The stringer is generally of a differing hardness to the surrounding unit; this can cause erratic drilling. E.g. a stringer of limestone in a shale bed. Interbedded: The term interbedded refers to beds of rock lying between or alternating with other beds of differing lithological composition. E.g. interbedded sand and shale.

Sedimentary rocks are characterised by formation at the earth’s surface by interaction between the earth’s crust and the hydrologic system (the system of moving water at the earth’s surface). Fossils are found in abundance in sedimentary rocks.

Porosity: Porosity is the percentage of the total volume of rock not occupied by mineral grains, i.e. the percentage that is pore space.

Sedimentary rocks are classified according to their lithological constituent and the size and shape of such. This results in there being two main sedimentary groups: • Clastic, formed from fragments of other rocks. Examples include conglomerates, sandstone, siltstone and shale. • Chemical & Organic, formed through chemical precipitation or biological activity. Examples include limestone, dolomite and anhydrite.

Permeability: Permeability is the ability of a formation to allow the age of pore fluid through the pore spaces. The fluid viscosity, hydrostatic pressure, size of the pore spaces and the degree to which these pore spaces are interconnected affects this characteristic. A rock can have high porosity but low permeability.

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Most oil and gas reserves are found in sandstone or porous limestone or dolomite reservoirs.

shelly limestones, to those with a biogenic or chemical origin, e.g. oolites and oozes.

Commonly Drilled Sedimentary Rocks:

Chalk is the purest type of limestone, containing only a fraction of sand or mud. It is mostly comprised of the remains of algae and broken shells and is typically very brittle and weak; it is white in colour.

Sandstone: Sandstone is typically composed of sand-sized quartz grains, smaller than 2mm but larger than 1/16mm. Other minerals may be present but quartz is usually the most dominant, due to its abundance in many other rock types and its high resistance to weathering and degradation through abrasion or chemical action.

Shelly limestones may contain a large proportion of the remains of crinoids, corals or brachiopods with the remainder being calcite or detrital mud. These shelly limestones form thick non-porous massive beds that are well ted and thus allow the percolation of ground water.

The shape of the grains can range from well rounded to very angular, and the size range distribution of these grains is classed on a scale from very well sorted to poorly sorted.

Reef limestones form from the hard skeletons of colonial corals, algae and associated reef dwelling organisms. These limestones exist as compact, rounded structures of hard nodular limestone among well-bedded layers. Reef limestone structures have a high porosity and as such are important as oil reservoirs.

Sandstone cement is typically iron oxide, calcite or quartz and is often used in the naming of the sandstone, e.g. sandstone with calcite rich cement can be described as calcareous sandstone. The colour of the sandstone is often determined by the nature of the cement, for example sandstones with calcitic or quartzitic cement are usually pale coloured, while iron oxide cement will produce brown or red coloured sandstone.

The formation of microcrystalline limestone occurs in quiet water environments where calcium carbonate precipitates as tiny needle-like crystals; these crystals then accumulate to form a limy mud. Compaction and recrystallisation modifies the crystals to form a dense very fine-grained microcrystalline limestone.

Conglomerate: A conglomerate is a consolidated deposit of gravel composed of rounded particles larger than 2mm in diameter, with variable amounts of interstitial mud and sand. Most conglomerates display crude stratification or layering and may include isolated pockets or lenses of sandstone. The parent rock or rocks are typically some distance from the site of deposition of the conglomerate; the rounding of the pebbles is due to water transportation and river or coastal wave-action. Conglomerates are indicators of slow encroachment of the sea over the land.

Oolitic limestone, limestone composed of oolites, form in highly calcareous shallow seas. As a nucleus, shell fragment for example, is rolled up and down the beach by wave action, it is repeatedly chemically coated in calcareous matter, and this resultant pellet is termed an oolite. Magnesian limestone forms as a result of chemical replacement of the original calcium carbonate by magnesium carbonate, resulting in a dirtier buff coloured limestone. Dolomitisation, replacement of calcium carbonate by magnesium carbonate, reduces rock volume by approximately 12% thus assisting in increasing porosity.

Breccia or Sharpstone: A breccia is a sedimentary rock composed of angular particles larger than 2mm in diameter. Breccias are usually found close to the site of the parent rock that led to their formation. Typically, they are formed as a result of accumulating scree at the base of a mountain slope, they may also occur along fault planes.

Igneous Rocks: Igneous rocks form as a result of the cooling and crystallising of liquid rock material, or magma. Igneous rocks are classified based on their texture and composition, from this one can gain information on the cooling history and the origin of the parent magma. Igneous rocks that remain below the earth’s surface as they cool are termed intrusive; those that cool at the surface are termed extrusive.

Chert: Chert is a sedimentary rock composed of chemically or biologically precipitated granular cryptocrystalline silica. The term flint is often used in place of chert – both are interchangeable. Salt: Salt deposits form through the evaporation of lake and seawaters in very dry climates. Salt undergoes plastic deformation when under stress and is often concentrated in to large dome-shaped masses; the rising salt dome deforming surrounding bedding can produce associated oil traps.

Examples of igneous rocks are basalt, granite and tuff.

Limestone: Limestones are a group of rocks rich in calcium carbonate. Limestones range from having a high proportion of calcium carbonate-rich skeletal matter, e.g. reef limestones or -2© ReedHycalog. All rights reserved.

Commonly Drilled Igneous Rocks: Basalt: Basalts are black, dense rocks associated with lava flows and the margins of minor intrusions. Basalts have a crystalline structure, with small individual crystals; they are essentially composed of calcium-rich plagioclase feldspar, augite and olivine, a ferromagnesian mineral. Granite: Granite is often termed an acid igneous rock and is associated with metamorphic rocks and areas of mountain formation, or orogeny. Granites have a coarse-grained granular crystalline structure; they contain a high proportion of quartz and orthoclase feldspar, with variable amounts of plagioclase, biotite and muscovite mica. Granite is a relatively light rock. Tuff: The term tuff refers to any volcanic rock formed from lithified ash layers or other volcanic ejectiles & pyroclastic material.

Metamorphic Rocks: Metamorphic rocks form as a result of the application of heat, pressure and the chemical action of pore fluids on existing material; this action results in a change in the rock’s chemical composition along with new textures and structures within the rock itself. Sedimentary, igneous and even previously metamorphosed rocks can all undergo metamorphism producing metamorphic rocks. A general characteristic of metamorphic rocks is their crystallinity: they consist of a fabric of interlocking crystal grains aligned in a planar structure, or foliation, developed in the direction of least stress. Occasionally, in metamorphic rocks with only one constituent mineral, such as marble or metamorphosed limestone, a strong foliation is not present, instead a granular or sucrose texture can be observed. Metamorphic rocks are classified based on presence or absence of foliation or planar texture. The process of metamorphism reduces a rock’s porosity. Examples of metamorphic rocks include slate, schist, gneiss, quartzite and marble.

References 1. 2. 3. 4. 5.

Press F. and Siever R. 1994: “Understanding Earth.” Hamblin W. K.1985: “The Earth’s Dynamic Systems, 4th Edition.” Deer, Howie & Zussman, 1996: “An Introduction To The Rock Forming Minerals”, 2nd Edition. Barnes J. 1993: “Basic Geological Mapping”, 2nd Edition. http://www.en.Wikipedia.org -3© ReedHycalog. All rights reserved.