Batuan & Properti Reservoir

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BATUAN & PROPERTI RESERVOIR

Sedimentary rocks are classified into threeclassified into three groups. These groups are clastic, chemical precipitate and biochemical (or bi i )biogenic).

Siliciclastic rocks are clasticnoncarbonate sedimentary rocksthat are almost exclusively silica-bearing, either as forms of quartz or other silicate minerals.quartz or other silicate minerals. All siliciclastic rocks are formed by inorganic processes, or d i d h hdeposited through some mechanical process, such as stream deposits that are s e depos s esubsequently lithified. They are sandstone based rocks

ti f b t 50 60% faccounting for about 50 - 60% of the world oil and gas exploration.p

SANDSTONESANDSTONE

Clastic sedimentary rocks may be regarded as falling along a scale of grain size withas falling along a scale of grain size, with shale being the finest with particles less than 0.002 mm, siltstone being a little bigger with particles between 0.002 to 0.063 mm, and sandstone being coarser still with grains 0 063 to 2 mm andstill with grains 0.063 to 2 mm, and conglomerates and breccias being more coarse with grains 2 to 263 mm. Breccia h h i l hil lhas sharper particles, while conglomerate is categorized by its rounded particles. Particles bigger than 263 mm are termedParticles bigger than 263 mm are termed blocks (angular) or boulders (rounded). Lutite, Arenite and Rudite are general t f di t k ith l / iltterms for sedimentary rock with clay/silt-, sand- or conglomerate/breccia-sized particles.p

The colour of a sedimentary rock is often mostly determined by iron, an element which has two major oxides: iron(II) oxide and iron(III) oxide. Iron(II) oxide only forms underIron(II) oxide only forms under anoxic circumstances and gives the rock a grey or greenish colour. I (III) id i ft i th f fIron(III) oxide is often in the form of the mineral hematite and gives the rock a reddish to brownish colour. In arid continental climates rocks are in direct contact with the atmosphere, and oxidation is an importantand oxidation is an important process, giving the rock a red or orange colour

The size, form and orientation of clasts or minerals in a rock is called its texture Theis called its texture. The texture is a small-scale property of a rock, but determined many of its large-scale properties, such as the density, porosity ordensity, porosity or permeabililty.Clastic rocks have a 'clastic t t ' hi h thtexture', which means they consist of clasts. The 3D orientation of these clasts is called the fabric of the rock. Between the clasts the rock can be composed of a matrican be composed of a matrixor a cement

Th f f l t bThe form of a clast can be described by using four parameters:p'surface texture' describes the amount of small-scale relief of th f f i hi h ithe surface of a grain which is too small to have influence on the general shape; g p'rounding' describes the general smoothness of the shape of a grain;grain; 'sphericity' describes the degree in with the grain approaches a sphere; and 'grain form' is used to describe the 3D shape of the grainthe 3D shape of the grain.

The mineralogy of a clastic rock is determined by the supplied materialdetermined by the supplied material from the source area, the manner of transport to the place of deposition

d th t bilit f ti land the stability of a particular mineral. The stability of the major rock forming minerals (their g (resistance to weathering) is expressed by Bowen's reaction series In this series quartz is mostseries. In this series, quartz is most stable, followed by feldspar, micasand other less stable minerals that will only be present when little weathering occurred.

The amount of weathering dependsweathering depends mainly on the distance to the source area, the local climate and the time it took for the sediment to b t t d th Ibe transported there. In most sedimentary rocks, mica feldspar and lessmica, feldspar and less stable minerals will have reacted to clay mineralslike kaolinite, illite or smectite.

The term diagenesis is used to describe all the chemical, physical, and biological changes, including cementationincluding cementation, undergone by a sediment after its initial deposition, p ,exclusive of surface weathering. Some of these processes cause the sediment to consolidate: a compact solid substancecompact, solid substance forms out of loose material.

Carbonates form a largeCarbonates Carbonates form a large proportion of all sedimentary rocks.

Carbonatessedimentary rocks.Carbonate rocks dominantly consist of carbonate minerals like calcite, aragonite or dolomite Both cement anddolomite. Both cement and clasts (including fossils and ooids) of a carbonate rock )can consist of carbonate minerals. Carbonates usually have an irregular structure.

The most common are calcite or calcium carbonate, CaCO3, the chief constituent of li t ( ll thlimestone (as well as the main component of mollusc shells and coralmollusc shells and coralskeletons); dolomite, a calcium-magnesium carbonate CaMg(CO3)2; and siderite, or iron (II)

b t F COcarbonate, FeCO3, an important iron ore.

Limestone is partially solubleLimestone is partially soluble, especially in acid, and therefore forms many erosional landforms. forms many erosional landforms. These include limestone pavements, pot holes, cenotes, caves and gorges. Such erosion landscapes are known as karsts. Limestone is less resistant thanLimestone is less resistant than most igneous rocks, but more resistant than most other sedimentary rocks. Limestone is therefore usually associated with hills and downland and occurs in regions with other sedimentary rocksrocks,

Special carbonate typesp yp

Chalk is a special form of limestone and is formed from the skeletons of small

t ( lith )creatures (cocoliths).Evaporites such as Salt (NaCl) and Anhydrite(NaCl) and Anhydrite (CaSO4) can also form in these environments.

Components deposited as discrete p pgrains or crystals

Contain lime mud, < 10% grains: (Lime) Mudstone Contain lime mud, (matrix supported) >10% grains: Wackestone Contains lime mud, grain supported: Packstone No lime mud, grain supported: G iGrainstone original components bound together during deposition by f k b ildi iframework building organisms, encrustation or sediment trapping mechanisms : BoundstoneBoundstoneDepositional texture not recognizable. Crystalline

Vuggy porosity is pore space that is within grains or crystals or that is significantly larger than grains or crystals; that is, pore space that is not interparticle. This definition deviates from the restrictive definition of vugs used by Choquette and Pray (1970) as C q y ( )nondescript, nonfabric-selective pores, but it is consistent with the Archie terminology and with theArchie terminology and with the widespread and less restrictive use in the oil industry of the term "vuggy porosity" in referring to"vuggy porosity" in referring to visible pore space in carbonate rocks

Rock PropertiesRock Properties

Rocks are described by three properties:y p pPorosity - quantity of pore spacePermeability- ability of a formation to flowflowMatrix -major constituent of the rock

Definition of PorosityDefinition of Porosity

Porosity is a measure of thePorosity is a measure of the void spaces in a material, and is a fraction of the volume of voids over thevolume of voids over the total volume, between 0–1, or as a percentage between 0–100%100%It is defined by the ratio:where VV is the v of void-space (such as fluids) and VTis the total or bulk volume of material, including the solid and void components

Porosity SandstonesThe porosity of a sandstone depends on the packing arrangement of its grains.The system can be examined using spheres

In a Rhombohedral packing, the pore space accounts for 26% of the total volume.

With a Cubic packing arrangement, the pore space fills 47% of the total volumefills 47% of the total volume.

In practice, the theoretical value is rarely reached p , ybecause:

a) the grains are not perfectly round, and

b) the grains are not of uniform size.

Well sorted (grains of approximately all one size)approximately all one size) materials have higher porosity than similarly sized poorly

t d t i l ( h llsorted materials (where smaller particles fill the gaps between larger particles). The graphic illustrates how some smaller grains can effectively fill the pores (where all water flow takes p (place), drastically reducing porosity and hydraulic conductivity, while only being aconductivity, while only being a small fraction of the total volume of the material.

Porosity and Grain Sizey

• A rock can be made up of small grains or large grains but have the same

itporosity.• Porosity depends

on grain packingon grain packing, not the grain size.

Primary porosity The main or original porosity system in a rock or unconfined alluvial deposit.

Secondary porosity A subsequent or separate porosity system in a rock often enhancingsystem in a rock, often enhancing overall porosity of a rock. This can be a result of chemical leeching of minerals or the generation of aminerals or the generation of a fracture system. This can replace the primary porosity or coexist with it (see dual porosity below).with it (see dual porosity below).

Macro porosity :Refers to pores greater than 50 nm in diameter. Flow gthrough macropores is described by bulk diffusion.

Meso porosity :Refers to pores greater than 2 nm and less than 50 nm in diameter Flow through mesopores isdiameter. Flow through mesopores is described by Knudsen diffusion.

Mi it R f t llMicro porosity :Refers to pores smaller than 2 nm in diameter. Movement in micropores is by activated diffusion.

DIAGENESIS

The environment can also involveThe environment can also involve subsequent alterations of the rock such as:Chemical changes,Diagenesis is the chemical alteration of a rock after b i l A l i thburial. An example is the replacement of some of the calcium atoms in limestone by magnesium toatoms in limestone by magnesium to form dolomiteMechanical changes - fracturing in a tectonically-active region.

Carbonate porosity is very heterogeneous. It is classified into a number of types: Intergranular porosity is calledinto a number of types: Intergranular porosity is called "primary porosity".

Interparticle porosity:Each grain is separated, giving a similar pore space arrangement as sandstone.

Intergranular porosity:Pore space is created inside the individualis created inside the individual grains which are interconnected.

Intercrystalline porosity:Produced y p yby spaces between carbonate crystals.

Mouldic porosity:Pores created by the dissolution of shells, etc.

Porosity created after deposition is called " d it ""secondary porosity".

Fracture porosity:Pore spacing created by the cracking of the rock fabriccracking of the rock fabric.

Ch l iChannel porosity:Similar to fracture porosity but larger.

Vuggy porosity:Created by the dissolution ofCreated by the dissolution of fragments, but unconnected.

Permeability Definition y

The rate of flow of a liquidThe rate of flow of a liquid through a formation depends on:Th d thThe pressure drop,the viscosity of the fluid and The permeability.

The pressure drop is a reservoir property.The viscosity is a fluid property.The permeability is a measureThe permeability is a measure of the ease at which a fluid can flow through a formation

Permeability and RocksRelationships exist between permeability and porosity for given formations, althoughand porosity for given formations, although they are not universal.A rock must have porosity to have any permeabilitypermeability.The unit of measurement is the Darcy.Reservoir permeability is usually quoted in

illid i ( d)millidarcies (md).

In formations with large grains, the permeability is high and the flow rate larger.gIn a rock with small grains the permeability is less and the flow lower.lower.

Reservoir rocks need two properties to betwo properties to be successful:Pore spaces able to pretain hydrocarbon.Permeability which ll th fl id tallows the fluid to

move.