Notes

11

Reservoir Rocks

Reservoir Rocks

Schlumberger 1999

Notes

22

Reservoir Rocks

The Earth

Crust

MantleCoreInner Core

solid

liquid

plastic

plastic

750 miles 1400 miles 1800 miles

10 miles

= 10.7 g/cc = 4.0 g/cc

The earth is made up of a number of components. At the centre is thesolid core which is Nickel - Iron ; around this is a liquid core of the samematerial. The next part is a liquid called the Mantle, composed of muchlighter materials. Finally there is a solid crust, a very thin sheath whencompared to the total diameter.

Notes

The crust is not one solid skin on the Mantle. It is broken into a number ofirregular plates . The plates can be large, the pacific plate, or relativelysmall, some of the Mediterranean plates. The centres of the plates arestable environments while the edges are the earthquake/volcano regionsof the earth. These plates move around driven by the convection currentsin the mantle.

33

Reservoir Rocks

The Earth 2

Notes

The mantle is plastic. It flows in convection currents from the very hotcore to the outer Mantle/crust. These currents cause the crust to move.The currents are continuous and are responsible for all the features on theearth's surface.

44

Reservoir Rocks

Plate Tectonics 1

Notes

Two types of features are caused by the movement of the plates. The firstset are compressional. Here two plates are pushed together. They cancreate a zone of mountains or one plate can go under the other creating atrench. Mountains are usually associated with trenching as well.

55

Reservoir Rocks

Compressional Features

Notes

On the other side of the currents tensional effects are found. Here theplate is stretched out thin creating faults and rifts and eventually a newplate.Both compressional and tensional features play a large role in thestructures of reservoirs.

66

Reservoir Rocks

Tensional Features

Notes

This diagram shows two ocean plates colliding in a compressional event.The trench is formed on one side while mountains (volcanoes) are createdon the other.

77

Reservoir Rocks

Ocean plate - Ocean Plate

Trench

Mountains

Notes

This is a typical trenching effect with the ocean plate being forced downunder the continental plate. The latter is forced up into a mountain chain,while there is a trench formed at the boundary.An example of this type of feature is found on the western side ofSumatra. The island has a range of volcanic mountains while offshore is adeep trench.The ocean plate is being driven by the creation of a mid-ocean ridge. Agood example of this types of feature is the Mid Atlantic Ridge whichstretches from Iceland to below Argentina.

88

Reservoir Rocks

Ocean plate - Continental plate

Mid Ocean Ridge Trench

Continentalplate

Magma Magma

Mountains

Ocean plate

Notes

Two continental plates colliding create a mountain between them.Compressional forces driving this effect. The entire region surroundingthe mountains with be heavily affected by faulting and fracturing.

99

Reservoir Rocks

Continental - Continental

Notes

This slide shows a number of the plates and other features of the IndianOcean. Several mid-ocean ridges are clearly visible delineating the edgesof the plates. The plates contain features such as basins and plateaus. thelatter are higher regions, some even forming island chains.At the edges of the plates are features such as the Java trench, createdwhere the ocean plate moving east is going under the continental plate.

1010

Reservoir Rocks

Plates

Notes

The rocks forming the earths crust are broken down into three majorclasses reflecting their origins.Igneous coming from molten material of the mantle, sedimentary rocksfrom sediments and metamorphic from the effects of heat and pressure ofboth of the others.

1111

Reservoir Rocks

Rocks General

There are three major classes of rock:

Igneous:

(e.g. Granite).

Sedimentary:

(e.g. Sandstone).

Metamorphic:

(e.g. Marble).

Notes

Volcanic rocks are those seen immediately after a volcanic eruption. Theycool quickly resulting in an amorphous structure. They have no texture.Plutonic rocks cool much slower as they come up from the Mantle andstop much deeper inside the crust. They have a crystalline structure.Continuing movements of the crust may bury the volcano and bring theplutonic rock to a shallower depth or even surface.

1212

Reservoir Rocks

Igneous RocksComprise 95% of the Earth's crust.

Originated from the solidification of moltenmaterial from deep inside the Earth.

There are two types:

Volcanic - glassy in texture due to fast cooling.

Plutonic - slow-cooling, crystalline rocks.

Notes

A granite has no porosity or permeability of its own, however tectonicforces may fracture the rock. Into these fractures hydrocarbons can flowto create a reservoir.The nature of volcanoes is to eject material which is mixed with thealready existing formations. This is what happened in some places wherethe sandstone of the reservoir has volcanic debris mixed into it.

1313

Reservoir Rocks

Igneous Rocks and Reservoirs

Igneous rocks can be part of reservoirs.Fractured granites form reservoirs in someparts of the world.

Volcanic tuffs are mixed with sand in somereservoirs.

Notes

The effect of heat and pressure is to transform the rock into a new form.In doing this it destroys all porosity and any hydrocarbon. Metamorphicrocks do not exist in reservoirs.

1414

Reservoir Rocks

Metamorphic Rocks

2) Metamorphic rocks formed by the action of temperatureand/or pressure on sedimentary origneous rocks.

Examples are

Marble - formed from limestoneHornfels - from shale or tuffGneiss - similar to granite but

formed by metamorphosis

Notes

Sedimentary rocks are formed from the material of other rocks whichcould be igneous, metamorphic or older sedimentary rocks. Theclassification splits those rocks which form from materials transportedfrom one place to another - clastic rocks, from rocks which are createdfrom materials in their place of formation ; no transportation - non clasticrocks.

1515

Reservoir Rocks

Sedimentary Rocks

The third category is Sedimentary rocks. Theseare the most important for the oil industry as itcontains most of the source rocks and cap rocksand virtually all reservoirs.

Sedimentary rocks come from the debris ofolder rocks and are split into two categories

Clastic and Non-clastic.

Clastic rocks - formed from the materials of older rocks by the actions of erosion, transportation and deposition.

Non-clastic rocks -from chemical or biological origin and then deposition.

Notes

The depositional environment often plays a vital part of the evaluation ofa well and a field. This often defines the major lithology and points to thepossibilities of minor minerals. For example the shallow fan of the deltain the slide produces a conglomerate , the deep water is showing shales (fine sediments ). Clues to the deposition come from a lot ofmeasurements in and around the well. Core data is invaluable for thefossils, something that cant be seen on logs. The analysis of Dipmetercurves was always one of the first steps to choosing the depositionalenvironment. Lately the imaging tools have made the process much easierwith high resolution borehole images.

1616

Reservoir Rocks

Depositional EnvironmentsThe depositional environment can be

Shallow or deep water.

Marine (sea) and lake or continental.

This environment determines many of thereservoir characteristics

Notes

The classical continental deposition of sand dunes produces an excellentreservoir quality reservoir rock. To create a reservoir the dune has to beburied with a source rock and cap rock providing the rest of the elementsof the reservoir.The sediments carried down rivers will be deposited once the energy ofthe river currents drops. Heavier particles will come out first, leaving thefine sediments to go into deep water.

1717

Reservoir Rocks

Depositional Environments 2Continental deposits are usually dunes.A shallow marines environment has a lot ofturbulence hence varied grain sizes. It can alsohave carbonate and evaporite formation.A deep marine environment produces finesediments.

Notes

An example of changing properties with deposition is water depth. Asclastic rocks are deposited by, for example a river, into the sea, the firstdeposits are the coarser sediments as they fall out of the river first. Finergrains are taken further out into the deep ocean.Sediments deposited in deep water will form poor quality reservoir rocksas the fine grains lead to poor permeability.

1818

Reservoir Rocks

Depositional Environments 3

The depositional characteristics of the rockslead to some of their properties and that of thereservoir itself.

The reservoir rock type clastic or non-clastic.The type of porosity (especially in carbonates) isdetermined by the environment plus subsequentevents.

The structure of a reservoir can also bedetermined by deposition; a river, a delta, a reefand so on.

This can also lead to permeability andproducibility. of these properties are oftenchanged by further events.

Notes

The tectonic forces do not stop. The sedimentary rock which wasdeposited flat will be pushed (or pulled) into different shapes. the foldsand faults resulting from these movements will provide the environmentfor the reservoir traps.

1919

Reservoir Rocks

Depositional Environment 4

The environment is not static.Folding and faulting change the structure.Dissolution and fracturing can change thepermeability.

Notes

The beginning of a sedimentary rock is a sediment in water, for examplea beach sand. The grains are separate .As this is buried deeper and deeperthe pressure forces out the water and creates the rock.

2020

Reservoir Rocks

Sedimentation

Sediments settle tothe bottom of thesedimentary basin.

As the sediments accumulate thetemperature andpressure increase

expelling water fromthe sediments.

Notes

The sedimentary can undergo a number of transformations during itslifetime. The continual movement of the plates will bury and layer andthen bring it back to surface. The rock will be changed by the differingforces and elements present in its environment.

2121

Reservoir Rocks

Sedimentation 2

Sedimentary muds become sedimentary rocks.

Calcareous muds become limestone.Sands become sandstone.

Another effect involves both the grains in thematrix and the fluids reacting to create newminerals changing the matrix and porosity.Fluids can also change creating a new set ofminerals.

This whole process is called Diagenesis.

Notes

The start and end of all rocks is the magma in the mantle . This is cooledto create igneous rocks. these can be broken down into sediments. Thesediments are turned into sedimentary rocks. These can be buried deeperwith heat and pressure, turning into metamorphic rocks. If these are thenheated we return to the magma. Inside this major cycle are subcycles.Igneous rocks can be heated to give metamorphic rocks. Any rocks can bebroken into sediments to give sedimentary rocks.

2222

Reservoir Rocks

Rock Cycle

Notes

Sands are a reservoir rock, while shales are a source rock and a cap rock.The shales are very fine grained and although the can contain fluids thiscan only leak out in geological time, very slowly.

Shales and silts also contain other minerals than Quartz. The sedimentsare buried to create the sedimentary rock, initially filled with water.

The source form which these rocks are formed can be of any type of rock.

2323

Reservoir Rocks

Clastic Rocks

Clastic rocks are sands, silts and shales. Thedifference is in the size of the grains.

Notes

Deltas can have huge extents. There are also a large number of potentialtraps in this environment, channels, bars and sheets of sands further out inthe deeper water. hence the delta is one of the most prolific hydrocarbonenvironments.

They are also complex with the structure ranging from shallow ,shoreline to deep water.

2424

Reservoir Rocks

Depositional Environment - Delta

Sediments are transported to the basins byrivers.A common depositional environment is the deltawhere the river empties into the sea.A good example of this is the Mississippi.

Notes

Ancient river beds below the current level can add up to a considerablethickness, although the individual river channels are small, they arenumerous leading to a commercial reservoir.

The shape of a river/channel type deposition is often complicated, causingproblems for well placement. Dipmeter analysis helps the geologistdetermine the direction of paleoflow and hence decide where to put thenext well.

2525

Reservoir Rocks

Rivers

Some types of deposition occur in rivers andsand bars.The river forms a channel where sands aredeposited in layers. Rivers carry sediment downfrom the mountains which is then deposited inthe river bed and on the flood plains at eitherside. Changes in the environment can cause thesesands to be overlain with a shale, trapping thereservoir rock.

Notes

Carbonates contain about half the worlds reserves in less than half of thereservoirs mainly due to the super giant fields of the Middle East.

2626

Reservoir Rocks

Carbonates

Carbonates form a large proportion of allsedimentary rocks.

They consist of:Limestone.

Dolomite.

Carbonates usually have an irregular structure.

Notes

Limestones and dolomites are usually reservoir rocks. A very dense, lowporosity limestone can, occasionally,dolomitisation is a very importantmechanism as it not only creates porosity but permeability paths vital tosome reservoirs.

Chalk reservoirs tend to have very high porosity and very lowpermeability.

The evaporites are important as they form good cap rocks.

2727

Reservoir Rocks

Carbonate types

Chalk is a special form of limestone and isformed from the skeletons of small creatures(cocoliths).

Dolomite is formed by the replacement of someof thecalcium by a lesser volume of magnesiumin limestone by magnesium. Magnesium issmaller than calcium, hence the matrix becomessmaller and more porosity is created.

Limestone CaCO3

Dolomite CaMg(CO3)2

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

Notes

A reef is the simplest carbonate deposition, the skeletons of the reefanimals.

In the shallow lagoons, Calcium Carbonate is deposited. Shells and so onare added to the mixture. Changes in sea level allow the deposition of saltor anhydrite as a seal.

Carbonate deposition is very complex as the rocks themselves have particle sizes ranging from whole shells to line mud. The basicdeposition is in shallow seas from biological and chemical action. CaCo3is soluble hence can be transported around as a solute and thenreprecipitated elsewhere.

In addition to the carbonates these environments also produce evaporitessuch as salt ( NaCl ) and anhydrate ( Ca So4 ) . Other rocks include pyrite( FeS2 ) and siderite ( FeCo3 ) and chert, microcrystalline quartz, thecarbonate reservoir is hence very complex.

2828

Reservoir Rocks

Depositional EnvironmentCarbonates

Carbonates are formed in shallow seascontaining features such as:

Reefs.

Lagoons.

Shore-bars.

Notes

These are the major petrophysical properties of the rocks, they determinehow much oil can be contained and how well it will flow.There are many other ways to describe a rock from a geologicalperspective.The minor constituents often determine how a rock behaves as areservoir, hence they are included in the description. For example theshale content of a sandstone and the type of shale will be used.

2929

Reservoir Rocks

Rock Properties

Rocks are described by three properties:

Porosity - quantity of pore space

Permeability - ability of a formation to flow

Matrix - major constituent of the rock

Notes

The amount of porosity gives the volume of the reservoir containingfluids. As it is a fraction it can be described as a number e.g. 0.25 orcommonly as a percentage, 25%. Porosity can range from zero to over50%. In normal reservoirs the range of 20% - 39%.

3030

Reservoir Rocks

Definition of Porosity

Notes

The two packing models shown represent some of the possibilities .Cubicpacking , with a porosity in excess of 47% is the theoretical maximumwhich is rarely reached.

These pictures are valid in a lot of cases as the sand sediments depositedare often of uniform size and shape. The addition of smaller grains willreduce the porosity.

Chalk often exhibits cubic packing.

3131

Reservoir Rocks

Porosity Sandstones

The porosity of a sandstone depends on thepacking arrangement of its grains.The system can be examined using spheres.

In a Rhombohedral packing, the porespace accounts for 26% of the totalvolume.

With a Cubic packing arrangement,the pore space fills 47% of the totalvolume.

In practice, the theoretical value israrely reached because:

a) the grains are not perfectly round,and

b) the grains are not of uniform size.

Notes

In a clastic rock the grain size ( same size grains ) does not affect theporosity. Thus a sand, a silt and a shale can have the same porosity .Thedifferences come in permeability where the grain size has a direct effect,large grains meaning higher permeability. This is the reason that auniversal porosity - permeability transform does not work; two rocks withthe same porosity but different grain sizes will not have the samepermeability. The saturation can occur even in the same sandstone layer in a reservoir in a sequence where the grain size has changed duringdeposition e.g.. a firing up sequence.

This implies that the silts and shales have porosity containing fluid. Thefluid is water as the pore size is so small that capilliary forces preventhydrocarbon from entering.

3232

Reservoir Rocks

Porosity and Grain Size

A rock can be made up of small grains or largegrains but have the same porosity.

Porosity depends on grain packing, not thegrain size.

Notes

Sedimentary rocks are subject to changes over time. If water of a differentchemical composition flows through the rock, reactions can occurchanging the rock type or dissolving some of it.Tectonic forces are always present. They crack the rock creatingfractures.

3333

Reservoir Rocks

Diagenesis

The environment can also involve subsequentalterations of the rock such as:Chemical changes.Diagenesis is the chemical alteration of a rockafter burial. An example is the replacement ofsome of the calcium atoms in limestone bymagnesium to form dolomite.

Mechanical changes - fracturing in atectonically-active region.

Notes

This set of porosity types is classified as fabric selective.

The porosity created due to changes in the composition.

Intercrystalline porosity can be due to dolomitisation.

Mouldic porosity is where specific shells have been leached from therock.

3434

Reservoir Rocks

Carbonate Porosity Types 1

Interparticle porosity:

Each grain is separated,giving a similar pore spacearrangement as sandstone.

Intergranular porosity:

Pore space is created insidethe individual grains whichare interconnected.

Intercrystalline porosity:

Produced by spaces betweencarbonate crystals.

Mouldic porosity:

Pores created by thedissolution of shells, etc.

Carbonate porosity is very heterogeneous. It isclassified into a number of types:

Notes

This set of porosities are not fabric selective, i.e.. they happen to theentire rock. Fractures crack through any of the types of mineral or shell in the rock.

These two sets are not the end of the classification of carbonate porositytypes, many more schemes exist.

From a petrophysical point of view the porosity can be classed by the twomain types.

3535

Reservoir Rocks

Carbonate Porosity Types 2

Fracture porosity:Pore spacing createdby the cracking of therock fabric.

Channel porosity:Similar to fractureporosity but larger.

Vuggy porosity:Created by thedissolution offragments, butunconnected.

Notes

Sandstones can also contain fractures and vugs, however this is rarer thanin the carbonates. In the case of vugs the latter are soluble whilesandstone is not.

3636

Reservoir Rocks

Carbonate Porosity

Intergranular porosity is called "primaryporosity".

Porosity created after deposition is called"secondary porosity".

The latter is in two forms:

Fractures

Vugs.

Notes

Fractures are classed as either being vertical or horizontal. Although theycan appear at almost any angle, the majority are vertical to sub vertical.They can penetrate from an oil column down into the water, and, as theyhave very high permeability, can cause production problems.

The amount of porosity in a fracture is very small as they are very fine, afew millimetres at most. Their most important property is thepermeability.

3737

Reservoir Rocks

Fractures

Fractures are caused when a rigid rock isstrained beyond its elastic limit - it cracks.The forces causing it to break are in a constantdirection, hence all the fractures are alsoaligned.

Fractures are an important source ofpermeability in low porosity carbonatereservoirs.

Notes

The full definition of vugs is more complicated. They are irregular holesin the rock. They have been caused by dissolution of shell (etc) fragmentsand also some of the matrix surrounding them. They can vary widely insize from a few microns to metres. In this context they are regarded asbeing a centimetres at most. In most cases the vugs are not connected toeach other in any producible manner and hence do not contribute to theformations productivity.

Carbonate rocks will frequently contain both vugs and fractures.

3838

Reservoir Rocks

Vugs

Vugs are defined as non-connected pore space.They do not contribute to the producible fluidtotal.Vugs are caused by the dissolution of solublematerial such as shell fragments after the rockhas been formed.They usually have irregular shapes.

Notes

The major difference in the two properties porosity or permeability is thatthe former is a static rock property while the latter is a dynamic rock andfluid property.The relationships between porosity and permeability are used inreservoirs to investigate changing permeabilities through the formation.They are often used as a lithofacies classification/description.

3939

Reservoir Rocks

Permeability Definition

The rate of flow of a liquid through a formationdepends on:

The pressure drop.The viscosity of the fluid.The permeability.

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

Relationships exist between permeability andporosity for given formations, although they arenot universal.A rock must have porosity to have anypermeability.The unit of measurement is the Darcy.Reservoir permeability is usually quoted inmillidarcies, (md).

Notes

The flow rate increases with increasing pressure drop; it decreases withincreasing length ; it increases with increasing surface area; it decreaseswith increasing viscosity. Putting this altogether gives an equation withthe unknown as the permeability, K.

The key term in this equation is k/, called the mobility. This determineshow a given reservoir will behave depending on the fluid in there.

4040

Reservoir Rocks

Darcy Experiment

The flow of fluid of viscosity m through aporous medium was first investigated in 1856 byHenri Darcy.He related the flow of water through a unitvolume of sand to the pressure gradient acrossit.In the experiment the flow rate can be changedby altering the parameters as follows:

Notes

Permeability is a metric ( but not SI ) unit.Note that the permeability depends directly on the flowrate, Q.

4141

Reservoir Rocks

Darcy Law

K = permeability, in Darcies.L = length of the section of rock, in centimetres.Q = flow rate in centimetres / sec.P1, P2 = pressures in bars.A = surface area, in cm2. = viscocity in centipoise.

Notes

The flow rate through the large pore spaces is high hence the permeabilityis high.

4242

Reservoir Rocks

Permeability and Rocks

In formations with large grains, thepermeability is high and the flow rate larger.

Notes

The flow rate through the small grained rocks is low hence thepermeability is low. The formation contrasts with the one in the previousslide; with the same porosity the permeabilities can differ dramatically.The ultimate contrast is between a very fine grained shale with zeropermeability and a coarse sandstone with a high permeability.

Due to bedding the permeability can change vertically to a clasticsequence . The vertical permeability kv is determined by the lowestpermeability layer. The horizontal permeability kh does not have thisproblem. The anisotropy , Kv/Kh describes the difference between thetwo. This ratio is always less than or equal to 1.

4343

Reservoir Rocks

Permeability and Rocks 2

In a rock with small grains the permeability isless and the flow lower.

Grain size has no bearing on porosity, but has alarge effect on permeability.

Notes

As noted a rock can have porosity but no permeability. If it has zeroporosity it will have zero permeability. In practical terms low porosityreservoirs ( < 10% ) exist.

4444

Reservoir Rocks

Reservoir Rocks

Reservoir rocks need two properties to besuccessful:Pore spaces able to retain hydrocarbon.Permeability which allows the fluid to move.

Notes

Sandstone reservoirs account for the majority of the worlds fields. Therewill always be bedding variations leading to differences in the quality ofthe reservoirs. The porosity and permeability are relatively simple toevaluate from core samples.Fractures may be important in low porosity reservoirs.

4545

Reservoir Rocks

Clastic Reservoirs

Sandstone usually has regular grains; and isreferred to as a grainstone.

PorosityDetermined mainly by the packing andmixing of grains.

PermeabilityDetermined mainly by grain size andpacking, connectivity and shale content.

Fractures may be present.

Notes

Carbonates reservoirs are difficult as their properties change in both thevertical and the horizontal directions, often in unpredictable ways.Fractures are nearly always present and can be essential to production.

4646

Reservoir Rocks

Carbonate Reservoirs

Carbonates normally have a very irregularstructure.

Porosity:Determined by the type of shells, etc. and

by depositional and post-depositional events(fracturing, leaching, etc.).

Permeability:Determined by deposition and post-deposition events, fractures.

Fractures can be very important in carbonatereservoirs.

Notes

The majority of cap rocks are shales as it is these rocks which arenormally present. Zero porosity carbonates not only form cap rocks butbarriers in the reservoir itself.

4747

Reservoir Rocks

Cap Rock

A reservoir needs a cap rock.

Impermeable cap rock keeps the fluids trappedin the reservoir.It must have zero permeability.Some examples are:

Shales.Evaporites such as salt oranhyhdrite.Zero-porosity carbonates.

Notes

The source rock is recognised on logs by a high GR (due to the highuranium content of organic material) and high resistivity. This is differentfrom the low resistivity of most shales.All of the processes associated with the source rock, sedimentation,hydrocarbon creation and migration take a very long time to happen.

4848

Reservoir Rocks

Source RocksHydrocarbon originates from minute organismsin seas and lakes. When they die, they sink tothe bottom where they form organic-rich"muds" in fine sediments.These "muds" are in a reducing environment or"kitchen", which strips oxygen from thesediments leaving hydrogen and carbon.The sediments are compacted to form organic-rich rocks with very low permeability.The hydrocarbon can migrate very slowly tonearby porous rocks, displacing the originalformation water.

Notes

The temperature at which the source rock has been cooked isimportant to the viability of the reservoir. It is closely related to the depthat which the rock was buried. As all this happened a long time in the pastthe geologist has to track the history of the source rock.

4949

Reservoir Rocks

Temperature Window

If the temperature is too low, the organicmaterial cannot transform into hydrocarbon.If the temperature is too high, the organicmaterial and hydrocarbons are destroyed.

Notes

Secondary migration is simple to understand with the higher hydrocarbonfloating to rest on top of the original water. The primary part of theprocess is much more complex. The exact mechanism is uncertain as theexperiment cannot be done in the laboratory ( high temperature andpressure and a very long time ).

5050

Reservoir Rocks

Hydrocarbon Migration

Hydrocarbon migration takes place in twostages:Primary migration - from the source rock to a porousrock.This is a complex process and not fully understood.It is probably limited to a few hundred metres.

Secondary migration - along the porous rock to the trap.This occurs by buoyancy, capillary pressure and hydrodynamicsthrough a continuous water-filled pore system.

It can take place over large distances.

Notes

Clastic rocks are classified initially by their grain size. There are manymore complex classifications for this type of rock but this is the simplest.In this list Conglomerates and Sandstones are reservoir rocks, Siltstonesand Shales are source rocks and shales are also cap rocks.Non- Clastics can be described by their chemical composition, there are,once again many more complex descriptions. Here limestone andDolomite are reservoir rocks and Silt and Anhydrite are cap rocks.

5151

Reservoir Rocks

Rock Classification

ClasticsRock type Particle diameter

Conglomerate Pebbles 2 - 64mmSandstone Sand .06 - 2mmSiltstone Silt .003 - .06mmShale Clay

Notes

The rocks compromising the reservoir undergo significant changes due totectonic movements. The most important is folding and faulting as it isthese alterations to the initial horizontal strata which create the structuresforming reservoir traps. The depositional environment contributes greatlyto the variety of trap. Shallow lagoons can have reefs as well as layers ofcarbonates.

5252

Reservoir Rocks

Reservoir Structure

There are many other types of structure.The criteria for a structure is that it must have:

Closure, i.e. the fluids are unable toescape.Be large enough to be economical.

The exact form of the reservoir depends on thedepositional environment and post depositionalevents such as foldings and faulting.

Notes

The key concepts are those of Net and Gross pay.Gross pay is always > Net pay. This can also be described by the Net -to -Gross ratio which is always less than or equal to one.The spill plane is the maximum level to which this particular reservoircan filled before the next anticline starts to be filled.

5353

Reservoir Rocks

Traps General

Notes

Structural traps describe all the large features and includes domes,anticlines and faults. These large scale reservoirs include most of theMiddle East giants.The difference between these two forms is in the manner of their creation,i.e. the forces applied to them.As they are created by tectonic pressures, there will invariably be faultsassociated with these structures.

5454

Reservoir Rocks

Structural TrapsThe simplest form of trap is a dome.This is created by upward movement or foldingof underlying sediments.

An anticline is another form of simple trap. Thisis formed by the folding of layers of sedimentaryrock.

Notes

Faulting is an important mechanism in most reservoirs. It forms reservoirsin its own right and also breaks other reservoirs down into specific blocks.Well testing helps determine the fault parameters such as distance from awell, angle and so on.Faulting of older blocks creating grabens also makes depositionalenvironments for new reservoir formation.In addition faults in reservoirs change the structure to give, as in thisdiagram, repeated layers and in the opposite type of fault, missingformations.

5555

Reservoir Rocks

Fault TrapsFaults occur when the rock shears due tostresses. Reservoirs often form in these faultzones.A porous and permeable layer may trap fluidsdue to its location alongside an impermeablefault or its juxtaposition alongside animpermeable bed.Faults are found in conjunction with otherstructures such as anticlines, domes and saltdomes.

Notes

Salt in creating the domes also adds faults and fractures due to the extrapressures on the rocks. The traps around the dome are difficult to find asanything below the Salt is invisible on the surface seismic. ( the contrastbetween the salt and anything else is too large ). However they can beexcellent reservoirs as the salts will have fractured them giving goodpermeability characteristics.

5656

Reservoir Rocks

Salt Dome TrapSalt Dome traps are caused when "plastic" saltis forced upwards.The salt dome pierces through layers andcompresses rocks above. This results in theformation of various traps:In domes created by formations pushed up bythe salt.Along the flanks and below the overhang inporous rock abutting on the impermeable saltitself.

Notes

Stratigraphic traps describe the traps associated with the depositionalenvironment. Reefs, channels and bars are from specific environments.Unconformities exist due to tectonic movements when a formation ;ananticline in the diagram is eroded ( it is above ground level ). It is thenburied and more sediments are added creating the seal and hence thereservoir.

5757

Reservoir Rocks

Stratigraphic Traps

Notes

Most reservoir maps in the world use m.s.l. as the reference. Depths ofthe layer increases away from the crest of the structure.The reference is needed because the drilling rig can be on top of amountain or an offshore platform. In each case the measured depth of thesame layer is different as the drilling reference is different.

5858

Reservoir Rocks

Reservoir Mapping

Reservoir contours are usually measured to bebelow Mean Sea Level (MSL).They can represent either the reservoirformation structure or fluid layers.