Petroleum Geoscience and Geophysics Chapter 6

7/23/2019 Petroleum Geoscience and Geophysics Chapter 6

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CHAPTER 6

CARBONATE RESERVOIRROCKS

Carbonates Reservoir

• Rock composed mainly of carbonate minerals.• 3 most common carbonates• minerals :

• – Calcite - CaCO 3 (Rhombohedral)

• – Aragonite - CaCO 3 (Orthorhombic)

• – Dolomite - CaMg(CO 3 )2 • (Rhombohedral) .

Textures: (3 primary textures)

A. Carbonate Grains -• Silt size or larger particles of calcite: • • c las ts - rock fragments derived• from weathering of limestones.

• • sk eleta l par t ic les - microfossilsor fragments or macrofossils

• – zooplankton (foraminifera),corals, molluscs.

• • oo ids - coated carbonate grains• with “seed” nucleus (ie., qtz grain, shell frag.)

that have a concentric or radial internal

structure.• – mainly aragonite.

• • pe lo ids - spherical aggregates ofmicrocrystalline calcite.

o o i d s

• B. Microcrystalline calcite - (micrite )• Clay size or smaller particles of calcite:• • m u d - needle shaped• aragonite crystals (1 - 5 μm)

• • nannofos s i l s (coc co l i ths ) -

• calcareous phytoplankton• (1 - 5 μm)

• C. Sparry calcite (spar)• • Large crystals of calcite (0.02 to

0.1 mm)• – limestones/marbles• • Primarily diagenetic in origin -• 1. Precipitation of secondary

calcite in voids• 2. Recrystallization of fossil calcite

Recrystallizedmollusc

Classification of Carbonates

• Divided into limestones (CaCO 3 ) and dolomites CaMg(CO 3 )2 .

• Two carbonate classification systems are in

common use today, one by R.L . Folk(1959,1962) and the second by R.J. Dunham(1962) .

• Dunham System : can be divided intoMudstones , Wackestones , Packstones ,Grainstones and Boundstones according to thelimestones depositional textures.

Classification of carbonates by texture(Dunham, 1962)

Examples of boundstones

Examples of grainstones

Photomicrograph of limestone under ordinary light. This is a well-sorted oolite grainstone from the Upper Jurassic PortlandLimestone, Dorset, UK

Examples of dolomite

Photomicrograph of dolomite under ordinary light. This is acoarsely crystalline variety from the Zechstein (Upper

Permian) of the UK North Sea. Some porosity (pale blue) is visible

• Folk System using the allochem/interstitialmaterial system is very systematic and straightforward. The allochem name is combined with

the interstitial name (micrite or spar).• Allochemica l rocks are those that contain

grains brought in from elsewhere (i.e. similar todetrital grains in clastic rocks). Orthochemica l rocks are those in which the carbonatecrystallized in place.

• Allochemical rocks have grains that may consistof fossiliferous material, ooids , peloids , orintraclasts . These are embedded in a matrix

consisting of microcrystalline carbonate (calciteor dolomite), called micrite , or larger visiblecrystals of carbonate, called sparite .

• Sparite is clear granular carbonate that hasformed through recrystallization of micrite, or bycrystallization within previously existing voidspaces during diagenesis.

Classification scheme developed by Folk

Pore Morphology

• The porosity, permeability and pore spacedistribution in carbonate reservoir rocks arerelated to both the depositional environment

and the diagenesis of the sediment.• They are most commonly of secondary(diagenetic) origin although residual primarypore space does occur.

• Carbonates have a large range of porestructures. The pore structures (porosity) havebeen classified by Choquette and Pray , 1970):

Pore Morphology

• Fabric-selective porosity includes:•Interparticle porosity.•Intercrystalline porosity - typical of dolomites.•Fenestral porosity - by solution along beddingplanes or joint surfaces.•Skeletal , framework , molding , or shelter

porosity - selective solution of, within, or aroundfossil material.•Oomoldic porosity - selective solution of ooliths.

Choquette & Pray (1970)

Depositionalorigin

DiageneticOrigin

Depositionalorigin

Pore Morphology

• Non fabric-selective porosity includes :•Fracture porosity - by stress or shrinkage.•Channel porosity - widening and coalescence

of fractures.•Vuggy or cavernous porosity.

• Fabric selective or not :

•Bioturbation porosity - from boring andburrowing.•Breccia porosity - in some cases, really highfracture porosity.

Mechanicalorigin

Diageneticorigin

Tectonic or solutioncollapse origin

Biogenic

Diagenetic origin

Choquette & Pray (1970)

Oolite gnst: depositionalintergranularporosity

Purely Diagenetic Porosity -Intercrystalline Pores inDolostone

Lower RE Higher RE

Pore Morphology

• Lucia s (1983) classification of carbonate pore types intovuggy and interparticle categories.

• This scheme is especially important because itemphasizes that interparticle (grains or crystals)porosity and separate or touching vuggy porosity haveprofound effects on such reservoir petrophysicalcharacteristics.

• Interparticle influence is reflected by the “ P d ” values in psia, which indicate the mercury displacement pressurerequired to enter the pore systems.

Pore Morphology

Lucia (1983) Classification

A New Classification ( Wayne M. Ahr, 2008 ) - Helpsidentify, correlate, & map readily traceable rock/stratigraphic

attributes that covary with genetic .

A New Classification (Wayne M. Ahr, 2008 )

Why Add Another Classification?Two main reasons:1. Methods for correlating & mapping pore types

and related „flow units at reservoir scale is notaddressed in previous schemes. “How do I predictspatial distribution of these pore types?” 2. Ways to assess contribution of genetic pore

types to reservoir performance (petrophysical rocktyping) has not been adequately developed andtested.

Example 1: Depositional Porosity

N Haynesville Smackover field, LA

Oolite gnst; depositionalintergranular porosity

Example 2: Purely Diagenetic Porosity -Intercrystalline Pores in Dolostone

Example 3: Fracture Systems

Corbett et al., (1991)

Carbonate DepositionalEnvironments

• Carbonates are predominantly shallow water(depths <10-20 m) deposits.

• Carbonate deposition in general only occurs inenvironments where there is a lack of siliciclasticinput into the water.• Most carbonate deposition also requiresrelatively warm waters which also enhance theabundance of carbonate secreting organisms anddecrease the solubility of calcium carbonate inseawater.

Carbonate Depositional

Environments• The principal carbonate depositional

environments are as follows: Carbonate Platforms and Shelves.

Warm shallow seas attached the continents, areideal places for carbonate deposition.

Tidal Flats. Tidal flats are areas that flood during high tides

and are exposed during low tides.

EnvironmentsDeep Ocean.

Carbonate deposition can only occur in theshallower parts of the deep ocean unless organicproductivity is so high that the remains oforganisms are quickly buried.

Non-marine Lakes. Carbonate deposition can occur in non-marine

lakes as a result of evaporation.

Environments Hot Springs.

When hot water saturated with calciumcarbonate reaches the surface of the Earth at hotsprings.

Diagenesis and Porosity ofCarbonates

• Carbonate diagenesis begins at deposition andcontinues during burial and uplift.

• Carbonates undergo cementation , leaching and

diagenesis (mineral alteration, mineralinversion, neomorphism).• When carbonates are brought into contact with

waters of varying chemical composition, theyhave a great susceptibility to mineralogical andtextural change , cementation and dissolution .

• During uplift , fracturing, additional cementationand leaching may occur.

• The diagenesis of carbonates can take place inmany settings : the marine environment duringdeposition of the sediment, near the sedimentsurface where fresh waters penetrate thesediments, or in brines of the deepersubsurface.

Purely diagenetic " in vadose-phreatic caves

• Porosity : the original primary porosity incarbonates may be totally destroyed duringdiagenesis and significant new secondary

porosity may be created.• The types of porosities encountered are quite

varied. Interparticle, intraparticle, growth-framework, shelter and fenestral porosities aredepositional porosities .

• Depositional porosity is a function of rocktexture , grain sorting and shape .

• Porosity formed during diagenesis may bemoldic, channel, inter-crystalline, fracture orvuggy porosity.

• The relationship between porosity anddiagenesis is complex and variable.

• The major diagenetic processes affectingporosity are dissolution , cementation anddolomitization .

Fractured reservoir

• Fractures are defined as naturally occurringmacroscopic planar discontinuities in rock dueto deformation or physical diagenesis (Nelson,

2001 ).• Most fractured reservoirs, especially in

carbonates, are brittle fractures .• In brittle behavior, different fracture types can

result depending on whether compression ,extension , or shear stresses caused failure.

Fractured reservoir

• Conjugate shear fractures are produced at anacute angle to the maximum principal stress σ 1 ,and a single extension fracture is oriented in a

plane parallel to σ 2 .• Extension fractures are always oriented parallel

to σ 1 and σ 2 and perpendicular to σ 3 and onlywhen principal stresses are compressive.

• Tension fractures have the same spatialorientation but occur only when σ 3 is negative.

Fractured reservoir

• The typical orientation of conjugate shear and extension fractures withrespect to the axes of maximum principal stress. When the maximumprincipal stress ( σ 1 ) is vertical, fractures typically occur in pairs calledconjugate shear sets.

Fractured reservoir

Fractured reservoir – Geneticclassification

• Nelson s (2001) genetic classification of naturalfractures identifies (1) tectonic fractures, (2)regional fractures, (3) contractional fractures,

and (4) surface – related fractures.• Stearns and Friedman (1972) focused attention

on the fracture sets associated with folds thatare important for exploration and developmentmodels. They pointed out that two main sets offractures are typical on anticlinal folds.

Fractured reservoir

(From Stearns and Friedman (1972)

Fractured reservoir

• A. A set of conjugate shear fractures and an extensionfracture indicating that σ 1 is oriented in the dip directionof the bedding on the fold limb, σ 1 and σ 3 are in theplane of bedding, and σ 2 is normal to bedding.

• B. The other fractures consist of a conjugate set ofshear fractures and an extension fracture, but theprincipal stresses are oriented differently.

• In this case, σ 1 is parallel to the strike of bedding and σ 3

is oriented in the dip direction of bedding on the foldlimb.

Fractured reservoir

• Corbett et al . (1991) classification :• Tectonic fractures commonly occur in

predictable patterns determined by the

geometry of the associated faults or folds.• The four structural types included anticlinal

folds, monoclinal flexures, listric normal faults,and graben-in-graben normal faults.

Fractured reservoir

(Corbett et al. (1991)

Fractured reservoir

• A fracture system may contain all of the porevolume for the reservoir as well as controllingthe permeability , or provide permeability for a

porous but otherwise low-permeability reservoir.• Open fractures can enhance the permeability of

an already permeable reservoir.• Conversely, closed fractures and faults with clay

smear or nonreservoir-to-reservoir juxtapositionwill increase the compartmentalization in areservoir.

Zagros Mountain – Upper beds of Asmari Limestone showing the beddingplane distribution and related variations in fracture density

Petroleum Geoscience and Geophysics Chapter 6

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