Chapter 6 Carbonate Reservoir Rocks PDF

MKPP 1213 APPLIED GEOSCIENCE & GEOPHYSICS LECTURER: DR. CHONG AIK SHYE CHAPTER 6 Carbonate Reservoir Rocks 3 Carbonates Reservoir • • • • Rock composed mainly of carbonate minerals. 3 most common carbonates minerals: – Calcite - CaCO3 (Rhombohedral) • – Aragonite - CaCO3 (Orthorhombic) • – Do...

MKPP 1213 APPLIED GEOSCIENCE & GEOPHYSICS LECTURER: DR. CHONG AIK SHYE CHAPTER 6 Carbonate Reservoir Rocks 3 Carbonates Reservoir • • • • Rock composed mainly of carbonate minerals. 3 most common carbonates minerals: – Calcite - CaCO3 (Rhombohedral) • – Aragonite - CaCO3 (Orthorhombic) • – Dolomite - CaMg(CO3)2 • (Rhombohedral). • Accessory minerals: ankerite, siderite, clay minerals. Textures: (3 primary textures) A. Carbonate Grains • Silt size or larger particles of calcite: • • clasts - rock fragments derived • from weathering of limestones. • • skeletal particles - microfossils or fragments or macrofossils • – zooplankton (foraminifera), corals, molluscs. Textures: (3 primary textures) • • ooids - coated carbonate grains • with “seed” nucleus (ie., qtz grain, shell frag.) that have a concentric or radial internal structure. • – mainly aragonite. ooids • • peloids - spherical aggregates of microcrystalline calcite. Textures: (3 primary textures) • • • • B. Microcrystalline calcite - (micrite) Clay size or smaller particles of calcite: • mud - needle shaped aragonite crystals (1 - 5μm) • • nannofossils (coccoliths)• calcareous phytoplankton • (1 - 5μm) Textures: (3 primary textures) • 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 Recrystallized mollusc Classification of Carbonates • Divided into limestones (CaCO3 ) and dolomites CaMg(CO3)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 into Mudstones, Wackestones, Packstones, Grainstones and Boundstones according to the limestones depositional textures. Classification of carbonates by texture (Dunham, 1962) Classification of carbonates by texture (Dunham, 1962) Classification of carbonates by texture (modified from Dunham, 1962) Examples of boundstones Examples of grainstones Examples of grainstones Photomicrograph of limestone under ordinary light. This is a wellsorted oolite grainstone from the Upper Jurassic Portland Limestone, Dorset, UK Examples of dolomite Photomicrograph of dolomite under ordinary light. This is a coarsely crystalline variety from the Zechstein (Upper Permian) of the UK North Sea. Some porosity (pale blue) is visible Classification of Carbonates • Folk System using the allochem/interstitial material system is very systematic and straight forward. The allochem name is combined with the interstitial name (micrite or spar). • Allochemical rocks are those that contain grains brought in from elsewhere (i.e. similar to detrital grains in clastic rocks). Orthochemical rocks are those in which the carbonate crystallized in place. Classification of Carbonates • Allochemical rocks have grains that may consist of fossiliferous material, ooids, peloids, or intraclasts. These are embedded in a matrix consisting of microcrystalline carbonate (calcite or dolomite), called micrite, or larger visible crystals of carbonate, called sparite. • Sparite is clear granular carbonate that has formed through recrystallization of micrite, or by crystallization within previously existing void spaces during diagenesis. Classification of Carbonates Classification scheme developed by Folk Pore Morphology of Carbonate • The porosity, permeability and pore space distribution in carbonate reservoir rocks are related to both the depositional environment and the diagenesis of the sediment. • They are most commonly of secondary (diagenetic) origin although residual primary pore space does occur. • Carbonates have a large range of pore structures. The pore structures (porosity) have been classified by Choquette and Pray, 1970): Pore Morphology • Fabric-selective porosity includes: •Interparticle porosity. •Intercrystalline porosity - typical of dolomites. •Fenestral porosity - by solution along bedding planes or joint surfaces. •Skeletal, framework, molding, or shelter porosity - selective solution of, within, or around fossil material. •Oomoldic porosity - selective solution of ooliths. Depositional origin Depositional origin Diagenetic Origin Diagenetic Origin Choquette & Pray (1970) 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 and burrowing. •Breccia porosity - in some cases, really high fracture porosity. Mechanical origin Tectonic or solution collapse origin Biogenic Diagenetic origin Diagenetic origin Choquette & Pray (1970) Oolite gnst: depositional intergranular porosity Lower RE Purely Diagenetic Porosity Intercrystalline Pores in Dolostone Higher RE Pore Morphology • Lucia’s (1983) classification of carbonate pore types into vuggy and interparticle categories. • This scheme is especially important because it emphasizes that interparticle (grains or crystals) porosity and separate or touching vuggy porosity have profound effects on such reservoir petrophysical characteristics. • Interparticle influence is reflected by the “Pd” values in psia, which indicate the mercury displacement pressure required to enter the pore systems. Pore Morphology Lucia (1983) Classification A New Classification (Wayne M. Ahr, 2008) - Helps identify, correlate, & map readily traceable rock/stratigraphic attributes that covary with genetic φ. A New Classification (Wayne M. Ahr, 2008) - Helps identify, correlate, & map readily traceable rock/stratigraphic attributes that covary with genetic φ. 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 not addressed in previous schemes. “How do I predict spatial distribution of these pore types?” 2.Ways to assess contribution of genetic pore types to reservoir performance (petrophysical rock typing) has not been adequately developed and tested. Example 1: Depositional Porosity N Haynesville Smackover field, LA Oolite gnst; depositional intergranular porosity Example 2: Purely Diagenetic Porosity - Intercrystalline Pores in Dolostone Example 3: Fracture Systems Corbett et al., (1991) Carbonate Depositional Environments •Carbonates are predominantly shallow water (depths <10-20 m) deposits. •Carbonate deposition in general only occurs in environments where there is a lack of siliciclastic input into the water. •Most carbonate deposition also requires relatively warm waters which also enhance the abundance of carbonate secreting organisms and decrease the solubility of calcium carbonate in seawater. Carbonate Depositional Environments • The principal carbonate depositional environments are as follows:  Carbonate Platforms and Shelves. Warm shallow seas attached the continents, are ideal places for carbonate deposition.  Tidal Flats. Tidal flats are areas that flood during high tides and are exposed during low tides. Carbonate Depositional Environments  Deep Ocean. Carbonate deposition can only occur in the shallower parts of the deep ocean unless organic productivity is so high that the remains of organisms are quickly buried.  Non-marine Lakes. Carbonate deposition can occur in non-marine lakes as a result of evaporation. Carbonate Depositional Environments  Hot Springs. When hot water saturated with calcium carbonate reaches the surface of the Earth at hot springs. Diagenesis and Porosity of Carbonates • Carbonate diagenesis begins at deposition and continues during burial and uplift. • Carbonates undergo cementation, leaching and diagenesis (mineral alteration, mineral inversion, neomorphism). • When carbonates are brought into contact with waters of varying chemical composition, they have a great susceptibility to mineralogical and textural change, cementation and dissolution. Diagenesis and Porosity of Carbonates • During uplift, fracturing, additional cementation and leaching may occur. • The diagenesis of carbonates can take place in many settings: the marine environment during deposition of the sediment, near the sediment surface where fresh waters penetrate the sediments, or in brines of the deeper subsurface. Diagenesis and Porosity of Carbonates Purely diagenetic " in vadose-phreatic caves Diagenesis and Porosity of Carbonates • Porosity: the original primary porosity in carbonates may be totally destroyed during diagenesis and significant new secondary porosity may be created. • The types of porosities encountered are quite varied. Interparticle, intraparticle, growth- framework, shelter and fenestral porosities are depositional porosities. • Depositional porosity is a function of rock texture, grain sorting and shape. Diagenesis and Porosity of Carbonates • Porosity formed during diagenesis may be moldic, channel, inter-crystalline, fracture or vuggy porosity. • The relationship between porosity and diagenesis is complex and variable. • The major diagenetic processes affecting porosity are dissolution, cementation and dolomitization. Fractured reservoir Fractured reservoir • Fractures are defined as naturally occurring macroscopic planar discontinuities in rock due to 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 an acute angle to the maximum principal stress σ1 , and a single extension fracture is oriented in a plane parallel to σ2. • Extension fractures are always oriented parallel to σ1 and σ2 and perpendicular to σ3 and only when principal stresses are compressive. • Tension fractures have the same spatial orientation but occur only when σ3 is negative. Fractured reservoir • The typical orientation of conjugate shear and extension fractures with respect to the axes of maximum principal stress. When the maximum principal stress ( σ 1 ) is vertical, fractures typically occur in pairs called conjugate shear sets. Fractured reservoir Fractured reservoir • The typical orientation of conjugate shear and extension fractures with respect to the axes of maximum principal stress. When the maximum principal stress ( σ1 ) is vertical, fractures typically occur in pairs called conjugate shear sets. Fractured reservoir – Genetic classification • Nelson’s (2001) genetic classification of natural fractures 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 that are important for exploration and development models. They pointed out that two main sets of fractures are typical on anticlinal folds. Fractured reservoir (From Stearns and Friedman (1972) Fractured reservoir • A. A set of conjugate shear fractures and an extension fracture indicating that σ1 is oriented in the dip direction of the bedding on the fold limb, σ1 and σ3 are in the plane of bedding, and σ2 is normal to bedding. • B. The other fractures consist of a conjugate set of shear fractures and an extension fracture, but the principal stresses are oriented differently. • In this case, σ1 is parallel to the strike of bedding and σ3 is oriented in the dip direction of bedding on the fold limb. 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 pore volume for the reservoir as well as controlling the 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 juxtaposition will increase the compartmentalization in a reservoir. Zagros Mountain – Upper beds of Asmari Limestone showing the bedding plane distribution and related variations in fracture density Next Class Chapter 7 Origin and Migration of Petroleum 57 THANK YOU In the Name of God for Mankind www.utm.my