6 PETROPHYSICS:RESERVOIR ROCK PROPERTIES TABLE 1.4 (CONTINUED) Graphite-C:carbon;gray to black;metallic luster;H=2. Gypsum-CaSO4.2H2O:hydrous calcium sulfate;transparent to white or gray; vitreous-pearly-silky;H 2. Halite-NaCl:sodium chloride;colorless to white;vitreous-pearly;H 2. Hematite-Fe2O3:iron oxide (the most important iron ore);reddish-brown to black or gray,H=6. Hornblende (Amphibole group)-Ca2Na(Fe2,Mg)(Al,Fe3,Ti)(Al,Si)s(O,OH)2: hydrous alkali,ferro-magnesium,aluminum silicates;dark green to black; exhibits cleavage.The iron and magnesium impart the dark color.H =5. Illite (Muscovite)-KAl2(AlSi,O10)(OH)2:hydrous potassium-aluminum silicate; clear to light green,vitreous;not chemically well-defined but with the approximate composition of muscovite;H 2.5 Kaolinite (Clay)-Al(SiOo)(OH4:hydrous aluminum silicate;light colored; H=1-2 Limonite (Goethite)-FeO(OH).H2O:hydrous iron oxide;yellow-brown to dark brown:H=5. Magnetite-FesOj:iron oxide;black;metallic luster;strongly magnetic iron ore; H=6 Montmorillonite (Smectite clay)-(CaNa)(Al,Fe,Mg)(Si,Al)a(OH8;generally light colored:H=1. Muscovite (Mica)-KAl2(AlSi3010)(OH)2:hydrous potassium-aluminum silicate; clear to light green:vitreous;rock-forming mineral;H=2.5. Olivine-(Fe,Mg)2SiO:ferro-magnesium silicate;clear to light green,various shades of green to yellow;vitreous (glassy)luster with crystals in the rock; H=7. Opal-SiO2.nH2O:hydrous silicon dioxide;variety of almost any color;glassy luster;H =5. Pyrite-FeS2:iron sulfide;pale yellow;bright metallic luster;H=6. Quartz-SiO2:silicon dioxide;clear (transparent)or with a variety of colors imparted by impurities (purple amethyst,yellow citrine,pink rose quartz, brown smoky quartz,snow white chert,multiple colored agate);glassy luster; H=7, Serpentine-Mg3Si2Os(OH:hydrous magnesium silicate;beige color;H=3. Siderite-FeCO3:ferrous carbonate;light colored to brown;H=3-4. Sphalerite-ZnS:zinc sulfide;yellow to dark brown or black;resinous luster; exhibits cleavage;zinc ore;H=3. Sulfur-S:yellow;resinous;H 1-2. Sylvite-KCI:potassium chloride;colorless to white;H 1-2. Talc-Mgs(SiO1o)(OH)2:hydrous magnesium silicate;green,gray,or white; soapy to touch;H=1. Topaz-Al2(SiO)(Fe,OH)2:yellow,pink,blue-green;exhibits cleavage;H=8. Turquoise-CuAl(PO)4(OH)8 2H2O:blue or green color;H =5. Vermiculite-Mg3SiO1o(OHD2nH2O:hydrous magnesium silicate; light colored:H 1
6 PETROPHYSICS: RESERVOIR ROCK PROPERTIES TABLE 1.4 (CONTINUED) ~~~ Graphite-C: carbon; gray to black; metallic luster; H = 2. Gypsum-Cas04 .2H20: hydrous calcium sulfate; transparent to white or gray; Halite-NaC1: sodium chloride; colorless to white; vitreous-pearly; H = 2. Hematite-FezO3: iron oxide (the most important iron ore); reddish-brown to black or gray, H = 6. Hornblende (Amphibole group)-Ca2Na(Fe2, Mg)4(Al, Fe3, TiXAl, Si)s(O, 0H)z: hydrous alkali, ferro-magnesium, aluminum silicates; dark green to black; exhibits cleavage. The iron and magnesium impart the dark color. H = 5. Illite (Muscovite)-KAt2(AlSi3 0 10)(OH)2 : hydrous potassium-aluminum silicate; clear to light green, vitreous; not chemically well-defined but with the approximate composition of muscovite; H = 2.5. Kaolinite (Clay)-&(Si4010)(0H)4 : hydrous aluminum silicate; light colored; Limonite (Goethite)-FeO(0H) . H20: hydrous iron oxide; yellow-brown to dark brown; H = 5. Magnetite-FejO4: iron oxide; black metallic luster; strongly magnetic iron ore; H = 6. Montmorillonite (Smectite clay)-(CaNa)(Al, Fe, Mg)&i, Al)s(OH)s; generally light colored; H = 1. Muscovite (Mica)-KAl~(AlSi30 lo)(OH)2 : hydrous potassium-aluminum silicate; clear to light green; vitreous; rock-forming mineral; H = 2.5. Olivine-@e, Mg)2Si04: ferro-magnesium silicate; clear to light green, various shades of green to yellow; vitreous (glassy) luster with crystals in the rock; H = 7. Opal-Si02 . nH20: hydrous silicon dioxide; variety of almost any color; glassy luster; H = 5. Pyrite-FeS2: iron sulfide; pale yellow; bright metallic luster; H = 6. Quartz-SiO2: silicon dioxide; clear (transparent) or with a variety of colors imparted by impurities (purple amethyst, yellow citrine, pink rose quartz, brown smoky quartz, snow white chert, multiple colored agate); glassy luster; H = 7. Serpentine--Mg$3205(0€€)4: hydrous magnesium silicate; beige color; H = 3. Siderite-FeCOj: ferrous carbonate; light colored to brown; H = 3-4. Sphalerite-ZnS: zinc sulfide; yellow to dark brown or black; resinous luster; exhibits cleavage; zinc ore; H = 3. Sulfur-S: yellow; resinous; H = 1-2. Sylvite-KC1: potassium chloride; colorless to white; H = 1-2. Talc-Mg3(Si4010)(OH)2: hydrous magnesium silicate; green, gray, or white; Topaz-A12(Si04)(Fe,OH)z: yellow, pink, blue-green; exhibits cleavage; H = 8. Turquoise-C~(PO4)4(OH)s e2H20: blue or green color; H = 5. Vermiculite--Mg3Si4010(OH)2nH20: hydrous magnesium silicate; light vitreous-pearly-silky; H = 2. H = 1-2. soapy to touch; H = 1. colored; H = 1
MINERAL CONSTITUENTS OF ROCKS-A REVIEW 7 complex and their chemical formulas differ in various publications;in such cases the most common formula reported in the list of references was selected. IGNEOUS ROCKS Igneous rocks (about 20%of all rocks)are the product of the cooling of molten magma intruding from below the mantle of the crust. Igneous(plutonic)rocks are divided into three easily recognizable rocks, which are subdivided by the rate of cooling (Figure 1.1).The granites are intrusive rocks that cooled slowly (at high temperature)below the surface,whereas gabbro is a rock resulting from more rapid (low temperature)cooling in the subsurface.Diorite is a rock that cooled below the surface at a temperature intermediate between granite and gabbro.The minerals differentiate during the slow cooling,forming large recognizable,silica-rich crystals with a rough(phaneritic)texture. The second classification is extrusive (volcanic)rock that has undergone rapid cooling on or near the surface,forming silica-poor basaltic rocks.Rhyolite,or felsite,is light colored and estimated to be produced on the surface at a lower temperature than the darker andesite that formed at a temperature intermediate between that of rhyolite and the dark-colored basalt.As a result of rapid cooling on the surface,these rocks have a fine (alphanitic)texture with grains that are too small to be seen by the unaided eye [5]. Minerals precipitating from melted magma,or melt,do not crystallize simultaneously.Generally,a single mineral precipitates first and,as the melt cools slowly,this is joined by a second,third,and so forth;thus the earlier-formed minerals react with the ever-changing melt composition. If the reactions are permitted to go to completion,the process is called equilibrium crystallization.If the crystals are completely or partially prevented from reacting with the melt (by settling to the bottom of the melt or by being removed),fractional crystallization takes place and the final melt composition will be different from that predicted by equilibrium crystallization.The mechanism by which crystallization takes place in a slowly cooling basaltic melt was summarized by Bowen [6]as two series of simultaneous reactions;after all of the ferro-magnesium minerals are formed,a third series of minerals begins to crystallize from the melt.From laboratory experiments Bowen discovered that the first two series of reactions have two branches: (a)The plagioclase grade into each other as they crystallize;the crystals react continually with the melt and change composition from an initial calcium plagioclase crystal to sodium plagioclase
MINERAL CONSTITUENTS OF ROCKS-A REVIEW 7 complex and their chemical formulas differ in various publications; in such cases the most common formula reported in the list of references was selected. IGNEOUS ROCKS Igneous rocks (about 20% of all rocks) are the product of the cooling of molten magma intruding from below the mantle of the crust. Igneous (plutonic) rocks are divided into three easily recognizable rocks, which are subdivided by the rate of cooling (Figure 1.1). The granites are intrusive rocks that cooled slowly (at high temperature) below the surface, whereas gabbro is a rock resulting from more rapid (low temperature) cooling in the subsurface. Diorite is a rock that cooled below the surface at a temperature intermediate between granite and gabbro. The minerals differentiate during the slow cooling, forming large recognizable, silica-rich crystals with a rough (phaneritic) texture. The second classification is extrusive (volcanic) rock that has undergone rapid cooling on or near the surface, forming silica-poor basaltic rocks. Rhyolite, or felsite, is light colored and estimated to be produced on the surface at a lower temperature than the darker andesite that formed at a temperature intermediate between that of rhyolite and the dark-colored basalt. As a result of rapid cooling on the surface, these rocks have a fine (alphanitic) texture with grains that are too small to be seen by the unaided eye [5]. Minerals precipitating from melted magma, or melt, do not crystallize simultaneously. Generally, a single mineral precipitates first and, as the melt cools slowly, this is joined by a second, third, and so forth; thus the earlier-formed minerals react with the everchanging melt composition. If the reactions are permitted to go to completion, the process is called equilibrium crystallization. If the crystals are completely or partially prevented from reacting with the melt (by settling to the bottom of the melt or by being removed), fractional crystallization takes place and the final melt composition will be different from that predicted by equilibrium crystallization. The mechanism by which crystallization takes place in a slowly cooling basaltic melt was summarized by Bowen [6] as two series of simultaneous reactions; after all of the ferro-magnesium minerals are formed, a third series of minerals begins to crystallize from the melt. From laboratory experiments Bowen discovered that the first two series of reactions have two branches: (a) The plagioclase grade into each other as they crystallize; the crystals react continually with the melt and change composition from an initial calcium plagioclase crystal to sodium plagioclase
c IGNEOUS ROCKS INTRUSIVE EXTRUSIVE COARSE-GRAINED FINE-GRAINED PETROPHYSICS:RESERVOIR GRANITE DIORITE GABBRO RHYOLITE (FELSITE) ANDESITE BASALT GLASS LIght Colored (Intermedlate) Dark Colored Light Colored (Intermedlate)Dark Colored [High Temperature Formation) (Low Temperature Formatlon) OBSIDIAN PUMICE GNESS AMPHIBOLITE GNEISS SCHIST SLATE AMPHIBOLITE GREEN SCHIST QUARTZ (Banded Felspar) (Hornblende)(Banded.Felspar)(MIca)(Flne Gralned)(Hornblende)(Green Mlca) A1 ROCK PROPERTIES L METAMORPHIC ROCKS Figure 1.1.Origin of tbe principal metamorpbic rocks
IGNEOUS ROCKS I i I INTRUSIVE COnRSEORAlNED I EXTRUSIVE FINE-GAAlNED I 1 GRANITE DIORITE (Hlgh Temperature Llght Colored 1 /\dla,?[!md Formatlon) (Low Temperature Formation) GNUSS AMPHIBOLITE A I 1 L,-,~,,,L,,,-L----L-,-I (Hornblende) (Banded Felspar) (Mlca) (Flne ralned) (Hornblende) (Green Mlca) E A A $. .f 3 .f I (Banded Felspar) ?- L,,,-,------.---L,-, METAMORbHIC ROCKS Figure 1.1. Origin of theprincipal metamorphic rocks
MINERAL CONSTITUENTS OF ROCKS-A REVIEW 9 (b)The other series of crystallization that is taking place simultaneously forms minerals that are compositionally distinct.The reaction series (olivine-pyroxene-amphibole-biotite)is discontinuous;thus the reaction between crystals and the melt occurs only during specific periods of the cooling sequence. (c)After all of the ferro-magnesium minerals and plagioclase are formed, the third series of minerals begins to crystallize as the melt continues to cool slowly.First potassium feldspar precipitates,followed by muscovite and finally quartz [7-9]. The Bowen series of specific crystallization occurs only for some basaltic magmas (a variety of different reaction series occurs within different melts),but the processes discussed by Bowen are significant because they explain the occurrence of rocks with compositions different from that of the original melted magma. METAMORPHIC ROCKS The metamorphic rocks (about 14%of all rocks)originate from mechanical,thermal,and chemical changes of igneous rocks 110]. Mechanical changes on or near the surface are due to the expansion of water in cracks and pores,tree roots,and burrowing animals.If the igneous rocks undergo deep burial due to subsidence and sedimentation, the pressure exerted by the overlying rocks,shear stress from tectonic events,and the increased temperature result in mechanical fracturing. When unequal shear stress is applied to the rocks as a result of continental motion of other force-fields,cleavage of the rocks (fracturing)occurs; alternatively,slippage of a regional mass of rocks and sediments(faulting) occurs.The pressure produced by overlying rocks is approximately 1.0 psi per foot of depth (21 kPa per meter of depth).The changes induced by overburden pressure occur at great depth in conjunction with other agents of metamorphism. Chemical metamorphosis of igneous intrusive rocks,aided by high pressure,temperature,and the presence of water,results in chemical rearrangement of the elements into new minerals.This produces foliated rocks with regularly oriented bands of mineral grains because the new crystals tend to grow laterally in the directions of least stress.This chemical metamorphism of granite yields gneiss:a foliated granite with large recognizable crystals of banded feldspars.Gabbro changes to amphibolite,whose main constituent is the complex mineral known as hornblende. The chemical metamorphosis of the extrusive rocks,rhyolite,basalt, etc.,produces changes to easily recognizable rocks.Rhyolite,light
MINERAL CONSTITUENTS OF ROCKS-A REVIEW 9 (b) The other series of crystallization that is taking place simultaneously forms minerals that are compositionally distinct. The reaction series (olivine-pyroxene-amphibole-biotite) is discontinuous; thus the reaction between crystals and the melt occurs only during specific periods of the cooling sequence. (c) After all of the ferro-magnesium minerals and plagioclase are formed, the third series of minerals begins to crystallize as the melt continues to cool slowly. First potassium feldspar precipitates, followed by muscovite and finally quartz [7-93. The Bowen series of specific crystallization occurs only for some basaltic magmas (a variety of different reaction series occurs within different melts), but the processes discussed by Bowen are significant because they explain the occurrence of rocks with compositions different from that of the original melted magma. METAMORPHIC ROCKS The metamorphic rocks (about 14% of all rocks) originate from mechanical, thermal, and chemical changes of igneous rocks [ 101. Mechanical changes on or near the surface are due to the expansion of water in cracks and pores, tree roots, and burrowing animals. If the igneous rocks undergo deep burial due to subsidence and sedimentation, the pressure exerted by the overlying rocks, shear stress from tectonic events, and the increased temperature result in mechanical fracturing. When unequal shear stress is applied to the rocks as a result of continental motion of other force-fields, cleavage of the rocks (fracturing) occurs; alternatively, slippage of a regional mass of rocks and sediments (faulting) occurs. The pressure produced by overlying rocks is approximately 1.0 psi per foot of depth (21 kPa per meter of depth). The changes induced by overburden pressure occur at great depth in conjunction with other agents of metamorphism. Chemical metamorphosis of igneous intrusive rocks, aided by high pressure, temperature, and the presence of water, results in chemical rearrangement of the elements into new minerals. This produces foliated rocks with regularly oriented bands of mineral grains because the new crystals tend to grow laterally in the directions of least stress. This chemical metamorphism of granite yields gneiss: a foliated granite with large recognizable crystals of banded feldspars. Gabbro changes to amphibolite, whose main constituent is the complex mineral known as hornblende. The chemical metamorphosis of the extrusive rocks, rhyolite, basalt, etc., produces changes to easily recognizable rocks. Rhyolite, light
10 PETROPHYSICS:RESERVOIR ROCK PROPERTIES colored volcanic rock,undergoes change principally to three types of metamorphic rock,depending on the environmental conditions inducing the changes:(1)gneiss,which has foliated bands of feldspars;(2)schist or mica;and (3)slate,which is a fine-grained smooth-textured rock. Basalt,the dark-colored volcanic rock,produces two main types of metamorphic rock:(1)amphibolite and (2)greenschist,or green mica, as illustrated in Figure 1.1. On a regional scale,the distribution pattern of igneous and meta- morphic rocks is belt-like and often parallel to the borders of the continents.For example,the granitic rocks that form the core of the Appalachian mountains in eastern United States are parallel to the east coast and those in the Sierra Nevada are parallel to the west coast. Igneous and metamorphic rocks are not involved in the origin of petroleum as source rocks.In some cases they do serve as reservoirs, or parts of reservoirs,where they are highly fractured or have acquired porosity by surface weathering prior to burial and formation into a trap for oil accumulated by tectonic events. SEDIMENTARY ROCKS All of the sedimentary rocks (about 66%of all rocks)are important to the study of petrophysics and petroleum reservoir engineering.It is possible to interpret them by considering the processes of rock degra- dation.The principal sedimentary rocks may be organized according to their origin (mechanical,chemical,and biological)and their composition,as illustrated in Table 1.3. Mechanical weathering is responsible for breaking large pre-existing rocks into small fragments.The most important mechanism is the expansion of water upon freezing,which results in a 9%increase of volume.The large forces produced by freezing of water in cracks and pores results in fragmentation of the rocks.Mechanical degradation of rocks also occurs when a buried rock is uplifted and the surrounding overburden is removed by erosion.The top layers of the rock expand when the overburden pressure is relieved,forming cracks and joints that are then further fragmented by water.Mechanical weathering produces boulder-size rocks,gravel,sand grains,silt,and clay from igneous and metamorphic rocks.These fragments remain in the local area,or they may be transported by winds and water to other sites to enter into the formation of conglomerates,sandstones,etc.,as shown in Table 1.3. Water is the principal contributor to chemical weathering,which occurs simultaneously with mechanical weathering.Mechanical weath- ering provides access to a large area for contact by water.Chemicals dissolved in the water,such as carbonic acid,enter into the chemical
1 0 PETROPHYSICS: RESERVOIR ROCK PROPERTIES colored volcanic rock, undergoes change principally to three types of metamorphic rock, depending on the environmental conditions inducing the changes: (1) gneiss, which has foliated bands of feldspars; (2) schist or mica; and (3) slate, which is a fine-grained smooth-textured rock. Basalt, the dark-colored volcanic rock, produces two main types of metamorphic rock: (1) amphibolite and (2) greenschist, or green mica, as illustrated in Figure 1.1. On a regional scale, the distribution pattern of igneous and metamorphic rocks is belt-like and often parallel to the borders of the continents. For example, the granitic rocks that form the core of the Appalachian mountains in eastern United States are parallel to the east coast and those in the Sierra Nevada are parallel to the west coast. Igneous and metamorphic rocks are not involved in the origin of petroleum as source rocks. In some cases they do serve as reservoirs, or parts of reservoirs, where they are highly fractured or have acquired porosity by surface weathering prior to burial and formation into a trap for oil accumulated by tectonic events. SEDIMENTARY ROCKS All of the sedimentary rocks (about 66% of all rocks) are important to the study of petrophysics and petroleum reservoir engineering. It is possible to interpret them by considering the processes of rock degradation. The principal Sedimentary rocks may be organized according to their origin (mechanical, chemical, and biological) and their composition, as illustrated in Table 1.3. Mechanical weathering is responsible for breaking large preexisting rocks into small fragments. The most important mechanism is the expansion of water upon freezing, which results in a 9% increase of volume. The large forces produced by freezing of water in cracks and pores results in fragmentation of the rocks. Mechanical degradation of rocks also occurs when a buried rock is uplifted and the surrounding overburden is removed by erosion. The top layers of the rock expand when the overburden pressure is relieved, forming cracks and joints that are then further fragmented by water. Mechanical weathering produces boulder-size rocks, gravel, sand grains, silt, and clay from igneous and metamorphic rocks. These fragments remain in the local area, or they may be transported by winds and water to other sites to enter into the formation of conglomerates, sandstones, etc., as shown in Table 1.3. Water is the principal contributor to chemical weathering, which occurs simultaneously with mechanical weathering. Mechanical weathering provides access to a large area for contact by water. Chemicals dissolved in the water, such as carbonic acid, enter into the chemical