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  • 1. Diagenesis and Reservoir QualitySyed A. Ali From the instant sediments are deposited, they are subjected to physical, chemicalSugar Land, Texas, USA and biological forces that define the type of rocks they will become. The combinedWilliam J. Clark effects of burial, bioturbation, compaction and chemical reactions between rock,William Ray MooreDenver, Colorado, USA fluid and organic matter—collectively known as diagenesis—will ultimatelyJohn R. Dribus determine the commercial viability of a reservoir.New Orleans, Louisiana, USAOilfield Review Summer 2010: 22, no. 2. The early search for oil and gas reservoirs cen- particle may undergo changes between itsCopyright © 2010 Schlumberger. tered on acquiring an overall view of regional source—whether it was eroded from a massiveFor help in preparation of this article, thanks to Neil Hurley, tectonics, followed by a more detailed appraisal body of rock or secreted through some biologicalDhahran, Saudi Arabia; and L. Bruce Railsback, TheUniversity of Georgia, Athens, USA. of local structure and stratigraphy. These days, process—and its point of final deposition.5 The1. There is no universal agreement on the exact definition however, the quest for reservoir quality calls for a water, ice or wind that transports the sediment of diagenesis, which has evolved since 1868, when deliberate focus on diagenesis. also selectively sorts and deposits its load accord- C.W. von Gümbel coined the term to explain postdeposi- tional, nonmetamorphic transformations of sediment. For In its broadest sense, diagenesis encompasses ing to size, shape and density and carries away an exhaustive discussion on the genesis of this term: all natural changes in sediments occurring from soluble components. The sediment may be depos- de Segonzac DG: “The Birth and Development of the Concept of Diagenesis (1866–1966),” Earth-Science the moment of deposition, continuing through ited, resuspended and redeposited numerous Reviews 4 (1968): 153–201. compaction, lithification and beyond—stopping times before reaching its final destination.2. Sujkowski Zb L: “Diagenesis,” Bulletin of the American short of the onset of metamorphism.1 The limit Diagenesis commences once a sedimentary Association of Petroleum Geologists 42, no. 11 (November 1958): 2692–2717. between diagenesis and metamorphism is not particle finally comes to rest.6 The nature and3. Krumbein WC: “Physical and Chemical Changes in precise in terms of pressure or temperature, nor rapidity of postdepositional changes depend on Sediments After Deposition,” Journal of Sedimentary Petrology 12, no. 3 (December 1942): 111–117. is there a sharp boundary between diagenesis the medium of deposition as well as the type of4. Worden RH and Burley SD: “Sandstone Diagenesis: The and weathering.2 Thus, the nebulous domain of sediment deposited.7 As a given lamina of sedi- Evolution of Sand to Stone,” in Burley SD and Worden RH diagenesis lies somewhere between the ill- ment is laid down, it becomes the interface (eds): Sandstone Diagenesis: Recent and Ancient. Malden, Massachusetts, USA: Wiley-Blackwell Publishing, defined borders of weathering at its shallow end between the transport medium and the previ- International Association of Sedimentologists Reprint and low-grade metamorphism at its deep end. ously deposited material, thus separating two Series, vol. 4 (2003): 3–44.5. The term “final deposition” refers to deposition immedi- These postdepositional alterations take place at distinctly different physicochemical realms. In ately preceding final burial of the sediment, in contrast to the relatively low pressures and temperatures its new setting, the sediment contains a variety of earlier phases of deposition, erosion, reworking and redeposition. For more: Choquette PW and Pray LC: commonly existing under near-surface conditions minerals that may or may not be in chemical “Geologic Nomenclature and Classification of Porosity in in the Earth’s lithosphere.3 equilibrium with the local environment, and Sedimentary Carbonates,” AAPG Bulletin 54, no. 2 (February 1970): 207–250. Diagenesis comprises all processes that con- changes in interstitial water composition, tem-6. The initial stage of diagenesis does not begin until the vert raw sediment to sedimentary rock.4 It is a perature or pressure can lead to chemical altera- sediment has finally come to a standstill within its current continually active process by which sedimen- tion of its mineral components. cycle of erosion, transportation and deposition. Changes or alterations that take place before this final deposition tary mineral assemblages react to regain equi- At or below the surface of this new layer, the are considered as adjustments of the particles to their librium with an environment whose pressure, sediment may be locally reworked by organisms environment rather than as diagenesis. For more on the initial stages of diagenesis: Shepard FP and Moore DG: temperature and chemistry are changing. These that track, burrow, ingest or otherwise redistrib- “Central Texas Coast Sedimentation: Characteristics of reactions can enhance, modify or destroy poros- ute the sediment, sometimes subjecting it to Sedimentary Environment, Recent History, and Diagenesis: Part 1,” Bulletin of the American Association ity and permeability. bacterial alteration. As deposition continues, the of Petroleum Geologists 39, no. 8 (August 1955): Prior to the onset of diagenesis, porosity and sedimentary lamination is buried beneath the 1463–1593.7. Krumbein WC and Sloss LL: Stratigraphy and permeability are controlled by sediment compo- depositional interface, forming successively Sedimentation, 2nd ed. San Francisco: WH Freeman, sition and conditions that prevailed during depo- deeper strata; there, it encounters continually 1963, as cited in de Segonzac, reference 1. sition. Even before it is laid down, a sedimentary14 Oilfield Review
  • 2. Summer 2010 15
  • 3. Lowstand Highstand Alluvial channel Distributary channel l leve sea and Highst level leve l sea sea and and Lo wst Lo wst Bas in f loo r fa n> Changes with sea level. The rise and fall of sea level influence the location of clastic sediment deposits and control the environments under whichcarbonates form. With decreasing sea level, higher-energy flows are able to carry sediments basinward, eventually depositing them in lowstand basin-floorfan complexes. Conversely, increasing sea level moves the coastline landward, with deposition closer to the coastline.increasing pressures and temperatures accompa- production engineers must contend with similar coarse-grained clastics are retained by fluvialnied by changing chemical and biological condi- phenomena to counteract the effects of fluid systems or deposited at the beach, rather than intions. These new conditions promote further incompatibility, mobilization of clays and reser- deep marine settings (above). It is the lowstandconsolidation and cementation of loose sediment voir compaction. This article discusses diagene- settings that are responsible for most of theand ultimately form lithified rock.8 sis as it affects conventional reservoirs, focusing coarse-grained siliciclastics deposited in deep- Important factors that influence the course of primarily on porosity and permeability changes water petroleum basins.10diagenesis are classified as either sedimentary or in siliciclastic and carbonate rocks. By contrast, the deposition of most carbon-environmental. Sedimentary factors include ates is largely controlled by marine biologicalparticle size, fluid content, organic content Setting the Stage activity, which is viable only within a narrowand mineralogical composition. Environmental Porosity and permeability are initially controlled range of light, nutrient, salinity, temperature andfactors are temperature, pressure and chemical by sedimentary conditions at the time of deposi- turbidity conditions. These requirements tend toconditions.9 Particles in a layer of sediment may tion but are subsequently altered through dia- restrict most carbonates to relatively shallow,be subjected to genesis. The environment of deposition sets the tropical marine depositional settings. Because• compaction, in which particles are moved into stage for diagenetic processes that follow. carbonate deposition is affected by inundation of closer contact with their neighbors by pressure Depositional environments for siliciclastic sedi- shallow marine platforms, most carbonate sedi-• cementation, in which particles become coated ments, from which sandstones 02 formed, differ Matt—Figure are ment is generated during highstands of sea level or surrounded by precipitated material greatly from those of carbonates, which can form and is curtailed during lowstands.11• recrystallization, in which particles change limestones. These rocks also differ in their reac- These differences in siliciclastic and carbon- size and shape without changing composition tions to changes in their environment. ate deposition can ultimately affect reservoir• replacement, in which particles change compo- Siliciclastics are primarily the product of ero- quality. Sand deposited during highstands may be sition without changing size or form sion from a parent source. They are transported eroded and transported downstream during low-• differential solution, in which some particles by some medium—fresh water, seawater, ice or stands. In contrast, carbonates deposited during are wholly or partially dissolved while others wind—to their depositional site. Sand deposition highstands may be uncovered during lowstands, remain unchanged is controlled by sediment supply, and the supply leaving them exposed to meteoric fluids that sub-• authigenesis, in which chemical alterations of coarser grains, in particular, is affected by ject them to chemical changes, reworking and cause changes in size, form and composition. energy of the transport medium. For water-driven porosity modifications such as karsting. Any one of these transformations can signifi- systems, energy is largely a function of sea level. A variety of outcrops and their uniquecantly impact porosity and permeability and thus During periods of relatively low sea level, or low- diagenetic environments have been studied andmodify reservoir volume and flow rate. These stand conditions, coarse-grained sediments can described extensively, leading geologists to rec-effects are therefore of great interest to petro- be carried beyond the continental shelf to be ognize similarities among various settings.leum geologists and engineers in their endeavors deposited in basinal marine settings. Conversely, Several schemes have been developed for classi-to optimize production. Indeed, drilling and during rises in sea level, or highstands, most fying diagenetic regimes. One method, proposed16 Oilfield Review
  • 4. by Machel, is applicable to all rock types.12 Thisclassification integrates mineralogic, geochemi-cal and hydrogeologic criteria from clastic andcarbonate rocks. It is divided into processes thatoccur in near-surface, shallow and intermediate-to deep-burial diagenetic settings, along withfractures and hydrocarbon-contaminated plumes.13 Eogenetic zone A different diagenetic model was outlined by Telogenetic zone Ne wl Fresh water yd Sea levelFairbridge in 1966. It emphasizes the geochemi- epo Fresh water sitecal aspect of diagenesis and recognizes three dis- Salt water d sed ime ntstinct phases: syndiagenesis, anadiagenesis andepidiagenesis. Each of these phases tends toward Salt water Water Burialequilibrium until upset by subsequent changes in tableenvironmental parameters.14 Another popular classification scheme relates Older carbonate rockscarbonate diagenetic regimes to the evolution of Mesogenetic zonesedimentary basins (right). This schema, origi- Uplift Upnally proposed by Choquette and Pray, is nowincreasingly being applied to clastic processes aswell.15 It is divided into three stages, some of M ph ph Metamorphic zonewhich may be bypassed or reactivated repeatedly. Eogenesis is the earliest stage of diagenesis, > Diagenetic regimes. The earliest phase of diagenesis occurs in the eogenetic zone. Sediments in thisin which postdepositional processes are signifi- zone are altered by near-surface processes, such as meteoric dissolution, which can occur on land as well as some distance downdip into the subsurface, even extending below sea level. Further burial willcantly affected by their proximity to the surface. drive those sediments into the mesogenetic zone, where they are no longer dominated by processesDuring this stage, the chemistry of the original directly related to the surface. With continued burial, the rock will become metamorphosed. However,pore water largely dominates the reactions. The with sufficient uplift, the rock will enter the telogenetic zone, where it is once again influenced byupper limit of the eogenetic zone is normally a meteoric waters. (Adapted from Mazzullo, reference 41.)depositional interface, but it may be a surface oftemporary nondeposition or erosion. The lowerlimit shares a gradational boundary with the nextstage and is not clearly defined because the effec- boundary is gradational and is placed at the Water is but one of many agents of diagenesis;tiveness of surface-related processes diminishes depth at which erosional processes become insig- organic-rich sediments in various states of decom-gradually with depth, and many such processes nificant. When a water table is present, the lower position introduce a host of chemical reactionsare active down to different depths. However, the limit of the telogenetic zone extends to that and bacteriological activities that consume allmaximum limit for eogenesis is estimated at 1 to point, which commonly serves as an effective available oxygen. This, in turn, leads to a chemi-2 km [0.6 to 1.2 mi], or 20°C to 30°C [68°F to lower limit of many weathering processes. cally reducing environment. Under pressure, the86°F].16 The greatest change in the eogenetic Dissolution by meteoric water is the major poros- gases of decomposition enrich the water with car-zone is probably the reduction of porosity from ity-forming process of the telogenetic zone. bon dioxide and lesser amounts of methane,cementation by carbonate or evaporite minerals. As with the above schema, most diagenetic nitrites and other dissolved organic products. Mesogenesis is the stage during which sedi- classifications are broadly based; some overlap   8. Krumbein, reference 3.ments or rocks are buried to such depths that with others and all contain exceptions to the rule.  9. Krumbein, reference 3.they are no longer dominated by processes 10. Kupecz JA, Gluyas J and Bloch S: “Reservoir Qualitydirectly related to the surface. This phase, some- Agents of Change Prediction in Sandstones and Carbonates: An Overview,” in Kupecz JA, Gluyas J and Bloch S (eds):times referred to as burial diagenesis, spans the Freshly deposited sediments—mixtures of chem- Reservoir Quality Prediction in Sandstones andtime between the early stage of burial and the ically unstable minerals and detrital materials— Carbonates. Tulsa: American Association of Petroleum Geologists, AAPG Memoir 69 (1997): vii–xxiv.onset of telogenesis. Cementation is thought to Matt—Figure 03 act as building blocks of diagenesis, while water 11. Kupecz et al, reference 10.be the major process affecting porosity in the and organic matter fuel the process. 12. Machel HG: “Effects of Groundwater Flow on Mineralmesogenetic zone, whereas dissolution is proba- Within a depositional system, changes in tem- Diagenesis, with Emphasis on Carbonate Aquifers,” Hydrogeology Journal 7, no. 1 (February 1999): 94–107.bly minor. perature and pressure can lead to the separation 13. Machel HG: “Investigations of Burial Diagenesis in Telogenesis refers to changes during the of different chemical compounds in unstable Carbonate Hydrocarbon Reservoir Rocks,” Geoscience Canada 32, no. 3 (September 2005): 103–128.interval in which long-buried rocks are affected mixtures. The liberation of unstable materials 14. Fairbridge RW: “Diagenetic Phases: Abstract,”by processes associated with uplift and erosion. from one area is accompanied by their introduc- AAPG Bulletin 50, no. 3 (March 1966): 612–613.Telogenetic porosity is strongly associated with tion elsewhere. Water plays a large role in diage- 15. Choquette and Pray, reference 5.unconformities. The upper limit of the teloge- netic processes, dissolving one grain, hydrating 16. Worden and Burley, reference 4. 17. Sujkowski, reference 2.netic zone is the erosional interface. The lower others. The chemical activity may even change the properties of the water medium itself over time.17Summer 2010 17
  • 5. whereas mechanical infiltration is the mode for Dispersed Floccule matrix continental sandstones. Detrital clay, of whatever mineral chemistry, occurs as tiny, ragged abraded grains and naturally accumulates in pore spaces, Mudstone rock forming tangential grain-coating and pore- fragment bridging fabrics. Intercalated lamina Authigenic clays, unlike allogenic clays, Biogenically Detrital mica develop within the sand subsequent to burial. introduced clay Pore-water chemistry and rock composition strongly influence the growth of authigenic Biogenic clays; connate water chemistry is modified over pellets time by new influxes of water, through dissolu- (may be Infiltraton altered to residues tion or precipitation of minerals and by cation glauconite) exchange.21 Various components of rock, such as > Allogenic clays. Sandstones may be infiltrated by a variety of detrital clays. lithic fragments, feldspars, volcanic glass and [Adapted from Wilson and Pittman, reference 19; reprinted with permission of ferromagnesian minerals—minerals containing SEPM (Society for Sedimentary Geology).] iron and magnesium—react with the pore water to produce clay minerals that may in turnThis fortified water becomes a strong solvent, sandstone or may accumulate to form thin lami- undergo subsequent transformation to other,increasing solubility of carbonates and in some nae. Clays can also flocculate into sand-sized more stable forms of clay. Authigenic clays cancases acting against silica in sandstones.18 aggregates.20 Another type of aggregate is clay or be recognized by their delicate morphology, Clays are also important to the diagenetic mud “rip-up” clasts eroded from previously which precludes sedimentary transport (belowequation. They are responsible for forming easily deposited layers. A similar mechanism is at work left). Authigenic clays in sandstone are typicallycompressible grains, cements and pore-clogging in reworked fragments of older shales or mud- found in four forms:22crystals. Some clays form prior to deposition and stone that are deposited as sand-sized or larger • Clay coatings can be deposited on the surfacesbecome mixed with the sand-sized mineral grains aggregates. Allogenic clays can also be intro- of framework grains, except at points of grain-during or immediately following deposition; duced into sands as biogenic mud pellets that are to-grain contact. In the interstices betweenothers develop within the sand following burial. produced through ingestion and excretion by grains, the coatings act as pore-lining clays.These clays are classified as allogenic and authi- organisms. These pellets may be retained in These clays may be enveloped during subse-genic clays, respectively. burrows or transported as detrital particles. The quent cementation by feldspar and quartz Allogenic, or detrital, clays originate as dis- biologic activity tends to homogenize the mud overgrowths. Chlorite, illite, smectite andpersed matrix or sand- to cobble-sized mud or and sand (above). mixed-layer clays typically occur as pore linings.shale clasts.19 These particles may be carried by All types of clay can occur as detrital compo- Pore linings grow outward from the grain sur-downward or laterally migrating pore waters to nents. Bioturbation, mass flow and soft-sediment faces and often merge with the linings oninfiltrate previously deposited sands. Individual deformation are other modes for introducing opposing grains in a process known as poreclay particles may be dispersed throughout a clays into the fabric of marine sandstones, bridging (below).23 Matt—Figure 01 20 µm > Pore-bridging clay. A grain contact is bridged 10 µm by mixed-layer illite-smectite clay (circled ) in this scanning electron microscope image. > Authigenic clays. Chlorite (left) grows in a finely foliated form, in contrast to Blocky quartz overgrowths cover adjacent grain surfaces. (Photograph courtesy of S.A. Ali.) the blocky form of kaolinite (right). (Photograph courtesy of W.J. Clark).18 Oilfield Review
  • 6. Kaolinite Quartz Quartz 20 µm 40 µm > Kaolinite booklets. Well-formed stacks of kaolinite are seen as pore-filling > Partial grain dissolution. This thin-section material, along with lesser amounts of quartz overgrowth cement. Kaolinite booklets are known for their propensity to migrate and plug pore throats. photograph highlights reservoir porosity (blue) (Photograph courtesy of S.A. Ali.) in this poorly sorted, very fine- to medium- grained sandstone. A feldspar grain (blue crystal, circled ) shows signs of partial grain dissolution. Secondary porosity in this form can marginally enhance reservoir producibility. (Photograph• Individual clay flakes or aggregates of flakes Sandstone Diagenesis courtesy of S.A. Ali.) can plug interstitial pores. These pore-filling Freshly deposited sand—the precursor of sand- flakes exhibit no apparent alignment relative stone—contains an assemblage of minerals that to framework grain surfaces (above). vary with local rock source and depositional• Clay minerals can partially or completely environment (right). Sand-sized grains create a replace detrital grains or fill voids left by dis- self-supporting framework at the time of deposi- solution of framework grains, sometimes pre- tion, finer particles form a detrital matrix and serving the textures of the host grains they the remaining volume is pore space. Framework replaced (above right). grains are detrital particles, chiefly of sand Grain• Clays can fill vugular pores and fractures. size—between 0.0625 and 2  mm [0.0025 to The interactions among clay, organic matter 0.08 in.] in diameter—commonly composed ofand water become even more important in the quartz, feldspars and rock fragments. The detri- Porecontext of sandstone and limestone porosity. tal matrix consists of mechanically transported fines—particles of less than 0.03  mm18. Sujkowski, reference 2.19. Wilson MD and Pittman ED: “Authigenic Clays in [0.001 in.]—that are predominantly clay miner- Sandstones: Recognition and Influence on Reservoir als.24 The constituent minerals of this assem- Properties and Paleoenvironmental Analysis,” Journal of Sedimentary Petrology 47, no. 1 (March 1977): 3–31. blage were formed under a specific range of20. Pryor WA and Van Wie WA: “The ‘Sawdust Sand’— temperature, pressure, pH and oxidation-state An Eocene Sediment of Floccule Origin,” Journal of conditions unique to each mineral. These condi- Sedimentary Petrology 41, no. 3 (September 1971): 763–769.21. Connate water is trapped within the pores of a rock as tions will have a bearing on the physicochemical the rock is formed. Formation, or interstitial, water, in stability of the mineral assemblage. Cement Matrix contrast, is water found in the pores of a rock; it may not have been present when the rock was formed. Connate Diagenetic processes are initiated at the > More than just sand. The volumetric water can be more dense and saline than seawater. interface between the depositional medium and components of sandstone may include22. Wilson and Pittman, reference 19. the previous layers of sediment. These processes framework grains, intergranular detrital matrix,23. Neasham JW: “The Morphology of Dispersed Clay in pore-filling cements and pore space. Sandstone Reservoirs and Its Effect on Sandstone are modified as the layer is buried beneath sedi- Shaliness, Pore Space and Fluid Flow Properties,” Matt—Figure 18 mentary overburden. With time, the sand Matt—Figure 15 paper SPE 6858, presented at the SPE Annual Technical Conference and Exhibition, Denver, October 9–12, 1977. responds to changing pressure, temperature and24. Any discussion of sands and clays is complicated by pore-fluid chemistry—eventually emerging as a ambiguities between grain size and mineral composition. sandstone, minus some of its original porosity but Sand grains range in size from 0.0625 to 2 mm. Any sedi- mentary particle within that range may be called a sand perhaps with gains in secondary porosity. grain, regardless of its composition. However, because the overwhelming majority of sand grains are composed of quartz [SiO2], it is typically implied that the term refers to quartz grains unless otherwise specified, such as carbonate sand. Clays are fine-grained particles of less than 0.0039 mm in diameter. The most common clay minerals are chlorite, illite, kaolinite and smectite.Summer 2010 19
  • 7. 1 2 3 4 The activities of flora and fauna, such as plant roots, worms or bivalves, can disturb the original fabric of sediment. Root growth and chemical uptake, along with walking, burrowing or feeding activities of fauna, redistribute the sediment. Slower sedimentation rates allow more time for organisms to rework a sedimentary layer. Bioturbation tends to have more impact in Quartz marine environments than in other settings. Slumping, or mass downslope movement, can result in a homogenization of sediments. This newly formed mixture of sand and clay has C substantially less porosity than the original sand layer. Sutured contact Soil creation can be an important diagenetic Quartz agent in environments such as alluvial fans, point bars and delta plains. Soil coverings contribute to the acidity of meteoric waters that percolate downward to underlying rock. Clay particles gen- erated through the formation of soil may be car- Unmodified grain margin ried in suspension by meteoric water to infiltrate previously deposited sand layers. There, individ- ual clay particles may disperse throughout a sandstone, accumulate to form thin laminae or Quartz attach as clay coatings on framework sand grains. Porosity loss during burial—Deeper burial is accompanied by the primary causes of poros- ity loss: compaction and cementation.25 Compaction reduces pore space and sand thick- ness (left). Cementation can reduce pore space or can hinder sand compaction and dissolution > Grain contacts. With continued pressure, intergranular contacts (top) at grain contacts. change from tangential (1) to flattened (2), concavo-convex (3) and sutured (4). During compaction, sand grains move closer The uniform size of Panels 1 to 4 highlights the reduction in sediment volume and porosity caused by compaction. The photomicrograph of a coarse- together under the load of overburden or tectonic grained sandstone (bottom) shows quartz grains that exhibit both sutured stress, destroying existing voids and expelling pore contacts and unmodified grain margins. Carbonate cement (C) also contributes fluids in the process. Chemically and mechanically to lithification of this sandstone. [Adapted from “An Atlas of Pressure unstable grains, such as clays and volcanic rock Dissolution Features,” http://www.gly.uga.edu/railsback/PDFintro1.html (accessed June 16, 2010). Reprinted with permission of L.B. Railsback of the fragments, tend to compact faster than more Department of Geology, University of Georgia.] stable grains, such as quartz. Compaction mecha- nisms include grain rotation and slippage, defor- mation and pressure dissolution. Grain slippage and rotation are typical All sands have intergranular porosity that amount of water or other fluids and their rate of responses to loading in which a slight rotation orchanges with diagenesis: Macropores become flow through the pore network govern the translation of grains permits edges of nondeform-micropores; minerals dissolve and create voids. amounts and types of minerals dissolved and pre- able grains to slip past adjacent grain edges,Other minerals dissolve, then precipitate as cipitated, which in turn can alter flow paths and 25. Rittenhouse G: “Mechanical Compaction of Sandscements that can partially or completely occlude rates. Diagenetic processes by which sandstone Containing Different Percentages of Ductile Grains:pore space. Initial porosity may be as high as 55%. porosity is lost or modified are outlined below. A Theoretical Approach,” The American Association of Petroleum Geologists Bulletin 55, no. 1 (January 1971):That pore space is occupied by fluids Matt—Figure Penecontemporaneous porosity loss—Those such as 04A 92–96.water, mineral solutions or mixtures thereof; processes that occur after deposition but before 26. Wilson TV and Sibley DF: “Pressure Solution and Porosity Reduction in Shallow Buried Quartz Arenite,”some pore fluids are inert, while others react consolidation of the enclosing rock are said to be The American Association of Petroleum Geologistswith previously precipitated cements, framework penecontemporaneous. Certain processes, such Bulletin 62, no. 11 (November 1978): 2329–2334.grains or rock matrix. as bioturbation, slumping and the formation of 27. Rittenhouse, reference 25. 28. Stylolites are wave-like or serrated interlocking com- Porosity and permeability are especially soil, fall into this category; although they may not paction surfaces commonly seen in carbonate andimportant parameters both for diagenetic devel- be important on a large scale, they can be respon- quartz-rich rocks that contain concentrated insoluble residues such as clay minerals and iron oxides.opment and its effects on reservoir rock. The sible for local reductions in sand porosity.20 Oilfield Review
  • 8. creating a tighter packing arrangement. Theamount of porosity that can be lost depends, inpart, on grain sorting, roundness and overburdenpressure. Porosity loss from compaction has beenestimated to range from 12% to 17% in variousoutcrop studies.26 Pisoid Ductile grain deformation—As ductile grainsdeform under load, they change shape or volume.Originally spherical or ovoid at the time of deposi-tion, ductile grains are squeezed between more- Styloliteresistant framework grains and deform intoadjacent pore spaces. This reduces porosity whiledecreasing stratal thickness.27 The extent ofcompaction and porosity loss depends on the Peloidal packstone 500 µmabundance of ductile grains and the load applied. Compaction-induced deformation is also > Limestone showing the effects of pressure dissolution along a stylolite.affected by cementation, timing and over­ Above the stylolite are large round pisoids—accretionary bodies commonlypressure. Sandstones containing ductile grains composed of calcium carbonate; below is a finer peloidal packstone. More thanundergo relatively little compaction if they are half of each pisoid has been dissolved, but the exact amount of section missingcemented before burial of more than a few on either side of the stylolite is unknown. The dark line along the stylolite is insoluble material. (Photograph courtesy of W.J. Clark.)meters or are strongly supported by pore fluidpressure in an overpressured subsurface setting.Whereas the load from increased overburden This substitution changes the mineral composi- dissolution of carbonate minerals, eventuallypressure is typically carried by grain-to-grain tion of the original sediment by removing unstable resulting in porosity exceeding that of the origi-contact, in an overpressured condition some of minerals and replacing them with more-stable nal sediment. On the other hand, porosity andthe stress is transferred to fluids within the pore ones. This process of equilibration can occur over permeability can be reduced by replacement ofsystem. Fluids normally expelled with increased the course of succeeding generations, whereby rigid feldspar minerals with ductile clay miner-pressure become trapped and carry some of one mineral begets another as environmental con- als, which are easily compacted and squeezedthe load. ditions change. into pore throats between grains. Brittle fossilized sediments also deform under Replacement opens the way to an assortment Some minerals are particularly susceptible toa load. Thin skeletal grains from fauna such as of porosity and permeability modifications. For replacement. Others, such as pyrite, siderite andtrilobites, brachiopods and pelecypods are sub- example, replacement of silicate framework ankerite, are on the other end of the spectrum:jected to bending stress because of their length. grains by carbonate minerals can be followed by They replace other cements or framework grains.When these grains break, they allow overlyinggrains to sag into tighter packing arrangements. Pressure dissolution—Points of contactbetween mineral grains are susceptible to disso-lution, typically in response to the weight of over- Dolomiteburden. Mineral solubility increases locally underthe higher pressures present at grain contacts.Stylolites are the most common result of this pro-cess (above right).28 Pressure dissolution can reduce bulk volumeand hence porosity. Dissolved material may be Calciteremoved from the formation by migrating inter- Matt—Figure 06stitial waters; alternatively, it may be precipi-tated as cement within the same formation.Grains composed of calcite, quartz, dolomite,chert and feldspar are commonly subjected to Anhydritepressure dissolution. 500 µm Replacement—This process involves thesimultaneous dissolution of one mineral and > Mineral replacement. Very coarsely crystalline calcite that filled the porethe precipitation of another (right). In this reac- space in a dolostone (dolomite crystals at top) is being replaced by anhydrite.tion to interstitial physicochemical conditions, Anhydrite is highly birefringent under the microscope’s crossed polarizers,the dissolved mineral is no longer in equilibrium which results in the bright light-blue and yellow colors. (Photograph courtesywith pore fluids, while the precipitated mineral is. of W.J. Clark.)Summer 2010 21
  • 9. High temperatures Least stable minerals Sandstone Cements Authigenic Clay Cements First minerals to form Olivine Calcium-rich Chamosite Fe2+3Mg1.5AlFe3+0.5Si3AlO12(OH)6 plagioclase Chlorite (Fe, Mg, Al)6(Si, Al)4O10(OH)8 Dickite Al2Si2O5(OH)4 Glauconite (K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2 Pyroxene Calcium-sodium Illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2•(H2O)] plagioclase Kaolinite Al2Si2O5(OH)4 Amphibole Smectite KAl7Si11O30(OH)6 Sodium-calcium plagioclase Carbonate Cements Biotite Calcite CaCO3 Dolomite CaMg(CO3)2 Sodium-rich plagioclase Siderite FeCO3 Potassium feldspar Muscovite Feldspar Cements Last minerals to form Quartz Orthoclase KAlSi3O8 Low temperatures Most stable minerals Plagioclase NaAlSi3O8> Weathering of minerals. The Bowen reaction series can be used to chart weathering of certain Iron-Oxide Cementssilicate minerals. High-temperature minerals become less stable as they move farther from the Goethite FeO(OH)conditions under which they were formed. Thus, in near-surface conditions, the minerals formed inhigh temperatures are more susceptible to weathering than those formed in lower temperatures. Hematite Fe2O3 Limonite Fe2O3•H2O Silica Cements The degree of susceptibility to replacement It is common for certain minerals to form Chert (microcrystalline quartz) SiO2normally follows an ordered mineral stability cements in sandstones. Over 40 minerals have Opal SiO2•n(H2O)series in which minerals removed from their zone been identified as cementing agents, but the Quartz SiO2of stability are readily replaced (above). However, most common are calcite, quartz, anhydrite,even the most stable minerals such as muscovite dolomite, hematite, feldspar, siderite, gypsum, Sulfate Cements Anhydrite CaSO4or quartz are not immune to replacement. clay minerals, zeolites and barite (right). Barite BaSO4 Cementation—Cements consist of mineral Calcite is a common carbonate cement, as are Gypsum CaSO4•2H2Omaterials precipitated chemically from pore dolomite and siderite. Framework grains offluids. Cementation affects nearly all sandstones carbonate rock fragments typically act as seed Sulfide Cementsand is the chief—but not the only—method by crystals that initiate calcite cementation. Marcasite FeS2which sands lithify into sandstone. Quartz typically forms cement overgrowths on Pyrite FeS2 Cementation can bolster porosity if it sup- framework quartz grains and tends to developports the framework before the sandstone is sub- during burial diagenesis at temperatures above Zeolite Cementsjected to further compaction. In this case, 70°C [158°F].29 Given sufficient space for enlarge- Analcime NaAlSi2O6•(H2O)remaining porosity is not lost to compaction, and ment, the overgrowth crystal will continue to Chabazite CaAl2Si4O12•6(H2O)excellent reservoir properties can be preserved grow until it completely masks the host grain sur- Clinoptilolite (Na2,K2,Ca)3 Al6Si30O72•24(H2O)to considerable depths. However, because cemen- face. Adjacent grains compete for diminishing Erionite (Na2,K2,Ca)2 Al4Si14O36•15(H2O)tation reaction rates generally increase with pore space, interfere with each other and gener- Heulandite (Ca,Na)2-3Al3(Al,Si)2Si13O36•12(H2O)temperature, subsequent increases in depth ally produce uneven mutual borders forming an Laumonite Ca(AlSi2O6)2•4(H2O)can promote cementation and corresponding interlocking mosaic of framework grains and Matt—Figure 08 Mordenite (Ca,Na2,K2)Al2Si10O24•7(H2O)decreases in porosity with depth. On the other their overgrowths. Phillipsite (Ca,K,Na)2(Si,Al)8O16•6(H2O)hand, cementation can lock fine-grained parti- Authigenic feldspar occurs in all types ofcles in place, preventing their migration during sandstones, mainly as overgrowths around detri- > Common sandstone cements. A number offlow that might otherwise block pore throats and tal feldspar host grains but occasionally as these cements are also found in carbonate rocks.reduce permeability. The amount and type of cement or newly formed crystal without a feld-cement in a sandstone depend largely on the spar host grain. Though common, feldsparcomposition of the pore fluids and their rate of cements are less abundant than carbonate,flow through the pores, as well as the time avail- quartz and clay cements.able for cementation and the kinetics of cement- Authigenic clay cements are common inprecipitating reactions. reservoir rocks of all depositional environments. The most common clay mineral cements are derived from kaolinite, illite and chlorite. Matt—Figure 0922 Oilfield Review
  • 10. Enhanced Porosity in Sandstones • Porosity created by dissolution of sedimentaryAll sands initially have intergranular pores. grains and matrix: Frequently, the soluble con-Primary porosity, present when the sediment is stituents are composed of carbonate minerals.deposited, is frequently destroyed or substan- Dissolution produces a variety of pore textures,tially reduced during burial. However, other dia- and pore size may vary from submicroscopicgenetic processes may also be at work, some of voids to vugs larger than adjacent grains.which may enhance porosity. • Dissolution of authigenic minerals that previ- Porosity that develops after deposition is ously replaced sedimentary constituents orknown as secondary porosity. It is typically authigenic cements: This process may begenerated through the formation of fractures, responsible for a significant percentage ofremoval of cements or leaching of framework secondary porosity. Replacive minerals aregrains and may develop even in the presence of typically calcite, dolomite, siderite, zeolitesprimary porosity. Secondary pores can be inter- and mixed-layer clays. 100 µmconnected or isolated; those pores that are inter- • Dissolution of authigenic cement: As with dis-connected constitute effective porosity, which solved grains, most dissolved cements are com- > Dissolution. This feldspar is partially dissolvedcontributes to permeability. In some reservoirs, posed of carbonate minerals: calcite, dolomite under an authigenic chlorite clay rim. Chlorite coatssecondary pores may be the predominant form of and siderite, though others may also be locally all grains. (Photograph courtesy of W.J. Clark.)effective porosity. important. These cements may have occupied Secondary porosity can be important from a primary or secondary porosity. This is perhapspetroleum system perspective. Most hydrocarbon the most common cause of secondary porosity. Porosity is seldom homogeneous within ageneration and primary migration take place The size, shape and distribution of pores in a given reservoir. It is often possible to find varia-below the depth range of effective primary poros- sandstone reservoir affect the type, volume and tions in porosity type across the vertical extent ofity. The primary migration path and the accumu- rate of fluid production. Three porosity types dis- a reservoir.lation of hydrocarbons are commonly controlled tinctly influence sandstone reservoir production:by the distribution of secondary porosity.30 Intergranular pores are found between detri- Carbonate Diagenesis Secondary porosity may develop during any of tal sand grains. Some of the most productive Most carbonate sediments are produced in shal-the three stages of diagenesis—before burial, dur- sandstone reservoirs have predominantly inter- low, warm oceans by marine organisms whoseing burial above the zone of active metamorphism granular porosity. skeletons or shells are built from the calciumor following uplift. However, burial diagenesis is Dissolution pores result from removal of carbonate they extract from seawater. Unlikeresponsible for most secondary porosity. In sand- carbonates, feldspars, sulfates or other soluble detrital sand deposits, carbonate sediments arestones, such porosity generally results from materials such as detrital grains, authigenic usually not transported far from their source, soreplacement of carbonate cements and grains or, mineral cements or replacement minerals their size, shape and sorting have little to do withmore commonly, from dissolution followed by (above right). When dissolution pore space is transport system energy. The size and shape offlushing of pore fluids to remove the dissolution interconnected with intergranular pores, the pores in carbonate sediments are more influ-products. Lesser amounts of porosity also result effectiveness of the pore system is improved. enced by skeletal materials, which can be asthrough leaching of sulfate minerals, such as anhy- Many excellent reservoirs are a product of car- varied as the assemblages of organisms that cre-drite, gypsum and celestite. In general, secondary bonates that have dissolved to form secondary ated them (see “Resolving Carbonate Complexity,”porosity is attributed to five processes:31 intergranular porosity. However, if there is no page 40).• Porosity produced through fracturing— interconnection, there is no effective porosity, Carbonate sediments—composed chiefly of whether it is caused by tectonic forces or by leaving the pores isolated, with no measurable calcite, aragonite (a less stable crystal varia- shrinkage of rock constituents: Should these matrix permeability. tion, or polymorph, of calcite), magnesian cal- fractures subsequently fill with cement, that Microporosity comprises pores and pore cite or dolomite—are made from minerals that cement may be replaced or dissolved, giving apertures, or throats, with radii less than 0.5 µm. are highly susceptible to chemical alteration.33 rise to second-cycle fracture porosity. In sandstones, very small pore throats are associ- The impact of Matt—Figure 11 biological and physical deposi-• Voids formed as a result of shrinkage caused by ated with microporosity, although relatively large tional processes, in combination with the diage- dehydration of mud and recrystallization of pores with very small pore throats are not uncom- netic overprint of metastable chemical deposits, minerals such as glauconite or hematite: mon. Micropores are found in various clays as 29. Worden and Burley, reference 4. Shrinking affects grains, matrix, authigenic well, and argillaceous sandstones commonly have 30. Schmidt V and McDonald DA: Secondary Porosity in cement and authigenic replacement minerals. significant microporosity, regardless of whether the Course of Sandstone Diagenesis. Tulsa: American Association of Petroleum Geologists, AAPG Course Note Pores generated through shrinkage vary in size the clay is authigenic or detrital in origin.32 Series no. 12 (1979). from a few microns across to the size of adja- Unless the sandstones have measurable matrix 31. Schmidt and McDonald, reference 30. cent sand grains. permeability, small pore apertures and high sur- 32. The term “argillaceous’’ is used to describe rocks or sediments that contain silt- or clay-sized particles face area result in high irreducible water satura- that are smaller than 0.625 mm. Most are high in clay- tion, as is often seen in tight gas sandstones. mineral content. 33. Kupecz et al, reference 10.Summer 2010 23
  • 11. Aspect Sandstones Carbonates Shallow-burial regime—Near-surface pro- Amount of primary porosity Commonly 25% to 40% Commonly 40% to 70% cesses can extend into the shallow-burial setting, Amount of ultimate, Commonly half or more of initial Commonly none or only a small fraction but the dominant process is compaction. Burial postdiagenetic porosity porosity: typically 15% to 30% of initial porosity: 5% to 15% leads to compaction, which in turn squeezes out Types of primary porosity Almost exclusively interparticle Interparticle commonly predominates; water and decreases porosity. Compaction forces intraparticle and other types important sediment grains to rearrange into a self-support- Pore diameter and Closely related to particle size Commonly bear little relation to particle throat size and sorting size or sorting ing framework. Further burial causes grain Uniformity of pore size, Fairly uniform Variable, ranging from fairly uniform to deformation, followed by incipient chemical shape and distribution extremely heterogeneous, even within a compaction in which mineral solubility increases single rock type with pressure. In this way, loading applied to Influence of diagenesis May be minor: reduction of primary Major: can create, obliterate or completely porosity by compaction, cementation modify porosity; cementation and solution grain contacts causes pressure dissolution. and clay precipitation important Expelled fluids will react with surrounding rock. Influence of fracturing Generally not of major importance Of major importance, when present Intermediate- to deep-burial regime—With Permeability-porosity Relatively consistent: commonly Greatly varied: commonly independent of depth, several diagenetic processes become interrelations dependent on particle size and sorting particle size and sorting active. Chemical compaction becomes more> Porosity comparison. In both sandstones and carbonates, porosity is greatly affected by diagenesis— prevalent with additional loading. Depending onperhaps more so in carbonates. (Adapted from Choquette and Pray, reference 5.) composition, clay minerals in the carbonate matrix may either enhance or reduce carbonate solubility. Pressure dissolution is further influ- enced by pore-water composition, mineralogy and the presence of organic matter. If the mate-can make the distribution of porosity and Updip from the marine setting, coastal areas rial dissolved at the contacts between grains ispermeability in carbonates much more hetero- provide an environment in which seawater and not removed from the system by flushing of poregeneous than in sandstones (above). In fact, fresh water can mix. In these groundwater mix- fluids, it will precipitate as cement in adjacentcalcium carbonate dissolves hundreds of times ing and dispersion zones, carbonate dissolution areas of lower stress.37faster than quartz in fresh water under normal creates voids that enhance porosity and permea- Dissolution is not just a pressure-driven pro-surface conditions. The dissolution and precipi- bility—sometimes to the extent that caves are cess; it can also result from mineral reactionstation of calcium carbonate are influenced by a formed. Other processes are also active to a much that create acidic conditions. In burial settingsvariety of factors, including fluid chemistry, rate lesser degree, such as dolomitization and the for- near the oil window, dissolution is active whereof fluid movement, crystal size, mineralogy and mation of aragonite, calcite or dolomite cements. decarboxylation leads to the generation of car-partial pressure of CO2.34 Further inland, near-surface diagenesis is bon dioxide, which produces carbonic acid in the The effects of mineral instability on porosity fueled by meteoric waters, which are usually presence of water. Acidic waters then react withmay be intensified by the shallow-water deposi- undersaturated with respect to carbonates. Rain the carbonates. If the dissolution products aretional setting, particularly when highstand car- water is slightly acidic because of dissolved atmo- flushed from the system, this process can createbonate systems are uncovered during fluctuations spheric CO2. Where the ground has a significant additional voids and secondary porosity.in sea level. Most diagenesis takes place near the soil cover, plant and microbial activity can With burial comes increasing temperatureinterface between the sediment and the air, fresh increase the partial pressure of CO2 in down- and pressure, and changes in groundwater com-water or seawater. The repeated flushing by sea- ward-percolating rainwater. This increases disso- position. Cementation is a response to elevatedwater and meteoric water is a recipe for diage- lution in the upper few meters of burial, thus temperatures, fluid mixing and chemical com-netic change in almost every rock, particularly as boosting porosity and permeability through rocks paction; it is a precipitation product of dissolu-solutions of different temperature, salinity or CO2 of the vadose zone. tion common to this setting. Burial cements incontent mix within its pores. In evaporitic settings, hypersaline diagenesis carbonates consist mainly of calcite, dolomite Porosity in near-surface marine diagenetic is driven by fresh groundwater or storm-driven and anhydrite. The matrix, grains and cements Matt—Figure 12regimes is largely controlled by the flow of water seawater that has been stranded upon the land’s formed at shallow depths become thermodynami-through the sediment. Shallow-burial diagenesis is surface. These waters seep into the ground and cally metastable under these changing condi-dominated by compaction and cementation with are subjected to evaporation as they flow seaward tions, leading to recrystallization or replacementlosses of porosity and permeability. The intermedi- through near-surface layers of carbonate sedi- of unstable minerals. In carbonates, commonate- to deep-burial regime is characterized by fur- ment. As they evaporate beyond the gypsum- replacement minerals are dolomite, anhydritether compaction and other processes, such as saturation point, they form finely crystalline and chert.dissolution, recrystallization and cementation. dolomite cements or replacive minerals. In some Dolomite replacement has a marked effect on Near-surface regime—Most carbonate rocks petroleum systems, these reflux dolomites form reservoir quality, though in some reservoirs it canhave primary porosities of as much as 40% to 45%, thin layers that act as barriers to migration and be detrimental to production. While some geolo-and seawater is the first fluid to fill those pore seals to trap hydrocarbons.36 gists maintain that dolostone porosity is inher-spaces. Filling of primary pores by internal sedi- ited from limestone precursors, others reasonments and marine carbonate cements is the first that the chemical conversion of limestone toform of diagenesis to take place in this setting, dolostone results in a 12% porosity increaseand it leads to significant reductions in porosity.3524 Oilfield Review
  • 12. because the molar volume of dolomite is smaller Destruction of pores Formation of poresthan that of calcite.38 The permeability, solubility Depositional environmentand original depositional fabric of a carbonate Synsedimentary cement High energyrock or sediment, as well as the chemistry, tem- Micrite Internal sediment Frameworkperature and volume of dolomitizing fluids, all 1. Initial Intraparticle Lime mud porosity Interparticleinfluence dolomite reservoir quality. Microdebris s Boring organisms esi In chemically reducing conditions, burial dia- Peloids iagen Burrowing Low energy ly dgenesis can generate dolomite by precipitating it Marine waters Ear organisms Fenestralas cement or by replacing previously formed Cement 2. Early Intramicrite Aragonite diageneticmetastable minerals in permeable intervals Magnesium- porosity calciteflushed by warm to hot magnesium-enriched nt mebasinal and hydrothermal waters.39 Temperatures Dissolution Ce Fresh water Vugsof 60°C to 70°C [140°F to 158°F] are sufficient ics Calcite Channels ctonfor generating burial dolomites, and these condi- Recrystallization d tetions can usually be met within just a few kilome- n an Intercrystalline Geologic timeters of the surface. In the deep subsurface, urdedolomitization is not thought to be extensive Overbbecause pore fluids and ions are progressivelylost with continued compaction. Few, if any, carbonate rocks currently exist asthey were originally deposited (right). Most are 3. Pressure- and Tectonic activity temperature- Fracturethe result of one or more episodes of diagenesis.40 related porosity Pressure tallizationSecondary Porosity in Carbonates solutionAs it does in sandstones, diagenesis in carbon- Compactionates can enhance reservoir properties through Recrysdevelopment of secondary porosity. Porosity inlimestones and dolomites may be gained throughpostdepositional dissolution. In eogenetic or telo- Infillings Fracture 4. Erosionalgenetic settings, dissolution is initiated by fresh porosity Breccla Calcite spar Jointswater. In mesogenetic settings, dissolution is s Dissolution esicaused by subsurface fluids generated through enmaturation of organic matter in the deep- Fissures ia g ld Vugs riaburial environment.41 e bu Caverns During eogenesis, development of secondary Latporosity is aided by a number of processes.Dissolution is dominated by meteoric freshwaters, which are undersaturated with respect to Porositycalcium carbonate. However, the extent of disso- > Carbonate porosity. During creation, deposition and diagenesis, carbonates undergo changes thatlution is determined by other factors, such as the can enhance or diminish reservoir porosity. Over the span of geologic time, these processes may bemineralogy of sediments or rocks, the extent of repeated many times and may be interrupted on occasion by periods of uplift (not shown), which canpreexisting carbonate porosity and fracturing, sometimes enhance porosity. [Adapted from Akbar M, Petricola M, Watfa M, Badri M, Charara M, Boyd A, Cassell B, Nurmi R, Delhomme J-P, Grace M, Kenyon B and Roestenburg J: “Classicthe acidity of the water and its rate of movement Interpretation Problems: Evaluating Carbonates,” Oilfield Review 7, no. 1 (January 1994): 38–57.]in the diagenetic system.42 During telogenesis, uplift exposes older, for-merly deep-buried carbonate rocks to meteoricwaters, but with less effect than during the eoge- 34. Longman MW: “Carbonate Diagenetic Textures 39. Land LS: Dolomitization. Tulsa: American Associationnetic phase. By this time, what were once carbon- from Near Surface Diagenetic Environments,” of Petroleum Geologists, AAPG Course Note Seriesate sediments have matured, consolidated and The American Association of Petroleum Geologists no. 24 (1982). Bulletin 64, no. 4 (April 1980): 461–487. 40. Land, reference 39.lithified to become limestones or dolostones. 35. Machel, reference 13. 41. Mazzullo SJ: “Overview of Porosity Evolution inThese older rocks have, for the most part, become 36. Machel HG and Mountjoy EW: “Chemistry and Carbonate Reservoirs,” Search and Discoverymineralogically stabilized. Soluble components Environments of Dolomitization—A Reappraisal,” Article #40134 (2004), http://www.searchanddiscovery. Matt—Figurenet/documents/2004/mazzullo/index.htm (accessed Earth-Science Reviews 23, no. 3 (May 1986): 175–222. 12Aof the eogenetic sediment—such as ooids or 37. Machel, reference 13. May 28, 2010).coral and shell fragments composed of arago- 38. For more on dolomites: Al-Awadi M, Clark WJ, 42. Longman, reference 34.nite—have probably dissolved during earlier Moore WR, Herron M, Zhang T, Zhao W, Hurley N, Kho D, Montaron B and Sadooni F: “Dolomite:phases. Having mineralogically evolved toward a Perspectives on a Perplexing Mineral,” Oilfield Review 21, no. 3 (Autumn 2009): 32–45.Summer 2010 25
  • 13. less soluble low-magnesium calcite, these rocks calcite or dolomite cement and may even be fairly detailed overview of the lithology along theare more resistant to dissolution than their eoge- capable of dolomitization in some cases.43 length of the borehole. The laboratory is reservednetic precursors. Further dissolution requires Creating secondary porosity in the mesogenetic for studying extremely small intervals—usuallyexposure to fluids that are undersaturated with region requires a different mode of dissolution. samples of core or formation cuttings—in ultra-respect to calcite. When this happens, vugs and With burial, the organic matter in source rocks microscopic detail.caverns may form. matures and is eventually converted to hydrocar- At the wellsite, the intricacies of various dia- Similar processes can, in part, explain how bons. During this process, organic acids, carbon genetic processes culminate in one central issue:secondary porosity forms in some dolomites. dioxide and hydrogen sulfide are expelled from the porosity. In particular, the ability to recognizeWhen partially dolomitized limestones undergo source rock; the gases combine with subsurface secondary porosity is critical to the formation-telogenesis, meteoric fluids dissolve any remain- waters, producing carbonic acid and sulfuric acid, evaluation process. Depending on the extent ofing calcite in the rock matrix or its component respectively.44 These acids migrate laterally and connectivity, secondary porosity can increase orparticles. The calcitic components, being more vertically, dissolving carbonates and creating decrease reservoir producibility.susceptible to freshwater dissolution than dolo- porosity along their paths. Once the acids are Although conventional porosity logs do notmite is, leave new pores in their place. spent, the fluids precipitate carbonate cements.45 directly measure secondary porosity, log analysts However, many carbonate reservoirs that can use an alternative method to obtain thiswere formed in deep waters have yet to be Diagenesis at Different Scales information. If neutron, density and compres-uplifted or exposed to meteoric waters; even Failure to recognize diagenetic effects on poros- sional sonic data are obtained, then noncon-without the benefit of eogenesis or telogenesis, ity can lead to serious errors in reservoir volume nected secondary porosity can be detected andthey are porous and permeable. Dissolution calculations and impact flow rate predictions. quantified. Both the neutron and density porositynonetheless requires that carbonates be Different aspects of diagenesis may be recog- tools respond to primary, interparticle matrixexposed to fluids that are undersaturated in cal- nized at the wellsite and investigated in the labo- porosity, as well as secondary, vuggy and fracturecium carbonate. Most connate fluids in deeper ratory. Developments in logging technology are porosity. However, these measurements do notenvironments, however, are saturated with continually improving the resolution of wireline distinguish between primary and secondaryrespect to calcium carbonate, leaving them and LWD tool measurements, and these data are porosities. Alternatively, the compressional sonicincapable of dissolving carbonate rocks and cre- valuable for high-grading formation intervals that slowness measurement responds to primaryating secondary porosity. In fact, just the oppo- merit further attention in the laboratory. In this porosity only.46site is the case: These fluids tend to precipitate capacity, well logs are quite good at providing a Porosity data can be compared after process- ing. The neutron and density porosity logs are corrected for environmental effects and matrix Density Porosity conditions, then crossplot porosity is calculated. 30 % –10 Sonic porosity from an acoustic tool is calculated Neutron Porosity utilizing the same rock and fluid properties used Deep Resistivity 30 % –10 Sonic Porosity in evaluating the neutron-density data. Any dif- 0.2 ohm.m 2,000 Gamma Ray Medium Resistivity 30 % –10 ference between sonic and crossplot porosities0 gAPI 100 0.2 ohm.m 2,000 Neutron-Density Crossplot Porosity represents the fraction of noneffective secondary Caliper Shallow Resistivity 30 % –10 porosity (left). However, if the secondary porosity6 in. 16 0.2 ohm.m 2,000 Secondary Porosity is connected, the sonic porosity and neutron- density porosity values will generally match and this method will not distinguish between the two. On the other hand, full suites of modern logs are fairly adept at detecting and evaluating sec- ondary porosity. Borehole imaging logs can indi- cate the type and occurrence of secondary porosity. Nuclear magnetic resonance (NMR) 43. Mazzullo, reference 41. 44. Mazzullo, reference 41. 45. Burruss RC, Cercone KR and Harris PM: “Timing of Hydrocarbon Maturation—Evidence from Fluid Inclusions in Calcite Cements, Tectonics and Burial History,” in Schneidermann N and Harris PM (eds): Carbonate Cements. Tulsa: Society for Sedimentary Geology, SEPM Special Publication 36 (1985): 277–289. 46. In rocks with vugular porosity, for example, the travel- time through the formation matrix is much faster than traveltime through fluids in the vugs. 47. For more on computed tomography applications in core analysis: Kayser A, Knackstedt M and Ziauddin M: “A> Sonic differentiation. A comparison of a neutron-density crossplot (black curve) and sonic Closer Look at Pore Geometry,” Oilfield Review 18, no. 1 (Spring 2006): 4–13.compressional (green curve) measurements in Track 3 shows a difference (light blue) attributed tosecondary porosity.26 Oilfield Review
  • 14. tools may be used to determine the pore throatsize and connectivity and infer secondary poros-ity from those relationships. Total porosity, com-puted from NMR data, is generally matrixindependent and is an indicator of incompatiblematrix inputs for neutron-density and sonicporosity logs. Multidimensional resistivity loggingtools can measure vertical and horizontal anisot-ropy for inference of secondary porosity. Dipoleand multiarray sonic data also help estimate 1.0 mmelastic anisotropy and reservoir connectivity.Spectroscopy logs can constrain lithologies todelineate zones of secondary porosity, thus pro-viding an effective matrix density, or grain den- > Sandstone pores and cements. An opacity filter is applied to a CT scan to render different featuressity, for computing porosity. of an eolian sandstone. Light-green quartz grains (upper left quadrant) are removed to reveal only blue The details of cementation, dissolution and pore space (upper right quadrant). The horizontal slice shows quartz (gray), pore space (blue), lateother diagenetic minutia are, for the most part, diagenetic barite cement (red) and carbonate cement (orange). (From Kayser et al, reference 47.)revealed in the laboratory by various forms ofmicroscopy or chemical analysis. Often thin-section study and scanning electron microscopy Thin-section examinations are routinely sup- analyzing the concentrations of carbon, oxygen(SEM) are combined to evaluate different poros- plemented with other sophisticated technology, and sulfur isotopes.ity types, determine rock texture and anticipate including SEM, X-ray diffraction (XRD) and Exploration and production companies havepotential reservoir problems. cathodoluminescence (CL). The SEM analysis used core analysis and petrophysical examina- Thin-section petrography is a basic technique couples extreme depth of focus with a wide range tion routines for years. The industry has madefor examining the textural characteristics of min- of magnification for identifying minerals or inves- many gains from these studies, includingeral grains in a rock. Rock samples are ground to tigating pore morphology and pore throat geom- improved drilling fluids, compatible completionextremely thin slices, which are polished and etry. The SEM technique enables geoscientists to fluids and a range of acid stimulation treat-impregnated with dyed epoxy resin to enhance photograph the distribution of detrital and ments—all designed specifically to overcome theporosity identification. These thin-section slides authigenic minerals and study the effects of effects of diagenetic cements.of rock samples are studied under filtered polar- cement and grain coatings on porosity. Other advanced formation evaluation tech-izing light using a petrographic microscope. X-ray diffraction can reveal much about the nologies, though less common, are makingGeoscientists use polarized light microscopy to crystallographic structure and chemical compo- inroads into the oil patch. One such advance isobserve optical properties caused by anisotropic sition of mudrocks and sandstones and their clay X-ray computed tomography (CT).47 By aiming amaterials that reveal details about the structure fractions. This technology works on the principle focused X-ray beam at a rock sample, geoscien-and composition of the rock. In some cases stains that each crystalline substance produces its own tists can obtain “virtual slices” that can beare applied to aid in identifying mineralogy such unique diffraction pattern. When a rock is ground resolved to a scale of microns (above). As aas feldspar grains and carbonate cements (below). to a powder, each component will produce a research tool, micro-CT scans are used for pore- unique pattern independent of the others, pro- space characterization. An increasingly impor- viding a fingerprint of the individual components tant tool in nondestructive testing, its application to enable identification of the mineralogical can be extended to laboratory testing of uncon- makeup of the sample. solidated or friable formation samples. Micro-CT In addition, quartz, feldspar and carbonate imaging may eventually lead to more-accurate minerals in sedimentary rocks emit Matt—Figure 19 visible, predictions of porosity-permeability trends and ultraviolet and infrared light when bombarded calculations of capillary pressure, relative per- by high-energy electrons. These emissions can meability and residual saturation. D be captured and displayed as color images in a Diagenesis has been a subject of research and cathodoluminescence microscope or petro- discussion since the 1860s, and the industry has graphic scope. The CL imaging results can help taken an increasing interest in the topic since 500 µm geoscientists evaluate the provenance of detrital the 1940s. In the field of diagenetic research, mineral grains. E&P companies have made great strides by> Stain applied to minerals in thin-section Other chemical investigative methods, such adopting evaluation techniques based on SEM,petrographic evaluation. The pink stain as stable isotope analysis, are facilitating inves- XRD and CL analysis. With the alternative to for-differentiates calcite cement from unstained tigations of pore waters and their effect on going comprehensive diagenetic evaluationswhite dolomite (D). In this sample, the original cementing minerals in sedimentary rocks. For becoming less attractive, operators are embrac-calcitic rock was dolomitized, then subsequently example, geoscientists can determine the ing these and other technologies in their attemptscemented by calcite. This thin sectionwas photographed in plane polarized light. marine or nonmarine origin of pore waters by to glean more information and gain greater(Photograph courtesy of W.J. Clark.) understanding of their reservoirs. —MVSummer 2010 27