Diagenesis and Reservoir Quality Syed A. Ali From the instant sediments are deposited, they are subjected to physical, chemical Sugar Land, Texas, USA and biological forces that define the type of rocks they will become. The combined William J. Clark effects of burial, bioturbation, compaction and chemical reactions between rock, William Ray Moore Denver, Colorado, USA fluid and organic matter—collectively known as diagenesis—will ultimately John R. Dribus determine the commercial viability of a reservoir. New Orleans, Louisiana, USA The early search for oil and gas reservoirs cen- particle may undergo changes between its Oilfield Review Summer 2010: 22, no. 2. Copyright © 2010 Schlumberger. tered on acquiring an overall view of regional source—whether it was eroded from a massive For help in preparation of this article, thanks to Neil Hurley, tectonics, followed by a more detailed appraisal body of rock or secreted through some biological Dhahran, Saudi Arabia; and L. Bruce Railsback, The 5 University of Georgia, Athens, USA. of local structure and stratigraphy. These days, process—and its point of final deposition. The 1. 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 the moment of deposition, continuing through ited, resuspended and redeposited numerous Concept of Diagenesis (1866–1966),” Earth-Science 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 42, no. 11 Association of Petroleum Geologists 6 (November 1958): 2692–2717. between diagenesis and metamorphism is not particle finally comes to rest. The nature and 3. 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 is there a sharp boundary between diagenesis the medium of deposition as well as the type of Petrology 12, no. 3 (December 1942): 111–117. 2 7 4. Worden RH and Burley SD: “Sandstone Diagenesis: The and weathering. Thus, the nebulous domain of sediment deposited. 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- 4 6. The initial stage of diagenesis does not begin until the vert raw sediment to sedimentary rock. 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. permeability are controlled by sediment compo- depositional interface, forming successively 7. Krumbein WC and Sloss LL: Stratigraphy and 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 sedimentary 14 Oilfield Review 31424schD5R1.indd 1 7/26/10 5:19 PM Summer 2010 15 31424schD5R1.indd 15 7/26/10 5:21 PM Lowstand Highstand Alluvial channel Distributary channel Highstand sea level l Lowstand sea level Lowstand sea leve Ba sin floor 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 which carbonates form. With decreasing sea level, higher-energy flows are able to carry sediments basinward, eventually depositing them in lowstand basin-floor fan 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 fluvial nied by changing chemical and biological condi- phenomena to counteract the effects of fluid systems or deposited at the beach, rather than in tions. These new conditions promote further incompatibility, mobilization of clays and reser- deep marine settings (above). It is the lowstand consolidation and cementation of loose sediment voir compaction. This article discusses diagene- settings that are responsible for most of the and 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.10 diagenesis 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 biological particle size, fluid content, organic content Setting the Stage activity, which is viable only within a narrow and mineralogical composition. Environmental Porosity and permeability are initially controlled range of light, nutrient, salinity, temperature and factors are temperature, pressure and chemical by sedimentary conditions at the time of deposi- turbidity conditions. These requirements tend to conditions.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 whichMatt—Figure sandstones 02are formed, differ 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