Diagenesis and Reservoir Quality

Oilfield Review Summer 2010: 22, no. 2. Copyright © 2010 Schlumberger. For help in preparation of this article, thanks to Neil Hurley, Dhahran, Saudi Arabia; and L. Bruce Railsback, The University of Georgia, Athens, USA. The early search for oil and gas reservoirs centered on acquiring an overall view of regional tectonics, followed by a more detailed appraisal of local structure and stratigraphy. These days, however, the quest for reservoir quality calls for a deliberate focus on diagenesis. In its broadest sense, diagenesis encompasses all natural changes in sediments occurring from the moment of deposition, continuing through compaction, lithification and beyond—stopping short of the onset of metamorphism. The limit between diagenesis and metamorphism is not precise in terms of pressure or temperature, nor is there a sharp boundary between diagenesis and weathering. Thus, the nebulous domain of diagenesis lies somewhere between the illdefined borders of weathering at its shallow end and low-grade metamorphism at its deep end. These postdepositional alterations take place at the relatively low pressures and temperatures commonly existing under near-surface conditions in the Earth’s lithosphere. Diagenesis comprises all processes that convert raw sediment to sedimentary rock. It is a continually active process by which sedimentary mineral assemblages react to regain equilibrium with an environment whose pressure, temperature and chemistry are changing. These reactions can enhance, modify or destroy porosity and permeability. Prior to the onset of diagenesis, porosity and permeability are controlled by sediment composition and conditions that prevailed during deposition. Even before it is laid down, a sedimentary particle may undergo changes between its source—whether it was eroded from a massive body of rock or secreted through some biological process—and its point of final deposition. The water, ice or wind that transports the sediment also selectively sorts and deposits its load according to size, shape and density and carries away soluble components. The sediment may be deposited, resuspended and redeposited numerous times before reaching its final destination. Diagenesis commences once a sedimentary particle finally comes to rest. The nature and rapidity of postdepositional changes depend on the medium of deposition as well as the type of sediment deposited. As a given lamina of sediment is laid down, it becomes the interface between the transport medium and the previously deposited material, thus separating two distinctly different physicochemical realms. In its new setting, the sediment contains a variety of minerals that may or may not be in chemical equilibrium with the local environment, and changes in interstitial water composition, temperature or pressure can lead to chemical alteration of its mineral components. At or below the surface of this new layer, the sediment may be locally reworked by organisms that track, burrow, ingest or otherwise redistribute the sediment, sometimes subjecting it to bacterial alteration. As deposition continues, the sedimentary lamination is buried beneath the depositional interface, forming successively deeper strata; there, it encounters continually 1. There is no universal agreement on the exact definition of diagenesis, which has evolved since 1868, when C.W. von Gümbel coined the term to explain postdepositional, nonmetamorphic transformations of sediment. For an exhaustive discussion on the genesis of this term: de Segonzac DG: “The Birth and Development of the Concept of Diagenesis (1866–1966),” Earth-Science Reviews 4 (1968): 153–201. 2. Sujkowski Zb L: “Diagenesis,” Bulletin of the American Association of Petroleum Geologists 42, no. 11 (November 1958): 2692–2717. 3. Krumbein WC: “Physical and Chemical Changes in Sediments After Deposition,” Journal of Sedimentary Petrology 12, no. 3 (December 1942): 111–117. 4. Worden RH and Burley SD: “Sandstone Diagenesis: The Evolution of Sand to Stone,” in Burley SD and Worden RH (eds): Sandstone Diagenesis: Recent and Ancient. Malden, Massachusetts, USA: Wiley-Blackwell Publishing, International Association of Sedimentologists Reprint Series, vol. 4 (2003): 3–44. 5. The term “final deposition” refers to deposition immediately preceding final burial of the sediment, in contrast to earlier phases of deposition, erosion, reworking and redeposition. For more: Choquette PW and Pray LC: “Geologic Nomenclature and Classification of Porosity in Sedimentary Carbonates,” AAPG Bulletin 54, no. 2 (February 1970): 207–250. 6. The initial stage of diagenesis does not begin until the sediment has finally come to a standstill within its current cycle of erosion, transportation and deposition. Changes or alterations that take place before this final deposition are considered as adjustments of the particles to their environment rather than as diagenesis. For more on the initial stages of diagenesis: Shepard FP and Moore DG: “Central Texas Coast Sedimentation: Characteristics of Sedimentary Environment, Recent History, and Diagenesis: Part 1,” Bulletin of the American Association of Petroleum Geologists 39, no. 8 (August 1955): 1463–1593. 7. Krumbein WC and Sloss LL: Stratigraphy and Sedimentation, 2nd ed. San Francisco: WH Freeman, 1963, as cited in de Segonzac, reference 1.