Diagenesis on clastic substance rocks: result on reservoir quality1. IntroductionAccording to Pettijohn, Potter and Siever (1972), diagenesis refers to all or any physical and chemical processes that are a part of post-depositional connected to fresh deposited grains/sediments before metamorphism’s realm begins. though there’s no clear data of the boundaries between diagenesis and geological process one will argue that diagenesis takes place at temperatures below 250 degrees, at pressures below one thousand bars and at most depths of thirty kilometer. Fig.1: Realm of diagenesis supported temperature and depthAccording Manzoor (2017), the degree to that sediments are subjects to those changes is ruled by multiple factors likethe makeup of sediments, for instance a rock consisting primarily of quartz that is a lot of resistant than one consisting largely of sand grains therefore the quantity of modification conjointly can take issue and therefore the rock wealthy in resistant mineral will resist more to physical alterations whereas the less resistant more prone. alternative factors thatmanagement diagenesis are: pressure applied by overburden strata, energy temperature, grain size sediments’ consistency, porousness and fluid flow quantity.
2. Summary of clastic substance rocks and reservoirsClastic diagenesis influences clastic sedimentary rocks that are those ensuing from weathering of pre-existing rocks specifically clasts or fragments of earth science detritus which are then transported and remodeled rock oncedeposition at a particular depth.Diagenetic processes that have an impression on clastic substance rocks are split into physical and chemical. Of which, the physical processes comprise of: compaction and pressure answer whereas chemical processes are cementation, replacement, recrystallization and dissolution. It is essential to understanding a couple of ideas before discussing its influence of reservoir quality. 1st and foremost, Russell (1960), claims that reservoir could be a word given to a locality containing fluids on an individual basis from its lithology, however a reservoir rock should be one that contains fluids and has the power of manufacturing them in amounts that are economical. These characteristics imply are connected to the pore area and porousness which suggests that a reservoir rock is one that is porous and leaky. wherever consistency relates to the empty areas during arock and porousness to property between pores or to the capability of fluid flow within the rock.Consequently, the standard of a reservoir rock depends totally on the way and distribution of consistency and porousness within the reservoir since it determines the extent to that oil are often retrieved and therefore the secondary recovery strategies that ought to be applied in the presumption of a operating rock oil system. However, it’s conjointlycrucial to worry that, porousness is directly associated with porosity whereas porosity is associate degree autonomous from permeability as a result of for any fluid flow to exist there should exist pores at first. So, a rock are often extremelyporous however with zero porousness. But, the broader and intensive the fractures or channels connecting the pores likewise, porousness are going to be favored and contrariwise the smaller the open area are can result permeability negatively.In addition thereto, consistent with Choquette and Pray (1970) as sited in Ali, Moore and Dibrus (2010) initial consistency in sandstones is 25-40%, the final word postdiagenetic consistency is 15-30%.3. Diagenetic processes and its result on reservoir quality Physical processesPhysical compaction: When sediments are at first deposited they’re loosely unpacked and uncemented so they need high porosities and high pore-water content. sequent deposition causes the older sediments to be buried deeper into the submersed which mighteven reach a couple of kilometers. underneath such circumstances temperature and pressure rises with depth (Manzoor, 2017). Now, the originally loose and uncemented grains answer the vertical shear compressional stress caused the burden of the overburden pressure by either fixing the way within which the grains are organized or fracturing or bending and therefore the result’s that the pore-water is expelled from amid the grains consequently inflicting the lithification of the sediments into rocks which successively significantly reduces the consistency and porousness. As associate degree example, once lithified, sandstones will decrease their size by 100% and in mudstones or shales the dimensions decrease is comparatively larger as a result of compared to their initial deposition eightieth of water and once burial this quantity is reduced to half-hour. Fig. 2: Compaction of grains because of overburden pressureThe type of lithology can confirm the degree to that the sediments are going to be compacted, for example, sedimentary rock compacts over sandstones because of their composition, clay/ silt grains are a lot of simply compacted than sands. And in cases wherever 2 rocks with completely different lithology are connected laterally, differential compaction takes place which ends up in a lateral variation of consistency and porousness still.Fig. 3: Differential compaction between mud and sandPressure dissolution / solution:This method happens as a response to the compaction of the grains. Since grains are connected by purpose contacts, the strain is targeted at these areas because the sediments are buried deeper. within the presence of pore-water, as grains are press against one another diffusion happens moving the fabric off from the contact and reprecipitating it on the free areas between grains and this is often the way within which the grains unfold the load created by the overburden weight. (Robin, 1978; cf. Bjorkum, 1996 as cited in Burley & Worden, 2003).Because the grains are currently closely packed along their grains modification from purpose contact to long contact or sutured contact for extreme overburden pressures wherever the sting of the grains bit one another, and this leads tograins being even closely packed that leads to a decrease in consistency and porousness. Fig.4: Grain contact in clastic rocks Chemical processesCementation: Refers to the expansion or mineral precipitation within the pores areas of sediments that consequently becomes lithified. Cement isn’t to be confused with matrix that is that the fine-grained material deposited at the side of the biggrains. Since there’s seldom sufficient material within the rock to account for the cement. The sources of cement are: connate saline water, expulsion from shales, mineral reaction, dissolution of simply soluble rocks and percolating groundwater. the material of the cement depends principally on the number of soluble minerals gift once cementation materialized or in alternative words the degree of supersaturation of the pore-fluid. Cementation will have a positive and negative impact on the reservoir quality of rock. If the cement will stand up to the framework of the sediments it can truly result in preservation of consistency upon any burial and lead excellent reservoirs. however it may also cause the grains to be coated or encircled by the precipitated material and scale back consistency and porousness dramatically by obstruction pores and therefore the channels connecting them (Ali, Moore & Dibrus (2010)).Fig 5. Four components of a stone (From Ali, Moore and Dibur (2010))According to Darling (2005), a clean arenaceous rock principally comprised of quartz minerals will have consistency up to four-hundredth and most porousness of five darcies. however the presence of clays and alternative mineral forming cements like carbonates as spar, oxide that type opaque gem and quartz or quartz and iron compounds can have an effect on the reservoir quality drastically. for example, mineral (Al2Si2O5(OH)4) that usually happens as polygoncrystals will fill the pores or replace detrital felspar minerals and reduce porousness. Similarly, mineral (Mg, Fe)6 AlSi3O10(OH) that forms in numerous structures usually coats the grains of sand at the side of the pore throats, illite (H3O, K) (Al4Fe4Mg4Mg6) (Si7Al)O22 (OH)4 once occurring as fine ribbon-like filaments of the grain sediments and replaces mineral has the tendency of obstruction the pores throats and montmorillonite (Na, K, Mg, Ca) Al2Si4O10(OH)2H2O once swelling will even result in drilling issues with fresh mud. Another relevant issue of cementation is overgrowth that happens once the crystal within the cements grow on the surface of associate degree existing mineral so making never-ending crystal form over the most grain. And since cementation lithifies the sediments it opens the chance for a rock to own high consistency however terribly low porousness by obstruction the pore throats between the grains. associate degree example is quartz overgrowth thatforms monocrystalline structures.Fig. 6: mineral and Illite coating quartz grains Fig. 7: Pore-filling mineralDissolution:According to Burley and Worden (2003), dissolution is that the method within which a mineral is destroyed as a results of the interaction with associate degree liquid solution/fluid and therefore the consequence of this is often a cavity or area on the mineral grain/ host sediment. This method doesn’t have a really intense result of clastic substance rocks because of the resistance of the clasts. However, oxide is a lot of soluble in hotter temperatures and underneath high hydrogen ion concentration (alkaline) conditions. Typically, quartz dissolution happens as pressure dissolution on the boundaries of the grains and therefore the material precipitates on associate degree empty area and this leads toneither increase nor decrease in consistency as a result of though some porosity was created in a region, another spaceconsistency has been shrivelled. however in cases wherever the fabric is faraway from the formation by migrating interstital waters this leads to a discount of the majority volume of the rock, more room between the grains so a rise in consistency could occur.Fig. 8: Quartz dissolutionRecrystallization:Recrystallization is outlined as a dissolution that’s followed by the in place precipitation with alterations within thecrystal size or specifications of a mineral chemistry irrespective pore-filling processes. (Burley and Worden ,2003). yet, the composition of the minerals doesn’t modification (Ali, Moore, Dibrus (2010)). as a result of this method ends up inchanges within the crystal size and type if minerals it’ll have an effect on the crystal volume and orientation. Therefore, if crystal have time enough time to grow consistency may be increased as a result of the grains can larger however if their growth is quick the result may terribly fine crystal that scale back empty area between grains.Replacement:Replacement refers to the expansion of a with chemicals completely different mineral of a pre-existing mineral. In alternative words, the particles modification its composition however maintain their original size and type (Ali, Moore & Dibrus (2010)). it’s typical of grain with biogenic origin or detrital mineral grains like feldspars. This method opens the prospect of consistency sweetening or reduction. for instance, if oxide minerals are replaced by carbonate mineral that then are dissolved this ends up in a rise in consistency on the opposite hand if the silica minerals are replaced by clay mineral that are simply compacted and squeezed between grains, a discount in consistency are going to be determined(Ali, Moore & Dibrus (2010)).References1. Russel (1951). Principles of economic geology (second edition). u. s. of America: MacGraw- Hill Book Company, Inc.2. Mazoor. M. W (2017). Diagenesis. Institute of earth science chemical element. LahoreRetrieved from: Ali. S.A, Clark. W.J, Moore. W.R and Dibrus J.R. (2010). Diagenesis and Reservoir Quality. Georgia: u. s. of America: field Review SummerRetrieved from: Burley S. D. and Worden R. (2003). arenaceous rock Diagenesis: recent and ancient. volume 3: International Association of Sedimentologists5. Darling T. (2005). Well work and formation analysis. Retrieved from: