According to Pettijohn, Potter and Siever (1972), diagenesis refers to all physical and chemical processes that are part of post-depositional connected to newly deposited grains/sediments before metamorphism’s realm begins. Although there is no clear knowledge of the limits between diagenesis and metamorphism one can argue that diagenesis takes place at temperatures below 250 degrees, at pressures below 1000 bars and at maximum depths of 30 km.
Fig.1: Realm of diagenesis based on temperature and depth
According Manzoor (2017), the degree to which sediments are subjects to these changes is governed by multiple factors such as the makeup of sediments, for example a rock consisting primarily of quartz which is more resistant than one consisting mostly of sand grains so the amount of modification also will differ and the rock rich in resistant mineral will resist more to physical alterations while the less resistant more susceptible.
Other factors which control diagenesis are: pressure applied by overburden strata, geothermal temperature, grain size sediments’ porosity, permeability and fluid flow amount.
Clastic diagenesis influences “clastic sedimentary” rocks which are those resulting from weathering of pre-existing rocks namely clasts or fragments of geological detritus which are then transported and transformed rock after deposition at a certain depth.
Diagenetic processes which have an effect on clastic sedimentary rocks are split into physical and chemical. Of which, the physical processes comprise of: compaction and pressure solution while chemical processes are cementation, replacement, recrystallization and dissolution.
It is essential to understanding a few concepts prior to discussing its influence of reservoir quality.
First and foremost, Russell (1960), claims that reservoir is a word given to a region containing fluids separately from its lithology, but a reservoir rock must be one which contains fluids and has the ability of producing them in amounts that are economical. These characteristics imply are linked to the pore space and permeability which means that a reservoir rock is one which is porous and permeable. Where porosity relates to the empty spaces in a rock and permeability to connectivity between pores or to the capacity of fluid flow in the rock.
Consequently, the quality of a reservoir rock depends mostly on the manner and distribution of porosity and permeability in the reservoir since it determines the extent to which oil can be retrieved and hence the secondary recovery methods that should be applied in the presumption of a working petroleum system. However, it is also crucial to stress that, permeability is directly related to porosity while porosity is an autonomous from permeability because for any fluid flow to exist there must exist pores initially. So, a rock can be highly porous but with zero permeability. But, the wider and extensive the fractures or channels connecting the pores likewise, permeability will be favored and vice versa the smaller the open space are will effect permeability negatively.
In addition to that, according to Choquette and Pray (1970) as sited in Ali, Moore and Dibrus (2010) initial porosity in sandstones is 25-40%, the ultimate postdiagenetic porosity is 15-30%.
When sediments are initially deposited they are loosely unpacked and uncemented therefore they have high porosities and high pore-water content. Subsequent deposition causes the older sediments to be buried deeper into the subsurface which can even reach a few kilometers. Under such circumstances temperature and pressure rises with depth (Manzoor, 2017). Now, the originally loose and uncemented grains respond to the vertical shear compressional stress caused the weight of the overburden pressure by either altering the manner in which the grains are arranged or fracturing or bending and the result is that the pore-water is expelled from amid the grains consequently causing the lithification of the sediments into rocks which in turn considerably reduces the porosity and permeability. As an example, after lithified, sandstones can decrease their size by 10% and in mudstones or shales the size decrease is relatively greater because compared to their initial deposition 80% of water and after burial this amount is reduced to 30%.
Fig. 2: Compaction of grains due to overburden pressure
The type of lithology will determine the degree to which the sediments will be compacted, for instance, shale compacts more than sandstones due to their composition, clay/ silt grains are more easily compacted than sands. And in cases where two rocks with different lithology are connected laterally, differential compaction takes place and that leads to a lateral variation of porosity and permeability as well.
Fig. 3: Differential compaction between mud and sand
This process occurs as a response to the compaction of the grains. Since grains are connected by point contacts, the stress is concentrated at these areas as the sediments are buried deeper. In the presence of pore-water, as grains are press against each other diffusion occurs moving the material away from the contact and reprecipitating it on the free spaces between grains and this is the manner in which the grains spread the load created by the overburden weight. (Robin, 1978; cf. Bjorkum, 1996 as cited in Burley & Worden, 2003).
Because the grains are now closely packed together their grains change from point contact to long contact or sutured contact for extreme overburden pressures where the edge of the grains touch each other, and this results in grains being even closely packed which leads to a decrease in porosity and permeability.
Fig.4: Grain contact in clastic rocks
Refers to the growth or mineral precipitation in the pores spaces of sediments which consequently becomes lithified. Cement is not to be confused with matrix which is the fine-grained material deposited along with the large grains. Since there is rarely sufficient material in the rock to account for the cement. The sources of cement are: connate saline water, expulsion from shales, mineral reaction, dissolution of easily soluble rocks and percolating groundwater. The fabric of the cement depends mainly on the amount of soluble minerals present when cementation took place or in other words the degree of supersaturation of the pore-fluid. Cementation can have a positive and negative impact on the reservoir quality of rock. If the cement can withstand the framework of the sediments it can actually lead to preservation of porosity upon further burial and lead very good reservoirs. But it can also cause the grains to be coated or surrounded by the precipitated material and reduce porosity and permeability dramatically by blocking pores and the channels connecting them (Ali, Moore & Dibrus (2010)).
Fig 5. Four elements of a sedimentary rock (From Ali, Moore and Dibur (2010))
According to Darling (2005), a clean sandstone mainly comprised of quartz minerals can have porosity up to 40% and maximum permeability of 5 darcies. But the presence of clays and other mineral forming cements such as carbonates as calcite, silica which form opal and chalcedony or quartz and iron compounds will affect the reservoir quality drastically. For instance, kaolinite (Al2Si2O5(OH)4) which generally occurs as hexagonal crystals can fill the pores or replace detrital feldspar minerals and decrease permeability. Similarly, chlorite (Mg, Fe)6 AlSi3O10(OH) which forms in various structures generally coats the grains of sand along with the pore throats, illite (H3O, K) (Al4Fe4Mg4Mg6) (Si7Al)O22 (OH)4 when occurring as fine ribbon-like filaments of the grain sediments and replaces kaolinite has the tendency of blocking the pores throats and montmorillonite (Na, K, Mg, Ca) Al2Si4O10(OH)2H2O when swelling can even lead to drilling problems with freshwater mud.
Another relevant factor of cementation is overgrowth which occurs when the crystal in the cements grow on the surface of an existing mineral therefore creating a continuous crystal shape over the main grain. And since cementation lithifies the sediments it opens the possibility for a rock to have high porosity but very low permeability by blocking the pore throats between the grains. An example is quartz overgrowth which forms monocrystalline structures.
Fig. 6: Chlorite and Illite coating quartz grains Fig. 7: Pore-filling kaolinite
According to Burley and Worden (2003), dissolution is the process in which a mineral is destroyed as a result of the interaction with an aqueous solution/fluid and the consequence of this is a cavity or space on the mineral grain/ host sediment. This process does not have a very intense effect of clastic sedimentary rocks due to the resistivity of the clasts. However, silica is more soluble in warmer temperatures and under high Ph (alkaline) conditions. Typically, quartz dissolution occurs as pressure dissolution on the boundaries of the grains and the material precipitates on an empty space and this results in neither increase nor decrease in porosity because although some porosity was created in an area, another area porosity has been decreased. But in cases where the material is removed from the formation by migrating interstital waters this results in a reduction of the bulk volume of the rock, more space between the grains therefore an increase in porosity may occur.
Fig. 8: Quartz dissolution
Recrystallization is defined as a dissolution that is followed by the in situ precipitation with alterations in the crystal size or specifications of a mineral chemistry disregarding pore-filling processes. (Burley and Worden ,2003). Nevertheless, the composition of the minerals does not change (Ali, Moore, Dibrus (2010)). Because this process leads to changes in the crystal size and form if minerals it will affect the crystal volume and orientation. Therefore, if crystal have time enough time to grow porosity could be enhanced because the grains will larger but if their growth is fast the result could very fine crystal which reduce empty space between grains.
Replacement refers to the growth of a chemically different mineral of a pre-existing mineral. In other words, the particles change its composition but maintain their original size and form (Ali, Moore & Dibrus (2010)). It is typical of grain with biogenic origin or detrital mineral grains such as feldspars. This process opens the chance of porosity enhancement or reduction. For example, if silica minerals are replaced by carbonate mineral that then are dissolved this leads to an increase in porosity on the other hand if the silica minerals are replaced by clay mineral which are easily compacted and squeezed between grains, a reduction in porosity will be observed (Ali, Moore & Dibrus (2010)).
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