Sedimentologic Controls on Reservoir Quality - Essay Example

SЕDIMЕNTОLОGIС СОNTRОLS ON RЕSЕRVОIR QUАLITY Name Institution Instructor Date Introduction The controls on the reservoir quality in sedimentary rocks with respect porosity and permeability involves taking a critical consideration of the influence of textual maturity of deposits. Major processes that influence porosity and permeability include compaction, cementation as well as dissolution. Composition maturity has got an influence in some of tis processes such as dissolution and compaction. The control on reservoir quality in sedimentary rocks also involves the evaluation of hydrocarbon reservoirs which occur in partially cemented rocks such as the sandstones. The major parameters involved in the control of reservoir quality are porosity and permeability. These parameters are usually however influenced various factors such as cementation, grain orientation, compaction as well as dissolution (Prikryl, 2007). The purpose of this essay is to give a description that explains parameters of the reservoir quality control as well as factors that influence the reservoir quality in sedimentary rocks. It contains on explanation of the factors such as permeability, porosity and diagenesis. Controls on Porosity Porosity refers to the volumetric portion of the bulk rock that is left unoccupied by the solid matter. Porosity is usually expressed as a percentage or as a fraction. ………………………………………………………………………. (I) Where; Ø = is the porosity V p = is the pore volume of the reservoir rock V B = is the bulk volume of the reservoir rock (Ketzer and De ros, 2012) However, porosity does not offer information regarding the distribution of pores, their sizes as well as their connectivity level. Therefore sedimentary rocks having similar porosities are capable of having physical properties that are very different. A very good example that can be used to demonstrate this is sandstone and carbonate rock. Each one of these two rocks could be having a porosity of the same magnitude, but the pores in carbonate rock are usually lack connection and this causes its permeability to reduce I comparison to that of the sandstone. A wide range of porosity definition is identified and put into use in the industry of hydrocarbon. Characteristics of porosity that influence the quality of reservoir in sedimentary rocks include the connected porosity which refers to the volumetric ratio of the pore that is connected to the entire pores (Worden & Morad, 2009). The porosity measurement of sedimentary rocks is varies from 5% to over 30%. This variation is majorly dependent on the nature of reservoir. When sample types of rocks that form part of the reservoir are subjected to porosity measurement for purposes of establishing the reservoir quality, it is found that dolomites have got the lowest porosity whereas the highest porosity is borne by sand stones at about 11-32%. The widest porosity range is however born by shale at 8-29%. The connected porosity is also known as the effective porosity. There is also the primary porosity which refers to the rock porosity which results from the depositional structure of origin. Inter granular porosity describes the porosity that results from the existence of pore volumes within the grains of the rocks whereas intra granular porosity results from the presence of voids within the grains of the rocks. Another feature of porosity that influences the reservoir quality of sedimentary rocks is the dissolution porosity that is brought about by the dissolution of grains in rocks. Fractures occurring in the rocks at various magnitudes result into a porosity that is known as fracture porosity. Inter crystal porosity there is the existence of micro porosity along the boundaries of inter crystals in carbonate rocks (Einsele, 2000). It is worth noting that incase the dry weight as well as the porosity and dry volume of a reservoir rock is identified, then the density of grain can be computed. The calculation of this parameter is commonly done from he collected data which is then subjected to comparison against the results of known rock densities. In order to achieve sedimentary rock quality control with regard to the quality of the reservoir, the initial porosity is influenced by three key microstructural aspects. These include grain packing, grain sizes, grain distribution as well as particle sizes. However, the porosity at initial stages is usually not present in real rocks just as they have been subjected to secondary porosity controls (Selley, 2000). Controls on Permeability Permeability refers to the parameter that describes the ability of soil particles as well as rocks to allow the flow of water through their voids or pores. Porosity is a very important property of the sedimentary rock reservoir and also a key feature in soil mechanics. The coefficient of permeability in sedimentary rock reservoir is commonly obtained through constant head permeability tests. The utilization of permeability coefficient is made in stability, settlement a well as filtration drainage calculations (Ketzer and De ros, 2012). The problems and issues that are associated with permeability calculations in the sedimentary reservoir quality control are very critical in the determination of the factors that influence the quality of reservoir. The utilization of empirical equations is made in the prediction of such critical parameters. However, the use of these equations is associated with certain uncertainties and limitations. Even though tests involving constant permeability head usually take a short time to perform, the relationship between the distribution of grain size and permeability parameter has been studied by various investigators. The formulas have also got the capability of performing the estimation of permeability in a reasonably precise way. Permeability is one of the important factors that influence the reservoir quality of the sedimentary rocks. Permeability is also important is therefore important in applications in that it affects the settlement rate of the soil that is saturated under load. The design of the dams in earth is mostly based on the soil permeability that is used. The slope stability as well as the retaining structures can be largely influenced by the rock permeability of the rocks involved. The design of the soil made filters is done on the basis of their permeability (Prikryl, 2007). Sedimentary rocks are made permeable due to the presence of the interconnected voids which can allow the flow of water from high energy points to low energy points. The study of water flow via permeable rock media is very critical in soil mechanics. The control of permeability as one of the factors influencing the reservoir quality involves the estimation of the underground seepage quantity that happens in various conditions of hydraulic. The estimation is performed for the purpose of investigation of problems that involve water pumping for underground construction and also for performing stability evaluations of earth retaining structures and earth dams that may be subjected to the forces of seepage (Ketzer and De ros, 2012). Controls on diagenesis Diagenesis refers to a combination of effects from processes such as bioturbation, cementation, replacement, recrystallization, burial, compaction as well as chemical reactions that take place the organic matter and the rock within the fluid. The quality as well as viability of sedimentary rock reservoir is ultimate influenced and controlled through a series of processes collectively known as diagenesis. From the time the deposition of sediments takes place, the sediments are subjected to biological, chemical and physical forces that assist in the determination of the type of rock that is eventually formed (Ketzer and De ros, 2012). The reservoir quality in sedimentary rocks with regard to is influenced and controlled by the factors which include depositional permeability and porosity which are largely affected by grain morphology, grain size, sorting as well as the ration of sand to mud. The other factors include that which is associated with the extent of chemical and mechanical compaction, the type and amount of cement that is used in pore filling and also the gradient of geothermal. The diagenesis that takes place in in early stages involves all the processes that take place very close to the surface of sediment deposition where the interstitial geochemistry waters is subjected to control basically through the depositional environment. Early diagenesis can also be described in terms of geothermal gradient, temperature as well as the depth of the higher limit of temperature. One of the noticeable characteristics of sedimentary rock diagenesis is the porosity control(Prikryl, 2007). The processes that are involved in the reservoir quality control include authigenic control, compaction as well as pressure solution through mineral reactions. Sedimentary reservoir diagenesis is subjected to control through a maze of processes that are interdependent and they include sediment texture and composition, rate of burial, tectonic and sedimentary environment, rates of chemical reactions as well as geothermal and hydrodynamic gradients. The determination of properties of textures is done through the physical sedimentation processes and basic mineralogical composition but may be influenced by burial history of the subsequent processes such as cementation, physical compaction and mineralogical alteration. The advance diagenesis processes largely involve cementation and compaction which are basically in control of the porosity. Cementation is one of the diagenesis processes through which the precipitation of minerals takes place in the sediment pore spaces. Compaction generally involve the simple rearrangement of grains during deformation as well as shallow burial of inter granular matrix and ductile grains (Worden & Morad, 2009). Diagenesis reservoir quality control processes like cementation compaction, cementation, replacement and recrystallization are widely described as mechanisms for the reduction of porosity in sedimentary rocks. The course of sedimentary diagenesis in a given pre-burial programmed basin, tectonic setting and depositional environments. These diagenesis interrelated factors and processes control the texture and composition of sand which then control the reaction of minerals as well as rates of fluid flow (Prikryl, 2007). During the process of burial, the three processes that are important in the diagenesis process of inter granular porosity modifications which include cementation, chemical compaction and mechanical compaction. Prediction of initial diagenesis needs estimation of the process of diagenesis as well as the outcomes on the basis of climate, composition, texture depositional environment and chemistry of pore water. In modern sedimentation the process of reservoir quality control is widely influenced by conditions of arid climate. Other key factors that play a significant role in the control of the reservoir quality include those that are influenced by mechanical as well as chemical weathering (Ketzer and De ros, 2012). Conclusion In conclusion, the reservoir quality control in sedimentary rocks is subject to influence by a number of factors which also help in describing the nature of sedimentary rocks reservoir. The major factors whose controls largely affect the quality of the reservoir in sedimentary rocks include the porosity, permeability as we diagenesis and the process involved therein. These factors are especially useful in describing the manner in which the quality of the reservoir is controlled. The permeability of the reservoir assists in presentation of the knowledge regarding the properties of the sedimentary rocks, plays an important role in the estimation of underground seepage quantity, assists in the analysis of the earth structures that are subjected to various forces and also helps in obtaining the solution to seepage pumping problems. Reference EINSELE, G. (2000). Sedimentary basins: evolution, facies, and sediment budget. Berlin [u.a.], Springer. KETZER, M., & DE ROS, L. F. (2012). Linking Diagenesis to Sequence Stratigraphy (Special Publication 45 of the IAS). New York, Wiley. PRIKRYL, R. (2007). Building stone decay: from diagnosis to conservation. London, Geological Society. SELLEY, R. C. (2000). Applied sedimentology. San Diego, Academic Press. MORAD, S., WORDEN, R., & MORAD, S. (2009). Clay Mineral Cements in Sandstones. Chichester, John Wiley & Sons. Read More