Carbonate Rocks-Classification of pores and Seismic Expression - Literature review Example

Table of Contents 1.0.Introduction 2 2.0.Understanding Geology of Carbonate Rocks 3 3.0.Observations of Seismic of Carbonate Rocks 4 4.0.Seismic Expressions 5 5.0.Classification of Pores 6 5.1.Intergranular Porosity 7 5.2.Intragranular Porosity (Sheltered Porosity) 9 5.3.Intercrystalline Porosity 11 5.4.Intergrain Porosity 12 5.5.Dissolution Porosity 12 Figure 1: P-Wave Velocity versus Porosity for Pore Types of Carbonates 4 Figure 2: Intergranular Porosity 8 Figure 3: Intergranular Porosity 9 Figure 4: Intragranular Porosity 10 Figure 5: Intercrystalline Porosity 11 Figure 6: Dissolution Porosity 13 Carbonate Rocks-Classification of pores and Seismic Expression 1.0. Introduction Researches and academic reports have been divided while presenting information regarding carbonate rocks, classification of pores and seismic expression. From the one hand, studies such as Weger et al. (2009) have attempted to add seismic as geophysics in the understanding of carbonate rocks. On the other hand, scholars such as Avseth, Mukerji and Mavko (2010) examined the argument by concentrating their evidences on carbonate porosities. While reports conducted by Avseth, Mukerji and Mavko (2010) has been significant in the general understanding of carbonate porosities, there is need to extend this position by evaluating what constitute seismic as geophysics, classification of pores and seismic expression. Just like it has been noted in other researches, conceptualization of seismic properties of carbonate rocks remain important as it provides platform for exploring both before and during production of hydrocarbons. Similarly, the current need to understand classification of pores and seismic expression call for a report on rock physics models; that has been understood to develop the total porosity as well as mineral compositions for rocks. However, carbonate rocks, classification of pores and seismic expression further need evidence-based researches that focus on geophysics. The main role of geophysics is processes and properties within the solid Earth but it also remains that one may not be able to access the regions of interest such as hydrocarbon reservoir while operating from subsurface thus need for the integration of seismic expression so that a knowledge is provided in the development of a method for synthetically generating data which can resemble that which a geophysicist would collect in the process of real surface-seismic exploration. Based on these uncertainties, this report presents case study and evidence-based evaluation of carbonate porosity. Additionally, the report integrates seismic expression that researches have linked to geophysical properties. Generally the report will provide argument and data on carbonate rocks, classification of pores and seismic expressions. 2.0. Understanding Geology of Carbonate Rocks Dolomite/dolostone, marble and limestone have often been documented as examples of carbonate rocks as they contain large amount of calcite or what is chemically known as calcium carbonate. As studies such as Lubis et al. (2015) has put it, sandstone, dolomite, shale and limestone are examples of sedimentary rocks though other researches have noted that sedimentary rocks stretches beyond sandstone, dolomite, shale and limestone (Burberry et al. 2016). While studies are still divided on the formation of carbonate rocks, the general consensus is that carbonate rocks come as a result of small pieces of materials that are joined together thus getting compacted into a rock (Burberry et al. 2016; Lubis et al. 2015). The research that took a case study on sandstone concluded that the rock is formed as a result of granite rock eroding thus releasing small grains of sand (quartz) thus the quartz pile and gets compacted together to form sandstone. This finding is in line with the report released by Müller-Huber et al. (2016) who noted that the formation of limestone resembles that of sandstone as it is formed when sea shells such as clams and snails are chemically dissolved, deposited and finally compacted together. The research on the formation of carbonate rocks have been presented in reports that investigated geology of carbonates. Report that investigated the formation of carbonate sediments while undertaking a study on Minnesota Geological Survey noted that carbonate rocks are formed at the period of deposition (Skalinski and Kenter 2015). 3.0. Observations of Seismic of Carbonate Rocks Studies that have documented seismic observations of carbonate rocks have reported an agreement on such observations. Firstly, data findings show that the most controlling aspect or factor for seismic is probably the porosity (Hafeez et al. 2016). This report consulted recent studies from Bouchaala et al. (2016) who expanded the knowledge on observations of seismic of carbonate rocks by P-wave velocity versus porosity. The study wanted consistency and as a result plotted the same information for different types of pore as presented in the figure 1 below. Explaining what Bouchaala et al. (2016) plotted; there appears a large scatter of P-waves as far as the exponential bets fit curve is concerned with existing difference being exhibited at about 2500m/s around the extreme values at about 40 percent porosity. Figure 1: P-Wave Velocity versus Porosity for Pore Types of Carbonates Source: Bouchaala et al. (2016) Relating figure one above, different studies that have focused on the carbonate core plug have indicated variations on ways in which the velocities can increase when pressure is increased. As the figure also provides, the different types of pore in carbonate rocks are evidence from measured pressure dependent velocities. From figure 1 above, there is clear understanding of the relationship between carbonate rocks, classification of pores and seismic expressions. 4.0. Seismic Expressions Studies have explored different aspects of seismic expressions with others focusing on issues such as seismic expressions of carbonate rocks and others investigating deeply to understand seismic expression related to Miocene prograde carbonate margin. Differently, Azbel et al. (2016) recognized that as far as seismic expression is concerned, subsurface data sets continue to increase in geology however; outcrop-based studies remain significant in the conceptualization of depositional systems since outcrops permit semi-continuous observations at all levels, including tens of miles to 1 mm. Nevertheless, seismic modeling with outcrops remains valuable approach when calibrating seismic images in carbonate depositional areas or environment. It is essential that the study of seismic expressions, carbonate rocks, classification of pores consider this position as it provides geophysicists with the information on the extraction of stratigraphic data by differentiating between real geologic information and intrinsic patterns of interference to the seismic methods. Recently, studies have concentrated on seismic expression by developing models that help in ascertaining how much of the real stratigraphy can be extracted from seismic data (Azbel et al. 2016; Ahr 2011). Further reports have indicated that current seismic models have helped in defining aspects such as critical frequency for imaging complicated clinoforms thus pointing to ways that helps in improving the seismic resolutions especially by varying the trace spacing. Seismic expression have further investigated the concept of noise-free synthetic seismograms as essential approach or tools for the investigation of different seismic artifacts as well as testing imaging of seismic especially for complex stratigraphy. In addition to this, there is need to report evidences that have focused on the vertical resolutions of methods of seismic expressions especially when dealing with carbonate systems, more so, when geophysicists are dealing with prograding of carbonate margins. In as much as this report remains brief as far as seismic expression is concerned, it provides and understanding on the connectedness between carbonates rocks and classification of pores. Important to note is that outcrop-based seismic models remain essential because of its resolution as well as its ability to confidently reconstruct stratigraphy. Furthermore, studies consider this option to be the best with regard to the calibration of the seismic signal. Still, there is the aspect of explosion of seismically generated or derived attributes which is outcrop-based models with ideal data sets to assist in the calibration of the stratigraphic importance of different attributes as well as evaluation of aspects such as seismic-inversion algorithms (Müller et al. 2010). 5.0. Classification of Pores Pore classification remains significant area of study when geophysicists investigate carbonate rocks. Besides, this report considers such classifications essential as it forms requisites of comprehensive understanding of porous materials or solids. Literatures including figure 1 above have indicated the existence of different classifications of pores however, it remains difficult to provide a consistent world classification of pores including adsorbents, catalysts, carbons, oxide, soils and organic polymers among others. Noting these concerns, this report incorporates the aspect of seismic expressions and carbonate rocks to organise pores in different classes by further grouping such pores basing on the widely used classification of porous materials. Secondly, studies have recognized that carbonate rocks remain complex in pore geometry and structure thus displaying heterogeneity with regard to all length scales. It therefore means that classification of pores will have to describe carbonate rocks on the premise of their pore geometry and content for use especially for researches that will be concerned with pore-scale modeling. One factor that remains in description of classes of pores in carbonate rocks is the porosities and grains which are further based on other carbonate-rock classification (Rezaee et al. 2006). As far as carbonate rocks are concerned, pores have been defined as the cavities found in carbonate rocks that are related to diagenetic, depositional, and tectonic processes. The following are classes of pores in carbonate rocks: 5.1. Intergranular Porosity Some studies have documented this class of porosity as primary intergranular porosity thus consisting of depositional multi-sized open spaces found between allochems that can be seen to be decreasing with their burial especially when the burial is found to be as a result of cementation and compaction (Vahrenkamp et al. 2004). Basing on figure 2 below, porosity of carbonate rock might differ in definition especially when the figure is related to Rezaee et al. (2006) definition of porosity in carbonate rocks. However, intergranular porosity brings the understanding that there are some rocks that exhibit pores as a fraction of the volume of existing spaces between the rocks (solid materials) of the rock to the total volume of the rock. It is for this matter that the pore’s classification in figure 2 below includes cracks, pores, intra-and inter-crystalline spaces and vugs. Figure 2: Intergranular Porosity Source: Rezaee et al. (2006) Figure 3 below further provides an understanding on the intergranular porosity. The blue spaces between what can be considered as quartz grains provide what Fournier and Borgomano (2007) termed as primary integranular porosity. The figure also shows a well-developed intergranular porosity (as shown in blue arrows). Figure 3: Intergranular Porosity Source: Fournier and Borgomano (2007) 5.2. Intragranular Porosity (Sheltered Porosity) Unlike intergranular porosity, intragranular or sheltered porosities are made of depositional open spaces that are further found within allochems (Zeng et al. 2011). According to Vanorio and Mavko (2011), the allochems tend to decrease as the rock is buried either as a result of cementation or compaction or both. Figure 4: Intragranular Porosity Source: Vanorio and Mavko (2011) From the figure below, there are voids formed under special points that are known as elongated clasts. The elongated clasts prevent the process of accumulation of thin-and fine-grained sediments. Vanorio, T. and Mavko (2011) noted that this type of porosity is often seen in bioclastic packstones that contain brachiopod valves and disarticulated bivalve. Conclusively, there are three issues that need to be noted as far this class of porosity is concerned. Firstly, intragranular porosity remains primary class of porosity that is found in clastic limestones (Vanorio and Mavko 2011). Still on this point, figure 4 above represents an example of shelter porosity that can be found in a number of oolitic grainstones that has coarse shell particles/fragments/intraclasts or carbonate mudstones that has platy skeletal particles. The second point is that the type of pores in carbonate rocks can be classified on the premise of the timing of porosity evolution. It is because of this timing that researches such as Zeng et al. (2011) have noted that original primary porosity (as it is the case with shelter porosity) can be destroyed totally in the process of diagenesis thus leading to secondary porosity. 5.3. Intercrystalline Porosity Intercrystalline porosity is a secondary level of porosity that defined by the level of pores they make on rocks. Also known as intercrystal porosity, they have open space between authigenic. In as much as this basic definition could be applied in a given situation or in strict sense, when it comes to almost all types of porosity in carbonate rocks, the type of porosity is understoond within the context of porosity between individual crystals of equal sizes as it is the case with a number of porous dolomites. In most cases, intercrystalline porosity may be secondary or primary (Skalinski and Kenter 2015). Figure 5 below has been adopted from Fournier and Borgomano (2007) research to provide an understanding on primary intercrystalline porosity. Figure 5: Intercrystalline Porosity Source: Fournier and Borgomano (2007) 5.4. Intergrain Porosity This type of carbon rock porosity refers to a situation when there are pore spaces existing between individual particles and grains of carbonate rock. Intergrain porosity remains to be the most commonly used concept between carbonates (such as interparticle porosity) and between-grain porosity such as that of sandstones. However, studies agree that interparticle porosity is not synonymous with primary porosity (Fournier and Borgomano 2007) instead; interparticle porosity remains to be non-genetic concept that means only the relative position, but not the time when it was formed of the pores. 5.5. Dissolution Porosity Dissolution porosity is a type of secondary porosity which is made up of open spaces between or within allochems that are formed by the removal of secondary and primary materials (Ahr 2011). Broad concepts regarding dissolution is that porosity within carbonate rocks result from subaerial meteoric exposure when it comes to eogenetic environment. In as much, creation of new porosity or enhancement of pre-existing porosity takes place by the process of dissolution in the depth burial within an environment categorized as mesogenetic. Based on figure 6 below, dissolution porosities occur either along stylolites or hydraulic fractures. However, there are some cases not covered in the figure below where there are some types of pores formed by mesogenetic dissolution. In such cases, such pores will mimic those that are created in the eogenetic environment (Ahr 2011). Another point to note regarding dissolution porosity is that mesogenetic dissolution are likely to affected by different environmental factors such as fluids, carbon dioxide and organic acids. Figure 6: Dissolution Porosity Source: Ahr (2011) References Ahr, W.M., 2011. Geology of carbonate reservoirs: the identification, description and characterization of hydrocarbon reservoirs in carbonate rocks. John Wiley & Sons. Avseth, P., Mukerji, T. and Mavko, G., 2010. Quantitative seismic interpretation: Applying rock physics tools to reduce interpretation risk. Cambridge university press. Azbel, K., Rodina, O., Baechle, G., Gouveia, W., Gracioso, D., Nogueira, F., Wang, P. and Centeno, R., 2016, April. De-risking Reservoir Quality and Presence through Seismic Inversion and Rock Physics Analysis in Brazil's Pre-salt Carbonates. In Third EAGE/SBGf Workshop 2016. Bouchaala, F., Ali, M.Y. and Matsushima, J., 2016. Estimation of seismic attenuation in carbonate rocks using three different methods: Application on VSP data from Abu Dhabi oilfield. Journal of Applied Geophysics, 129, pp.79-91. Burberry, C.M., Jackson, C.A.L. and Chandler, S.R., 2016. Seismic reflection imaging of karst in the Persian Gulf: Implications for the characterization of carbonate reservoirs. AAPG Bulletin, 100(10), pp.1561-1584. Fournier, F. and Borgomano, J., 2007. Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines). AAPG bulletin, 91(2), pp.235-258. Hafeez, A., Wibisono, A., Rochmad, K.I., Guntoro, A. and Octavia, G., 2016. Developing a Versatile Method for Rock Physics Modeling in a Carbonate Reservoir by Integrating Rock Mechanic Laboratory Results, Petrophysical Analysis and Computational Rock Physics: A Case Study from The Kais Formation in the Salawati Basin. Kazatchenko, E., Markov, M. and Mousatov, A., 2006. Simulation of acoustic velocities, electrical and thermal conductivities using unified pore-structure model of double-porosity carbonate rocks. Journal of Applied Geophysics, 59(1), pp.16-35. Lubis, L.A., Bashah, S. and Ghosh, D.P., 2015. Comparison of Different Rock Physics Models to Evaluate the Impact of Pore Types on Velocity—Porosity Relationship in Carbonates of Central Luconia Sarawak. In ICIPEG 2014 (pp. 387-393). Springer Singapore. Müller, T.M., Gurevich, B. and Lebedev, M., 2010. Seismic wave attenuation and dispersion resulting from wave-induced flow in porous rocks—A review. Geophysics, 75(5), pp.75A147-75A164. Müller-Huber, E., Schön, J. and Börner, F., 2016. Pore space characterization in carbonate rocks—Approach to combine nuclear magnetic resonance and elastic wave velocity measurements. Journal of Applied Geophysics, 127, pp.68-81. Rezaee, M.R., Jafari, A. and Kazemzadeh, E., 2006. Relationships between permeability, porosity and pore throat size in carbonate rocks using regression analysis and neural networks. Journal of Geophysics and Engineering, 3(4), p.370. Skalinski, M. and Kenter, J.A., 2015. Carbonate petrophysical rock typing: integrating geological attributes and petrophysical properties while linking with dynamic behaviour. Geological Society, London, Special Publications, 406(1), pp.229-259. Vahrenkamp, V.C., David, F., Duijndam, P., Newall, M. and Crevello, P., 2004. Growth architecture, faulting, and karstification of a middle Miocene carbonate platform, Luconia Province, offshore Sarawak, Malaysia. Vanorio, T. and Mavko, G., 2011. Laboratory measurements of the acoustic and transport properties of carbonate rocks and their link with the amount of microcrystalline matrix. Geophysics, 76(4), pp.E105-E115. Weger, R.J., Eberli, G.P., Baechle, G.T., Massaferro, J.L. and Sun, Y.F., 2009. Quantification of pore structure and its effect on sonic velocity and permeability in carbonates. AAPG bulletin, 93(10), pp.1297-1317. Zeng, H., Loucks, R., Janson, X., Wang, G., Xia, Y., Yuan, B. and Xu, L., 2011. Three-dimensional seismic geomorphology and analysis of the Ordovician paleokarst drainage system in the central Tabei Uplift, northern Tarim Basin, western China. AAPG bulletin, 95(12), pp.2061-2083. Read More

Figure 1: P-Wave Velocity versus Porosity for Pore Types of Carbonates Source: Bouchaala et al. (2016) Relating figure one above, different studies that have focused on the carbonate core plug have indicated variations on ways in which the velocities can increase when pressure is increased. As the figure also provides, the different types of pore in carbonate rocks are evidence from measured pressure dependent velocities. From figure 1 above, there is clear understanding of the relationship between carbonate rocks, classification of pores and seismic expressions. 4.0.

Seismic Expressions Studies have explored different aspects of seismic expressions with others focusing on issues such as seismic expressions of carbonate rocks and others investigating deeply to understand seismic expression related to Miocene prograde carbonate margin. Differently, Azbel et al. (2016) recognized that as far as seismic expression is concerned, subsurface data sets continue to increase in geology however; outcrop-based studies remain significant in the conceptualization of depositional systems since outcrops permit semi-continuous observations at all levels, including tens of miles to 1 mm.

Nevertheless, seismic modeling with outcrops remains valuable approach when calibrating seismic images in carbonate depositional areas or environment. It is essential that the study of seismic expressions, carbonate rocks, classification of pores consider this position as it provides geophysicists with the information on the extraction of stratigraphic data by differentiating between real geologic information and intrinsic patterns of interference to the seismic methods. Recently, studies have concentrated on seismic expression by developing models that help in ascertaining how much of the real stratigraphy can be extracted from seismic data (Azbel et al.

2016; Ahr 2011). Further reports have indicated that current seismic models have helped in defining aspects such as critical frequency for imaging complicated clinoforms thus pointing to ways that helps in improving the seismic resolutions especially by varying the trace spacing. Seismic expression have further investigated the concept of noise-free synthetic seismograms as essential approach or tools for the investigation of different seismic artifacts as well as testing imaging of seismic especially for complex stratigraphy.

In addition to this, there is need to report evidences that have focused on the vertical resolutions of methods of seismic expressions especially when dealing with carbonate systems, more so, when geophysicists are dealing with prograding of carbonate margins. In as much as this report remains brief as far as seismic expression is concerned, it provides and understanding on the connectedness between carbonates rocks and classification of pores. Important to note is that outcrop-based seismic models remain essential because of its resolution as well as its ability to confidently reconstruct stratigraphy.

Furthermore, studies consider this option to be the best with regard to the calibration of the seismic signal. Still, there is the aspect of explosion of seismically generated or derived attributes which is outcrop-based models with ideal data sets to assist in the calibration of the stratigraphic importance of different attributes as well as evaluation of aspects such as seismic-inversion algorithms (Müller et al. 2010). 5.0. Classification of Pores Pore classification remains significant area of study when geophysicists investigate carbonate rocks.

Besides, this report considers such classifications essential as it forms requisites of comprehensive understanding of porous materials or solids. Literatures including figure 1 above have indicated the existence of different classifications of pores however, it remains difficult to provide a consistent world classification of pores including adsorbents, catalysts, carbons, oxide, soils and organic polymers among others. Noting these concerns, this report incorporates the aspect of seismic expressions and carbonate rocks to organise pores in different classes by further grouping such pores basing on the widely used classification of porous materials.

Read More