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→Carbonate reservoirs
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Petrographic analysis of carbonate reservoirs provides description of depositional facies, reconstruction of diagenetic history, and documentation of the porosity system.
Petrographic analysis of carbonate reservoirs provides description of [[depositional facies]], reconstruction of [[Diagenesis|diagenetic]] history, and documentation of the [[porosity]] system.
Depositional facies of reservoir rocks can be inferred on a microscale if diagenesis has not obliterated original carbonate textures. Petrographers recognize a spectrum of original textures that range from mostly carbonate mud (low energy environments) through mostly sand-sized or larger carbonate grains (high energy environments). In fact, this spectrum of textures is the basis for the two most commonly used carbonate classifications, those of Folk<ref name=pt05r56>Folk, R. L., 1959, [http://archives.datapages.com/data/bulletns/1957-60/data/pg/0043/0001/0000/0001.htm Practical petrographic classification of limestones]: AAPG Bulletin, v. 43, p. 1–38.</ref> and Dunham,<ref name=pt05r50>Dunham, R. J., 1962, [http://archives.datapages.com/data/specpubs/carbona2/data/a038/a038/0001/0100/0108.htm Classification of carbonate rocks according to depositional texture], in Ham, W. E., ed., Classification of Carbonate Rocks: AAPG Memoir 1, p. 108–121.</ref> summarized in [[:file:thin-section-analysis_fig2.png|Figure 2]]. In certain instances, variation in reservoir quality (porosity and permeability) can be explained on the basis of textural variation related to distribution of depositional facies within the carbonate reservoir.
Depositional facies of reservoir rocks can be inferred on a microscale if diagenesis has not obliterated original [[carbonate]] textures. Petrographers recognize a spectrum of original textures that range from mostly carbonate mud (low energy environments) through mostly sand-sized or larger carbonate grains (high energy environments). In fact, this spectrum of textures is the basis for the two most commonly used carbonate classifications, those of Folk<ref name=pt05r56>Folk, R. L., 1959, [http://archives.datapages.com/data/bulletns/1957-60/data/pg/0043/0001/0000/0001.htm Practical petrographic classification of limestones]: AAPG Bulletin, v. 43, p. 1–38.</ref> and Dunham,<ref name=pt05r50>Dunham, R. J., 1962, [http://archives.datapages.com/data/specpubs/carbona2/data/a038/a038/0001/0100/0108.htm Classification of carbonate rocks according to depositional texture], in Ham, W. E., ed., Classification of Carbonate Rocks: AAPG Memoir 1, p. 108–121.</ref> summarized in [[:file:thin-section-analysis_fig2.png|Figure 2]]. In certain instances, variation in reservoir quality ([[porosity]] and [[permeability]]) can be explained on the basis of textural variation related to distribution of depositional facies within the carbonate [[reservoir]].
Diagenetic history of carbonate reservoir rocks is important to reconstruct because it influences the volume, size, shape, and distribution of pores. Diagenesis may involve porosity-reducing cementation, porosity-enhancing dissolution, and recrystallization, which may result in either reduction or enhancement of porosity. An important goal of carbonate petrography is to establish the sequence of such events, or paragenesis, of the reservoir. Careful reconstruction of reservoir paragenesis can provide a perspective of the porosity system at the time of hydrocarbon [[accumulation]], thereby enhancing the geologist's understanding of how reserves may be distributed relative to diagenetic facies.
Diagenetic history of carbonate reservoir rocks is important to reconstruct because it influences the volume, size, shape, and distribution of pores. Diagenesis may involve porosity-reducing cementation, porosity-enhancing dissolution, and recrystallization, which may result in either reduction or enhancement of porosity. An important goal of carbonate petrography is to establish the sequence of such events, or paragenesis, of the reservoir. Careful reconstruction of reservoir paragenesis can provide a perspective of the porosity system at the time of hydrocarbon [[accumulation]], thereby enhancing the geologist's understanding of how reserves may be distributed relative to diagenetic facies.
Documentation of the porosity system within a carbonate reservoir provides a clear understanding of the origin and three dimensional distribution of pores. This information is typically collected by classifying individual pores into discrete categories<ref name=pt05r34>Choquette, P. W., Pray, L. C., 1970, [http://archives.datapages.com/data/bulletns/1968-70/data/pg/0054/0002/0200/0207.htm Geological nomenclature and classification of porosity in sedimentary carbonates]: AAPG Bulletin, v. 54, p. 207–250.</ref> and by evaluating the degree to which the various pore types are interconnected. (For more on carbonate porosity types, see [[Porosity#Carbonate pore systems|Table 1]] and [[:file:porosity_fig3.png|Figure 3]].)
Documentation of the porosity system within a carbonate reservoir provides a clear understanding of the origin and three dimensional distribution of [[Pore system fundamentals|pores]]. This information is typically collected by classifying individual pores into discrete categories<ref name=pt05r34>Choquette, P. W., Pray, L. C., 1970, [http://archives.datapages.com/data/bulletns/1968-70/data/pg/0054/0002/0200/0207.htm Geological nomenclature and classification of porosity in sedimentary carbonates]: AAPG Bulletin, v. 54, p. 207–250.</ref> and by evaluating the degree to which the various pore types are interconnected. (For more on carbonate porosity types, see [[Porosity#Carbonate pore systems|Table 1]] and [[:file:porosity_fig3.png|Figure 3]].)
This analysis results in a conceptualization of the three-dimensional pathways that hydrocarbons must follow from their original location in the virgin reservoir to the wellbore. Knowing, for example, that porosity in a particular reservoir is selective to a specific depositional facies would allow a geologist to plan enhanced recovery by siting injection and withdrawal locations on the basis of facies distribution. In contrast, knowing that porosity is mostly not fabric selective (e.g., a combination of [[fracture]] and vuggy) would likely result in a very different plan for siting injection and withdrawal locations. Documentation of the porosity system also provides information that is fundamental to planning optimum reservoir stimulation procedures.
This analysis results in a conceptualization of the three-dimensional pathways that hydrocarbons must follow from their original location in the virgin reservoir to the wellbore. Knowing, for example, that porosity in a particular reservoir is selective to a specific depositional facies would allow a geologist to plan enhanced recovery by siting injection and withdrawal locations on the basis of facies distribution. In contrast, knowing that porosity is mostly not fabric selective (e.g., a combination of [[fracture]] and vuggy) would likely result in a very different plan for siting injection and withdrawal locations. Documentation of the porosity system also provides information that is fundamental to planning optimum reservoir stimulation procedures.