There is commonly ample evidence of porosity creation due to reaction between rock and meteoric water beneath unconformity surfaces. Fig. Therefore. 
KAPPA Engineering; Horne, R.N., 1995. (For more on summation of fluids, see Overview of routine core analysis.). 25. Many apparently sutured boundaries between quartz grains are observed on close inspection by cathodoluminescence imaging to consist of highly comminuted detrital quartz particles that have been cemented by authigenic quartz (Milliken, 1994b, 2001; Dickinson and Milliken, 1995a). Schematic of MINC grid showing the division of original cubic blocks into groups, each containing a fracture block and a set of nested matrix blocks (more matrix blocks are possible than the two shown). Secondary porosity development through dolomitization is common in association with evaporative marine waters and may also occur where normal marine waters are flushed through limestone sequences by thermal convection or marine water movement associated with Kohout convection (Vahrenkamp and Swart, 1994). MINC, Multiple interacting continua. The theoretical effects of grain size and packing on porosity were investigated by Graton and Fraser[7] who computed the porosity of various packing arrangements of uniform spheres. Bulk volume can also be determined by immersing a small sample in a nonwetting fluid. Facies MRSL pore size characterization (50m Scale). Fracture-vug model. Characteristic pressure derivative for dual-porosity model: initial IARF flat derivative (flow in fractures only), followed by transition (dip) then second IARF flat derivative (flow in fractures and matrix). Vinegar, H. J., 1986, X-ray, CT, and NMR imaging of rocks: Journal of Petroleum Technology, v. 38, p. 257259, DOI: Wellington, S. L., and H. J. Vinegar, 1987, X-ray computerized tomography: Journal of Petroleum Technology, v. 39, n. 8, p. 885898, DOI: Amaefule, J. O., D. K. Keelan, D. G. Kersey, and D. M. Marschall, 1988, Reservoir descriptiona practical synergistic engineering and geological approach based on analysis of core data: 63rd SPE Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Houston, TX, October 25, SPE 18167. Matrix blocks are connected only to the adjacent matrix blocks, or to the fracture block in the case of the largest matrix block. [10] In general, porosity decreases as sphericity increases due to tighter packing arrangements associated with spherical grains. Primary porosity formed during initial deposition. The bulk volume is calculated from Archimedes' principle. ScienceDirect is a registered trademark of Elsevier B.V. ScienceDirect is a registered trademark of Elsevier B.V. Epikarst: An Important Aquatic and Terrestrial Habitat, Reference Module in Earth Systems and Environmental Sciences, Advanced analytical pressure-transient analysis relevant to geothermal wells, Some reservoirs have distinct primary and. Berg, R. R., 1970, Method for determining permeability from reservoir rock properties: Transactions Gulf Coast Association of Geological Societies, v. 20, p. 303317. If hydrocarbons cannot be removed with toluene, toluene/methanol (azeotrope), chloroform/methanol (azeotrope), methylene chloride or carbon disulfide may be used. 26. Silica concentration in pore fluids versus depth for Cenozoic units of the Gulf of Mexico Basin. Secondary porosity ranges from 0.3% to 6.7% (average 2.7%) and is mainly due to the dissolution of feldspar grains and volcanic rock fragments. Figure 2.6. Some reservoirs have distinct primary and secondary porosity. Fig. The reduction in resolution has reduced the number of observable nanopores and as such the observable porosity that they can contribute. In the reservoir, however, the resolved stress component is uniaxial. Porosity decreases with increasing net overburden pressure (lithostatic pressure minus pore pressure), and in clastic rocks, stress sensitivity generally increases with increasing clay and decreasing cement content. Fig. 36. Fig. However, proof that a particular mineral has dissolved during diagenesis is often more difficult to come by. 23.8B). Dissolution of either grains or cements in a sandstone leads to the development of secondary porosity. Secondary porosity increases as scale increases, highlighting the importance of factoring in the contributions of secondary pore types in pore network modeling. There are some cases where the epikarst layer becomes more open and uncovered, such as the stone forests of China (Williams, 2008). Under reducing conditions, burial diagenesis (6070C) generates dolomite by replacing earlier minerals at a depth of a few kilometers. In practice the earlier secondary porosity (fracture) response is often masked by wellbore storage and only the later response including the primary porosity (matrix) is observed (Fig. Reproduced courtesy of BP. Pore volume can be measured directly by resaturating a clean, dry rock with a fluid. Residual oil saturation was evaluated in the Erlich model24 on a 12 12 array by a random process according to the intercommunication probability. In total combined secondary pore type microporosity equals: Fig. 39. The MINC method effectively takes a grid and divides each block into a group of blocks: a fracture block and within this a number of nested matrix blocks (up to 5 nested matrix blocks) (Fig. Because these variables directly affect porosity, changes in grain size indirectly affect porosity. Fig. The new equilibrium pressure in the system is monitored, and the pore volume is calculated from Boyle's law: Very accurate measurements of pore volume can be achieved with a Boyle's law porosimeter if the boot conforms to the sample. Facies ILRS pore size characterization (200m Scale). Fig. In an anthropomorphic twist, this became known as the John the Baptist Hypothesis porosity created ahead of the oil coming. 23.8. It is clear that the relative importance of pressure solution as a mechanism of IGV decline increases markedly with depth. This page was last modified on 20 January 2022, at 14:09. This new model accounts for the transient behaviour of flow between the matrix and fractures (Valdes-Perez et al., 2013). Secondary porosity can develop before the burial, after the burial, or as a result of other tectonic activity. Fig. The dual-porosity concept was later generalised for numerical simulation by Pruess and Narasimhan (1985) with the multiple interacting continua (MINC) method. Copyright 2022 Elsevier B.V. or its licensors or contributors. Facies OMRS pore type characterization (50m Scale). The gravimetric determination of bulk volume is similar to the saturation procedure used to determine pore volume. Gerard, R. E., C. A. Philipson, F. M. Bellentine, and D. H. Marshall, 1991, Petrographic image analysis, in Polaz, I., Sengupta, S. K., eds., Automated Pattern Analysis in Petroleum Exploration: New York, Springer-Verlag. Another technique available for the determination of porosity in addition to those mentioned here is point counting pore space occupied by blue epoxy in thin sections (see Thin section analysis). Fracture nanopores contribute between 0.07% and 0.32% toward total porosity, where present, fractures are predominately nanoporous. For example, pressure solution appears to be widespread in sandstones of plutonic derivation (Thomas et al., 1993; Oelkers et al., 1996; Sptl et al., 2000), but not in the largely volcanogenic Cenozoic sandstones of the Gulf of Mexico basin (Land et al., 1987; Land and Milliken, 2000). A mechanism for mobilizing quartz by chemical means rather than directly by pressure has been proposed as the primary cause for the dissolution of quartz along pressure solution seams (Bjrkum, 1996; Renard et al., 1997). Analysis of reservoir pore complexes, https://wiki.aapg.org/index.php?title=Porosity&oldid=27156, Porosity within individual particles or grains, Porosity formed by selective removal of an individual constituent of the rock, Pores larger than grain-supported interstices (interparticle), Porosity created by the sheltering effect of large sedimentary particles, Porosity created by in-place growth of a carbonate rock framework, Pores larger than 1/16 mm in diameter and somewhat equant in shape, Saturated samples available for further testing, Saturation time is dependent on permeability, Boyle's law porosity: grain volume determination, Rapid technique (after cleaning and drying), Boyle's law porosity: pore volume determination, Porosity can be determined at reservoir stress, Permeability can be run on same apparatus to avoid stress hysteresis, Porosity and saturation determined on sample splits, Porosity can be determined on irregular, fractured, and/or vuggy samples, Grain, cement, matrix, and pore relationships can be established, Porosity can be determined on irregular shapes, Precise determination of visible porosity. Fig. Fig. Facies ILRS pore type characterization (200m Scale). Understanding of karst geomorphology concepts is a challenge particularly because much of the karst system lies below the surface, where direct observation is hardly possible. A range of dissolution mechanisms were proposed to explain this. Many of these discrepancies, however, can be explained by noting differences in the definition and assessment of porosity (Figure 1). After Houz, O., Viturat, D., Fjaere, O., 2012. Porosity can be determined by measuring two of three variables: pore volume (Vp), bulk volume (Vb), or grain volume (Vg). This is also a function of the irreducible water saturation. This is not surprising given the kinetic and nucleation limitations on quartz precipitation (Oelkers et al., 1996; Walderhaug, 1996) and the fact that the silica geothermometer does not work well in sedimentary basins (Land and Macpherson, 1992a). 
31. Effective porosity measure of the interconnected pore spaces in a reservoir rock as a function of its bulk volume. Thus, The summation of fluids method is a rapid analytical technique of determining porosity by using the assumption that the total volume of the oil, water, and gas in a material constitutes the pore volume of that material. The improved porosity is then commonly (partially) retained during reburial of the sequence (Figure 9). Cathodoluminescence evidence supports the notion that detrital quartz (outside of pressure solution) is essentially inert to volumetrically significant reaction with pore fluids even into the early stages of metamorphism. This makes it difficult to identify dual-porosity reservoirs. Porosity values for other packing arrangements (Figure 4) can be calculated. Fig. 23.8D). Fig. At this scale, combined dissolution-based porosity can only be demonstrated from the DOMS and the DSND microfacies with porosities of 0.13% and 2.12%, of which nanopores contribute 0.05% and 1.59%, respectively. Vugular reservoirs contain many small cavities, which can either be primary, forming at the same time as the host rock, or secondary, forming due to subsequent processes. 5.9) may not be present. Changes in grain size, however, affect grain shape and sorting. 22. Facies VSND pore size characterization (50m Scale). Factors influencing sample cleaning include the types of hydrocarbon present, the presence of salts precipitated from pore waters, rock mineralogy, degree of cementation, and time constraints. This possibility has received little attention since it was suggested by Fchtbauer (1967). Fig. Facies DSND pore size characterization (200m Scale). Uniaxial stress is less than hydrostatic stress, and consequently, the hydrostatic strain measured in the laboratory should be converted to an equivalent reservoir (uniaxial) strain. Most subsurface fluids are supersaturated with respect to quartz (Figure 11). Shale microfacies (OMRS) contributes 0.21% toward total porosity, representing 1.36% of total network porosity. The relative permeability of fractures and vugs forming the secondary porosity of an intergranular limestone was examined through a simplified model developed by Erlich24. Siltstone microfacies (ILRS, OM/ILS, MRSL, and DOMS) contribute 0.140.64% toward total porosity, representing 2.239.22% of the pore network. However, it can be determined by disaggregating the samples. By continuing you agree to the use of cookies. Such porosity could then be occupied by petroleum. Figure 9. Fig. Carbonate reservoir models: facies, diagenesis, and flow characterization, Core-log transformations and porosity-permeability relationships, Geological nomenclature and classification of porosity in sedimentary carbonates, Influence of texture on porosity and permeability of unconsolidated sand, Relationship between porosity, median size and sorting coefficients of synthetic sands, Permeability-porosity patterns and variations in some Holocene sand bodies, 10.1306/D4269602-2B26-11D7-8648000102C1865D, Petrographic image analysis, 1. Fig. Figure 5.10. The fully saturated sample is first weighed in air, and reweighed while immersed in the wetting fluid. Shrinkage crack nanopores are not a common pore type, in microfacies MRSL and VSND they only contribute 0.11% and 0.06% toward total porosity. Thus, secondary porosity formed within detrital quartz in the subsurface is not observed. Helium from a reference cell at known pressure is then expanded into the pore volume. The total porosity of the sandstones was enhanced by dissolution. The boundary between epikarst and the percolation zone can be gentle or abrupt (see Fig. Bulk volume can be determined by direct measurement, by fluid displacement, or gravimetrically. 5.3 is the version for cubic or spherical matrix blocks) (Horne, 1995). Recrystallization of the detrital quartz imparts a major textural reorganization as textures related to grains, cements, and their intervening pore spaces are erased (Holness and Watt, 2001). This indicates that grain loss is far smaller than estimates based on light microscopy. Primary porosity of carbonate rock is 4050%, and sea water sediments fill these pore spaces; this is the first stage of diagenesis. Fig. Total porosity from a density log would equate with the disaggregated oven-dried total porosity from cores. Grain volume can also be measured by the Boyle's law method. Meteoric water normally plays an important role in dissolution. Secondary porosity creation beneath an unconformity during exposure and weathering, Upper Jurassic Magnus Member Sandstone, North Sea. The total porosity of the sandstones was enhanced by dissolution.
19. 23.8C). If the disaggregated rocks contain smectite, the technique used to dry the samples can affect porosity values and the oven-dried total porosity will be larger than the humidity-dried total porosity (see Effective porosity below). Removal of carbonate, sulfate, and feldspar creates dissolution pores. Of this percentage, micropores only constitute 0.010.02%. Proportionally, secondary pore typecontributions toward total porosity across all microfacies at the two separate scales are as follows: Unlike primary pore types, secondary pore types are predominantly nanoporous in size and consist of fractures, shrinkage cracks, framboidal pores, dissolution pores and mica-based pore types. After Warren, J.E., Root, P.J., 1963. Fig. Rocks containing gypsum and smectite require special low temperature cleaning techniques to minimize removal of structural and bound water.[14]. This true in practice as not all pores are connected. (C) Quartz dissolution (SEM; well X41, depth 1066.3m; LF 1). Karst terrains develop where chemical dissolution dominates over mechanical processes, commonly with well-developed secondary porosity. (A) Feldspar dissolution along cleavage planes (PPL; well X41, depth 1009.1m; LF 2). Eng. Different solvents and cleaning techniques can be used to remove hydrocarbons from rocks. Quite apart from the difficulties of creating sufficient acid and getting it to the reservoir where secondary porosity is required, it remains difficult to find convincing evidence of large-scale, large-volume mineral dissolution in the deep subsurface.
Fig. : New York, Harper and Row, p. 628. However, in the VSND larger microporous shrinkage cracks are present. Discrepancies often exist between laboratory determined porosity values and porosities derived from downhole logs. Facies DSND pore type characterization (200m Scale). 311326., 10., 2118/10681-PA. Pittman, E. D., 1979, Porosity, diagenesis, and productive capability of sandstone reservoirs, in P. A. Scholle, and P. R. Schluger, eds., Aspects of Diagenesis: Society Economic Paleontologists and Mineralogists Special Publication 26, p. 159173. 24. The neutron log, however, would enlarge the definition to include structural hydroxyl chemistry. The theoretical maximum porosity for a cubic packed rock, regardless of the value assigned to grain radius, is 47.6%. Various laboratory techniques are available to determine porosity. 35. Chemical heterogeneity of detrital quartz grains, and also differences in cathodoluminescence between grains and cements, are ultimately homogenized during metamorphism (Sprunt et al., 1978) as quartz undergoes massive recrystallization, the last of the volumetrically significant detrital phases to do so. Thus. 1). Oven-dried core analysis porosity includes the void space of all interconnected pores plus the volume of water bound to smectite. Nanoporous mica-based pores contribute between 0.02% and 0.35% toward total porosity. Pressure is then applied to the outside of the rubber boot to seat it against the sample. Shrinkage crack porosity ranges from 0.04% to 0.1% of which 0.020.09% is microporous. The interface between block groups is by the fracture blocks only. Sample type, pore types, time constraints, and accuracy requirements are generally used to determine the best analytical technique (Table 2). 5.8B). Most porosity analysis techniques require removal of soluble hydrocarbons before sample analysis. Fig. Influence of textural parameters on porosity, Summation of fluids (Retort Porosity) technique, Effects of confining pressure on porosity. Figure 8. Evidence that silt particles in shale undergo changes in aspect ratio during progressive burial has been reported (Evans, 1990). (B) Illite replacing dissolved feldspar (SEM; well X52, depth 1159.6m; LF 1). Total porosity includes all void space regardless of whether the pores are interconnected or isolated. Secondary porosity can be generated by dissolution of sedimentary grains or authigenic minerals. Sandstone microfacies (VSND and DSND, excluding the dolomitic dissolution pores in the DSND), contribute 0.06% and 0.78% toward total porosity, representing 0.7% and 21.1% of pore network porosity. Helium, from a reference cell at known pressure, is then expanded into the sample chamber. Babs Oyeneyin, in Developments in Petroleum Science, 2015. Figure 5.8. In this case there are 312 possible fracture locations. Intergranular porosity exists as space between detrital grains. Carbonate sediments form in shallow warm oceans by marine organisms. Pet. Facies OMRS pore size characterization (200m Scale). Fig. Methanol is used to remove salts formed from the evaporation of saline pore waters. Schematic of dual-porosity concept: (A) actual reservoir showing matrix and fractures and (B) dual-porosity model reservoir with matrix blocks and fractures. (D) Dissolution of volcanic dust (PPL; well X40, depth 1071.5m; LF 1). 20. There is a flat derivative associated with this (and semilog straight line). Facies DOMS pore size characterization (50m Scale). Facies ILRS pore type characterization (50m Scale). The only requirement is that the connate (formation) water that surrounds the grains is undersaturated with respect to the mineral in question.
Sadiq J. Zarrouk, Katie McLean, in Geothermal Well Test Analysis, 2019. A. Frumkin, in Treatise on Geomorphology, 2013. According to the models of Gabrovek (2004), it is nearly universal in its occurrence. Facies VSND pore type characterization (200m Scale). A practical method for modelling fluid and heat flow in fractured porous media. Petroway Inc. After Pruess, K., Narasimhan, T., 1985. 7612, p. 113. The dissolution of volcanic dust also took place (Fig. With deep burial, organic material matures in source rocks and produces hydrocarbons. After the pressure stabilizes, the fully saturated sample is removed from the saturator, immediately rolled on an absorbent material to remove the surface film of saturating fluid, and weighed. 42. This is an excellent method to determine the grain volume regardless of the samples shape or surface characteristics. The depth of epikarst varies with soil and rock conditions, but Klimchouk (2004) states that it can be up to 15m deep. 5.8A) is represented by regularly spaced matrix blocks, connected by straight and evenly spaced fractures (Fig. Most fluids are oversaturated with respect to quartz (1ft=0.3048m).
[1][2] (Also see Reservoir quality.). 25 (1), 1426. A Type of network and probability factor f. BMatrix and fracture-vug, case f=0.5. Advocates of secondary porosity often claimed the wholesale dissolution of mineral cements (particularly calcite) during deep burial, rendering once cemented, low-porosity sandstones highly porous and permeable. (Erlich24, courtesy AIME). An example of a fracture-vug network superimposed on a matrix with a fracture probability factor of f=0.5 is drawn in figure 4.48B. Shrinkage of certain minerals creates fractures, and porosity increases. 29. [8] In theory, porosity is independent of grain size. 16. 1416), with shrinkage cracks and fractures displaying elongate but diametrically constrained morphologies. The equilibrium pressure of the system is monitored and Boyle's Law is used to calculate the grain volume. Shrinkage cracks also produce negligible quantities of porosity and microporosity where present. Toluene is generally an effective solvent for most liquid hydrocarbons. Dissolution porosity is the pore space formed from the partial to complete dissolution of framework grains and/or cements. Coneybeare, C. E. B., 1967, Influence of compaction on stratigraphic analysis: Canadian Petroleum Geology Bulletin, v. 15, p. 331345. Pettijohn, F. J., 1975, Sedimentary rocks, 3rd ed. Dissolution of quartz is less common and is mostly restricted to the edge of the quartz phases (Fig. The first reservoir response is from the fractures as they have greater transmissivity and are connected directly to the wellbore. Both X-ray computerized tomography (CT) and nuclear magnetic resonance (NMR) have applications to determining porosity. This dual-porosity model is equally applicable to fractured or vugular reservoirs. Facies DOMS pore type characterization (200m Scale). Shrinkage cracks in microfacies MRSL are completely nanoporous. Facies ILRS pore size characterization (50m Scale). It is defined as the ratio of void space, commonly called pore volume, to bulk volume and is reported either as a fraction or a percentage. The dual-porosity model has a characteristic pressure derivative response (Fig. Facies MRSL pore size characterization (200m Scale). Assumptions of the dual-porosity model include pseudosteady-state matrix-fracture flow, uniform block size and uniform fracture spacing (Fig. Porosity is generated after deposition, and secondary porosity is very important for hydrocarbon. 30. (B) Sponge spiculite sandstone in which many of the spicules have dissolved (blue pore space) and microcrystalline quartz has precipitated in the original pore space (stained brown by oil), Jurassic Alness Spiculite, North Sea; plane-polarized light photomicrograph. This technique is not suited for noncylindrical samples. Soc. The model includes isolated vugs inside the matrix, and vugs connected with fractures in one or two directions (figure 4.48A). J. Gluyas, in Encyclopedia of Geology, 2005. If clay minerals, especially smectite, are present, humidity drying (45% relative humidity, 63C336.15K 145.4F 605.07R) is required to prevent removal of clay-bound water. Although appealing and superficially elegant, there is scant evidence to support such a hypothesis. Dehydration of mud and recrystallization of minerals produces secondary porosity. Controls that appear to favor the development of pressure solution include abundant potassium-rich micaceous debris, a history of meteoric water incursion, and elevated temperature (>100C?). Fig. Primary porosity is the original porosity of the rock when it formed, and secondary porosity is the pore space created by subsequent processes such as fracturing. Facies VSND pore type characterization (50m Scale). Facies DSND pore size characterization (50m Scale). More recently, Valdes-Perez et al. The DSND microfacies, which has a dolomite dominated intergranular pore space, has a dissolution-based porosity of 0.43% of which microporosity contributes 0.22%, representing 51.1% of dolomite dissolution-based porosity (Figs. Ethan J. Richardson, Michael Montenari, in Stratigraphy & Timescales, 2020. In Developments in Petroleum Science, 1982. This is outside the scope of this discussion but is comprehensively covered in the literature (e.g., Vinegar,[17] and Wellington and Vinegar[18]).
Equations 1, 2, or 3 are then used to compute porosity: Porosity can also be determined by adding (using summation of fluids) the individual ratios of gas volume to bulk volume (Gb), oil volume to bulk volume (Ob), and water volume to bulk volume (Wb) (Equation 4). 5.8B). SEM and thin-section analyses show that feldspar grains were mostly dissolved along cleavage planes (Fig. Popular amongst the various processes invoked for such widespread dissolution was appeal to organic acids created during the initial phases of oil source rock maturation. Porosity determines reservoir storage capacity. Although some of these pore type may be plentiful within the pore network, as confirmed by earlier results in pore abundance, they occupy a very small area (Figs. These analyses indicate that significant differences exist between clastic and carbonate pore types.