Skill target: Quantifying multiple porosity types in a carbonate reservoir (Lucia,1983, 1995, 2007)
Porosity in carbonates
Classifying and quantifying porosity textures, as done in Figure 1, allows us to understand better the process and geometries of porosity evolution and flow unit groupings. What Lucia's work (1983, 1995, 2007) does is better quantify and predict the poroperm paradigm based on carbonate rock textures. A standard poroperm plot based on core-measured carbonate rock properties typically generates a cloud of complexity without a rock-based understanding Figure 2a). It makes better predictive sense when carbonate rock property measurements are broken out according to particle/crystal size criteria quantified by Lucia (Figures 2b).
Lucia's classification of carbonate porosity (Figures 2, 3) evolved from work he did at Shell Oil and Shell Development Companies during the 1960s and the earlier work done by others on poroperm relationships to grain-size in sandstone. Although the influence of Archie's quantitative work is evident in Lucia's classification, Lucia's division of carbonate pore types into vuggy and interparticle categories distinguishes it (Figure 3). Like the Archie classification, the primary aim of Lucia's classification is to provide a practical field and laboratory method for a meaningful visual description of porosity in carbonate rock samples.
For rocks with interparticle and separate vug porosity, Archie's m factor can be estimated when the particle size, amount of separate-vug porosity, and total porosity are known (water saturation page). Lucia's emphasis on the relationship between porosity, permeability, inferred capillary displacement pressure, and particle size, were also recognised by Craze (1950) and Bagrintseva (1977). Lucia's classification offers a quantitative predictive method for "rock typing" or ranking reservoir zones based on observed petrophysical characteristics.
1. Carbonate porosity variablity tied to diagenesis (in part after Akbar, 1995).
2. Poroperm crossplots and the influence of particle/crystal size in a carbonate succession. A) All data. B) Same data classified according to particle or crystal size as per Lucia (1995, 2007)
3. Carbonate porosity classification developed by Lucia (1983, 1995)
The foundation of the Lucia classification, like the Archie (1952) classification, is the quantitative concept of pore-size distribution, whereby the spatial distribution of pore sizes within a rock, controls permeability and saturation and the pore-size distribution is related to rock fabric. To relate carbonate rock fabrics to pore-size distribution, it is crucial to determine if the pore space belongs to one of the three major pore-type classes, interparticle, separate-vug, or touching-vug. Each class has a different type of pore-size distribution and interconnection (Figures 3, 4). The classification developed by Lucia (1983, 1995) is based on quantitative petrography integrated with capillary pressure (MICP) measurements.
Lucia distinguishes between:
(1) Pore space located between grains or crystals (interparticle porosity), where interparticle porosity can be quantified in terms of pore-size distribution or particle-size distribution (Figure 5). The term interparticle in Lucia’s classification is a quantitative textural descriptor and encompasses both the interparticle and intercrystal descriptors of Choquette and Pray (1970)
(2) All other pore space (vuggy porosity). Vugs are commonly present as dissolved grains, fossil chambers, or large irregular cavities.
Vuggy pore space is further subdivided into (Figure 6):
• Separate vugs (vugs are interconnected only through the interparticle pores). Separate vugs are fabric selective in their origin. Intra-fossil and mouldic pore space are typical,
touching vugs (vugs form an interconnected pore system).
• Touching vugs are typically non-fabric selective in origin. Cavernous, breccia, and solution-enlarged fracture pore types commonly form an interconnected pore system.
Essentially, Choquette and Pray (1970) divide all carbonate pore space into two classes: fabric selective and non-fabric selective. Mouldic and intraparticle pore types are classified as fabric selective porosity by and grouped with interparticle and intercrystalline pores. However, Lucia (1983) showed that mouldic and intraparticle pores exert different effects on measured petrophysical properties ad show need separate groupings, while interparticle and intercrystalline pores are similar and, thus should be grouped under the term interparticle (Figure 4).
Pore-type terms of the two classifications are listed in Figure 4. Although most of the terms overlap, the terms interparticle and vug porosity have different definitions in the two classifications. Lucia (1983) demonstrated that pore spaces located both between grains (intergrain porosity) and between crystals (intercrystal porosity) are petrophysically similar, and uses the term “interparticle.” In contrast, the classification of Choquette and Pray (1970) does not use a single term to encompass these two petrophysically similar poroperm responses. In the Choquette and Pray (1970) classification, the term “interparticle” is used instead of “intergrain.”
Vuggy porosity, as defined by Lucia (1983, 2007), is pore space that is within grains or crystals or that is significantly larger than grains or crystals (>2.5 times average grain diameter); that is, vuggy in Lucia’s classification is pore space that is not interparticle (Figures 3, 4). Vugs sizes are wide-ranging; vugs can be present as dissolved grains, fossil chambers, fractures, or large irregular cavities (Figure 6). Although depositional or diagenetic processes may not form fractures, fracture porosity is included because it defines a unique type of porosity in carbonate reservoir rocks. This definition of vug deviates from the restrictive interpretation of vugs used by Choquette and Pray (1970) as nondescript, non-fabric selective pores. Still, it is consistent with the Archie (1952) terminology and with the widespread and less restrictive use in the oil industry of the term “vuggy porosity” to refer to visible pore space in carbonate rocks.
6. Examples of vuggy porosity at the macro-scale
4. Terminology of pore types compared.
5. Multiple interparticle porosity types at the microscale in carbonate reservoirs. A) Interparticle porosity with pendant cement located below a significant unconformity. B) Interparticle (intercrystalline) porosity with high permeability in a Permian dolomite reservoir. C) Interparticle (intercrystalline) microporosity in a low permeability Pliocene dolomite.
7. Continuum of rock fabrics and associated porosity-permeability transforms in non-vuggy carbonates. A) Fabric continuum in nonvuggy limestone. B) Fabric continuum in nonvuggy dolostone. C) Rock-fabric numbers ranging from 0.5 - 4 defined by class-average and class-boundary porosity-permeability transforms (after Lucia, 2007)
Interparticle (non-vuggy) pore classification and response
Non - vuggy or interparticle pores are classified by Lucia as visible or not visible in cuttings (as per Archie (1952). Visible pores are grouped according to particle size as fine ( < 20μm), medium (20 - 100 μm), and large ( > 100 μm) (Figure 7). Unlike the Choquette and Pray (1970) scheme, there are no hierarchical modifiers or categories for time and direction of pore alteration. Rather, Lucia’s classification is defined by the current state of reservoir properties and so provides a basis for estimating the displacement pressure (mercury injection capillary pressure) for each particle size range in interparticle porosity (Figure7). This is important as it gives clues as to the ease with which fluids can move through rocks of different particle (grain or crystal) sizes and hence is a useful concept for reservoir rock typing for both limestones and dolomites.
The relationship between displacement pressure and particle size (Figure 8) is hyperbolic, and a compilation of laboratory measurements suggests important particle-size boundaries at 100 and 20 microns. Based on results illustrated in Figure 8, Lucia (1983) demonstrated that three permeability fields are defined using particle-size boundaries of 100 and 20 microns; this relationship that appears to be limited to particle sizes less than 500 microns (Figure 2).
The three permeability fields (class 1, 2 an 3) initially were based on intercrystalline porosity in dolostone reservoirs of west Texas. Subsequent work that included other dolomites and considerably more limestone and grainier fabrics showed that permeability fields are similar across other types of interparticle porosity (Figures 7, 9). Poroperm plot fields are better described in geologic terms if sorting as well as particle size is considered, and the interpreted plot fields range globally across limestone and dolomites (Figures 7, 9 ).
9. Lucia’s global transform equations (Lucia 2007)
8. Relationship between mercury displacement pressure and average particle size for nonvuggy carbonate rocks with permeability greater than 0.1 md (Lucia, 1983). The displacement pressure is determined by extrapolating the capillary pressure curve to a mercury saturation of zero.
The approach to size and sorting used in Lucia's petrophysical classification (Figure 10) is similar to the grain/mud-support principle upon which Dunham's (1962) classification is built. Dunham's classification, however, is focused on depositional texture, whereas Lucia's petrophysical classification is focused on contemporary rock fabrics, which include depositional and diagenetic textures. Therefore, minor modifications are made to Dunham's classification so it can be applied to a petrophysical classification. Instead of dividing fabrics into grain support and mud support as in Dunham's classification, fabrics are divided into grain-dominated and mud-dominant (Figure 10).
These slightly modified terms are meant to emphasize the fabric elements that control pore size (Figure 10). The most critical attributes of grain-dominated fabrics are the presence of open or occluded intergrain porosity and a grain-supported texture. The essential quality of mud-dominated fabrics is that the areas between grains are filled with mud even if the grains appear to form a supporting framework. Grainstone is clearly a grain-dominated fabric, but Dunham's packstone class bridges a boundary between large intergrain pores in grainstone and small interparticle pores in wackestones and mudstones. Some packstones have both intergrain pore space and mud (reasonable reservoir), others have intergrain spaces filled with mud (poor reservoir). Thus the packstone textural class must be divided into two rock-fabric categories: grain-dominated packstones that have intergrain pore space or cement and mud-dominated packstones that have intergrain spaces filled with mud (Figure 10).
10. Geological/petrophysical classification of carbonate interparticle pore space based on size and sorting of grains and crystals (after Lucia, 1983, 2007). The volume of interparticle pore space is important because it relates to pore-size distribution in a carbonate reservoir
Vuggy porosity (Lucia, 1983, 2007)
The segregation of interparticle from vuggy is a particularly important attribute of Lucia's classification when dealing with the effects of multiple porosity types on reservoir properties, especially the petrophysical significance of separate and touching vugs. The addition of vuggy pore space to interparticle pore space alters the petrophysical characteristics by changing how the pore space is connected, with all pore space being connected in some fashion.
In Lucia's definition, vugs are pores larger than surrounding framework grains. Separate vugs are defined as pore space that is interconnected only through interparticle pore space. Touching vugs are defined as pore space that forms an interconnected pore system independent of interparticle pore space (Fig. 11). Vugs may have been initially mouldic or interparticle pores that were diagenetically modified to become vugs. Dissolution does not follow a predictable pattern in most cases; consequently, the size, shape, and spatial distribution of vugs may be quite irregular. They may begin as fabric-selective dissolution (Figure 12c, d, 13c, d) or non - fabric - selective enlargement of fractures by leaching (Figure 13a, b).
Flow between separate vugs has to pass through matrix porosity and permeability to drain the vugs; therefore the contribution of separate vugs to total reservoir porosity and permeability can be estimated if matrix characteristics and total porosity are known. The only way to obtain that kind of information is by direct observation of rock samples — such as cores — that are large enough to display vugs that may be centimetre-scale in size. Fluid flow through touching vugs is much less affected by matrix permeability and behaves more like flow through open fractures (Figure 11). Because most vugs, particularly touching vugs, are larger than rotary drill cuttings, they may be overlooked during sample examination, which again emphasises the importance of examining full - diameter cores when working with carbonate reservoir.
11. Geological/petrophysical classification of vuggy pore space based on vug interconnection. The volume of separate-vug pore space is important for charac-terizing the pore-size distribution (after Lucia, 2007)
Separate-vug pore space
Separate-vug pore space is defined as pore space that is 1) either within particles Figure 12) or is significantly larger than the particle size (generally >2x particle size) (Figures 6a, 11) and is 2) interconnected only through interparticle pore space (hence the term separated). Separate vugs are typically fabric-selective in their origin. Intrafossil pore space, such as the living chambers of a gastropod or bryozoan shell (Figure 12a, b); grain moulds, such as oomoulds or skeletal moulds (Figure 12c, d); and intragrain microporosity are examples of intraparticle, fabric-selective separate vugs. Moulds of evaporite crystals and fossil-moulds found in mud-dominated fabrics are examples of fabric-selective separate vugs that are significantly larger than the particle size (Figure 13c). Mouldic separate-vug porosity in a potential reservoir tends to give elevated porosity values without a corresponding increase in permeability (Figure 14a)
In mud-dominated fabrics, shelter pore space is typically much larger than the particle size. It is classified as separate-vug porosity, whereas in grain-dominated fabrics, shelter pore space is related to particle size and is considered intergrain porosity. In grain-dominated fabrics, crushing of grains with large intragrain pores by overburden pressure may improve the connection between intragrain and intergrain pore space by fracturing the walls of the grains. In the extreme case, the grains may be crushed beyond recognition and the distinction between intra- and inter-grain pore space blurred, in which case the grain fragments become particles of diagenetic origin. Similarly, the centres of dolomite crystals may be selectively dissolved and the skeletal dolomite crystals crushed to form diagenetic particles composed of pieces of dolomite crystals.
Grain-dominated fabrics may contain grains with intragrain microporosity (Pittman 1971, Keith and Pittman, 1983; Moshier, 1989; Cantrell et al., 1999; Rahman et al., 2011). Even though the pore size is small, intragrain microporosity is classified as a type of separate vug because it is located within the particles of the rock (Figure 14b). Mud-dominated fabrics may also contain grains with microporosity, but they present no unique petrophysical condition because of the similar pore sizes between the microporosity in the mud matrix and the grains.
12. Examples of separate porosity at the within-particle microscopic scale (after Schools, 2003) A) Mid. Pliocene Up. Haurangi Lst., Wairarapa District, New Zealand. This porous, bryozoan-rich, temperate-water limestone has separate vug porosity within bryozoans along with interparticle porosity between bioclasts. B) Recent sediment, Grand Cayman, Cayman Islands, showing separate vug porosity in living chambers of this colonial scleractinian coral. C) Up. Jurassic (Oxfordian) Up. Smackover Fm. (2,366 m) depth showing some isolated vug porosity within partially dissolved ooids, but the bulk of the porosity is interparticle, D) Up. Permian (Kazanian?) Wegener Halvø Fm., Jameson Land, East Greenland. An example of an oolitic grainstone that has undergone essentially complete porosity inversion so that visible porosity (blue) is dominantly separate vug porosity.
13. Multiple styles of separate vug (greater than 2.5 times grain diameter) porosity at the microscopic scale - see Figure 7 for meso-scale examples . A) Pleistocene meteoic leaching, Barbados. B) Late stage leaching, Jurassic Smackover Fm., USA. C) Moulds after anhydrite, Permian USA. D) Permian strandzone, fenestral dolograinstone, USA. (in part after Scholle, 2003)
14. Cross plot illustrating the effect of separate-vug porosity on air permeability. A) Grainstones with separate-vug porosity in the form of grain molds plot to the right of the grainstone field in proportion to the volume of separate-vug porosity. B) Dolograinstones with intra-grain microporosity plot in the class 2 field.(after Lucia, 2007).
Touching-vug porosity
Examples of touching-vug pore types are illustrated in Figures 15 and 16 and have little relationship to rock fabric. Touching vugs can increase permeability well above what would be expected from the interparticle pore system (Figure 18,19). Lucia (1983) illustrated this fact by comparing a plot of fracture permeability versus fracture porosity to the three petrophysical fields (lower right in Figure 18).
Figure 17 shows that rock fabrics or petrophysical classes cannot quantify permeability in touching-vug fracture pore systems. Permeability in touching-vug fracture systems is related principally to fracture width, and so is very sensitive to minimal changes in the volume or type of cement in fracture porosity (Figure 16a, b).
Estimating the permeability of all touching-vug systems is difficult because some vugs can be larger than the wellbore and so these vugs are not reliably captured in standard core plug or whole core measurement. The best information about flow properties for larger-scale touching vug (and fractured) reservoirs comes from production data.
In a predictive sense, core-based measurements are not meaningful due to the large size of most touching-vug systems. For example, standard core plug analysis of the cavernous Niagaran core from northern Michigan (Figure 15a) gives a permeability < 0.1 md in the unaltered grey matrix. This is not a useful whole reservoir value as it measures matrix permeability, not the solution-channel enlarged mesoscale permeability.
However, permeability of smaller touching-vug fabrics composed of micro-fractures and grain moulds is sampled in core plugs or whole core and so can be measured by routine methods. For example, porosity-permeability cross-plots from two microfracture fabrics are captured in Figure 18 and suggest a permeability enhancement of 5x to 10x over what would be expected from a simple interparticle pore system (Lucia and Ruppel 1996). Effects of somewhat coarser touching-vugs are captures using whole core analyses from the Sacroc field, west Texas (Figure 19). In both examples, the impact of increased touching vug porosity is to move values into higher permeabilities as the vugs increasingly interconnect across what is otherwise lower levels of matrix permeability.
There is no hard data on saturation characteristics of large touching vugs. It is thought that large touching vugs most likely have initial water saturations near zero. The initial water saturation of the microfracture fabrics, however, is probably similar to matrix saturation values because the microfractures occupy a small percentage of the pore volume and posses much narrower pore throat diameters (a separate discussion).
15. Mesoscale core examples of toughing vug porosity. A) Silurian Niagaran of the Michigan Basin, where the unaltered grey matrix has permeability < 0.1 mD. B) Permian San Andres dolomite, west Texas, showing touching vug porosity tied to leached anhydrite nodules and cements.
16. Microscale examples of touching vug porosity. A) Cretaceous limestone, Zakinthos, Ionian Islands, Greece showing solution enhanced channel porosity. B) Permian (Guadalupian) Road Canyon Fm., Brewster Co., Texas showing multiple generations of cement-reduced fracture porosity in a platform limestone. Note offset of an earlier generation of completely filled fractures by later, partially filled ones. (after Scholle, 2003)
17. Theoretical fracture air permeability-porosity relationship compared to the rock-fabric/petrophysical porosity, permeability fields (Lucia 1983, 2007). W = fracture width, Z = fracture spacing.
18. Permeability enhancement due to microfractures. A) A factor of 5 permeability enhancement due to microfracturing of a mud-dominated limestone. (b) A factor of 5 permeability enhancement in a class 2 medium crystalline dolowackestone due to microfractures connecting fusumoulds.
19. Poroperm plot field form Pennsylvanian Sacroc field in the Carbonifwerous (Pennsylvanian) Horseshoe Atoll West Texas. The presence of mesoscale-scale touching vug porosity is to move values into higher permeabilites as vugs interconnect across what is otherwise class 1 matrix permeability.
References
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Bagrintseva , K. I., 1977, Carbonate Rocks, Oil and Gas Reservoirs: Moscow, Izdated ’ stvo Nedra, 231 p.
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Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture, in W. E. Ham, ed., Classification of Carbonate Rocks Tuls, OK, AAPG Memoir No. 1, p. 108-121.
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Scholle, P. A., and D. S. Ulmer-Scholle, 2003, A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, v. 77: Tulsa, Okla, American Association of Petroleum Geologists Memoir, 459 p.
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