Skill target: Quantifying carbonate porosity -  Choquette and Pray (1970) textures and subsequent modifications

Classification of carbonate porosity

Three carbonate porosity classifications are currently in widespread use in the oil and gas industry (Figure 1); Archie (1952), Choquette and Pray (1970) and Lucia (1983, 1995). The most useful in relating well-log character to reservoir quality is that of Lucia (1995, 2007). Lucia’s classification of porosity in a carbonate follows on from the work of Archie (1952) and his quantitative documentation that pore throat size distribution is critical to reservoir quality understanding. Lucia’s porosity classification is most useful when applied during the production-stage of reservoir characterization when significant volumes of rock-property data are available for study. It is more difficult to use in earlier phases of geological exploration and modelling where quantitative rock-based understanding is minimal, and poroperm geometries must be inferred using minimal data. If the data is there, I tend to use both to garner the best understanding of subsurface rock property distribution.

1. Petrophysical classifications of carbonate pore types as first defined by Archie (1952) compared with the fabric-selectivity concept of Choquette and Pray (1970) and Lucia (1983).

Porosity - Choquette and Pray (1970)

Choquette and Pray (1970) recognise 15 different basic porosity types, as shown in Figure 2. Each type is a physically or genetically distinct and differentiated by variations in pore size, shape, genesis, or relationship to other elements of the fabric, Carbonate porosity is described in detail by Ahr 2008, Moore 2009; see below) and listed as fabric/pore elements in Figure 1.

Most of the pore types in the Choquette and Pray classification are self-explanatory, but a few of the more esoteric types require further explanation. Growth framework porosity encompasses the primary pore space between reef formers as the biota builds a bioherm or boundstone. Fenestral porosity refers to penecontemporaneous or primary pores larger than the surrounding grain-supported fabric. Fenestral pores can develop in beach sands or carbonate muds as a result of air or gas bubbles in the sediment. Umbrella or shelter porosity describes pores preserved beneath cones disarticulated bivalve shells, for example. Neither fenestral or umbrella (shelter) pores contribute large volumes to comic levels of porosity in a reservoir. Mouldic porosity forms when a fabric element dissolves, such as a shell or a crystal cluster of a particular mineral (e.g., dolomite rhombs or anhydrite nodules).

Channel porosity refers to large somewhat irregular elongate pores cutting across rock fabric generally associated with the solution enlargement of fractures. Vug porosity in their definition refers to irregular pores cutting across rock fabric elements such as grains, cement, or crystals and generally range from cm to decimetres in diameter.

Fabric selectivity is the critical component of the Choquette and Pray (1970) classification (Figure 2). Fabric is defined as the solid depositional and diagenetic constituents of a sediment or rock. These solid constituents consist of various types of primary grains, such as ooids and bioclasts; later-formed diagenetic constituents, such as calcite and dolomite cements; and recrystallisation or replacement components, such as dolomite and calcium sulphate crystals (Figure 3). If a dependent relationship exists between porosity and particular fabric elements, that porosity is referred to as fabric-selective, which can be primary or secondary (Figure 2). If no relationship between fabric and porosity can be established, the porosity is classed as not fabric-selective. Then there’s the third grouping of textures that may or may not be fabric selective (Figure 2).

Two factors are essential for interpreting fabric selectivity: the configuration of pore boundaries and the position of pores relative to fabric (Choquette and Pray, 1970). In most primary porosity, pore boundary shapes and pore locations are determined entirely by fabric elements. Primary intergranular pore space in unconsolidated sediments, therefore, is fabric-selective because its configuration is determined solely by depositional particles. The same is true for primary intragranular porosity, which is controlled by the shape and location of cavities determined by the nature of growth of the organism that produced the particle.

In secondary pore systems, however, porosity may be either fabric-selective or not, depending primarily on diagenetic history. As an example, mouldic porosity is commonly fabric-selective because of preferential removal of particular fabric elements from the rock, such as early removal of aragonitic ooids and bioclasts during mineral stabilisation. Another example is the later removal of anhydrite, gypsum, or even calcite from a dolomite matrix during diagenesis. On the other hand, cavern and fracture development typically cut across fabric elements. In rocks without significant tectonic overprint, fracture orientation is controlled primarily by joint systems. Fracture porosity is therefore not fabric-selective and tends to link up other forms of porosity, which is why many giant oil fields in carbonates have significant levels of their effective porosity contributed by fractures.

Primary porosity refers to any porosity present in a sediment or rock at the termination of depositional processes. Secondary porosity can develop at any time after deposition, and the length of time involved may be enormous. The locus and geometry of processes creating secondary porosity generation may be divided into zones based on differences in the porosity-modifying processes occurring in shallow subsurface versus those encountered during deep burial (eogenetic versus mesogenetic; see skill set).

Proportions of porosity contributed by various elements of the classification can be quantified by the use of a ternary diagram and so define pore facies (Figure 4; Ahr, 2008). “ Pore facies ” are constructed from petrographic and stratigraphic data, as described by Kopaska - Merkel and Mann (1993) with the concept further developed by Nazemi et al. (2018). Reservoir pore types are mapped based on their depositional and diagenetic characteristics so that pore facies maps can be constructed to pinpoint the spatial distribution of the pore types with highest corresponding permeability and lowest resistance to fluid flow and that can be correlated stratigraphically at the field scale.

Pore facies, as outlined in Figure 4, encompass particular characteristics such as fluid-flow, pore-throat size distributions and reservoir properties for various reservoir rocks (Nazemi et al., 2018). Generally, pore facies encompass several pore types, but they also may comprise only a single pore type (Bahrami et al., 2017). Identified pore facies were categorised into three main groups in the units studied in Pars Field, offshore Iran (Nazemi et al., 2018). The dominant three clusters are; PF1-dominated by primary or sedimentary pores (interparticle, intraparticle and fenestral); F2-fabric selective pores (mouldic and intercrystalline formed by fabric retentive dolomitization); and PF3- non-fabric selective pores (vuggy, cavernous, fracture, channel, and intercrystalline), with pore facies4 > pore facies2 > pore facies 1). 

2. Porosity classification of Choquette and Pray (1970

3. Some grain-scale porosity examples, according to Choquette and Pray (1970). A) Entrada Fm (Jurassic) mature quartz arenite with interparticle porosity. B) Entrada Fm (Jurassic) mature quartz arenite, Polyhedral (intercrystal) porosity due to quartz overgrowth cement. C) Tansill Fm (Permian) green alga (Mizzia) with intraparticle porosity D) Lower Coralline Lst Fm (Oligocene) with intraparticle porosity in large benthic foram shells. E) Skinner Ranch Fm (Permian) dolomite with polyhedral intercrystalline porosity. F) Ronda unit (Jurassic) with crystal-mouldic porosity (black) due to the dissolution of dolomite rhombs (images courtesy of Scholle et al. (2003) and Ulmer-Scholle et al., (2014).

4. Ternary porosity plot utilising classification of Choquette and Pray (1970) and designed to quantify proportions of relevant porosity fabrics in a carbonate reservoir (after Kopaska-Merkel and Mann, 1993; Nazemi et al., 2018). This methodology requires petrographic study in order to quantify fabric elelments and can be supplemented by core calibrated image log (FMI or UBI log) interpretation

5. Capillary pressure curves based on MICP measurements from Jurassic Smackover rocks at Walker Creek Field, Arkansas. The capillary pressure curves clearly discriminate between good, intermediate, and poor reservoir quality. Good reservoirs have high oil saturation at low capillary pressure, “ Marginal ” reservoirs have lower oil saturation at higher pres-sures, and nonreservoir rocks have high irreducible water saturation and low oil saturation even at high capillary pressure. (adapted from Ahr, 2008).