Rock-Fabric/Petrophysical Relationships

Petrophysics of Interparticle Pore Space

Limestone Rock Fabrics

Examples of nonvuggy limestone petrophysical rock fabrics are illustrated in Fig. 7. In grainstone fabrics, the pore-size distribution is controlled by grain size; in mud-dominated fabrics, the size of the micrite particles controls the pore-size distribution. In grain-dominated packstones, however, the pore size distribution is controlled by grain size and by the size of micrite particles between grains. Figure 8 illustrates all the data for limestones compared with the permeability fields. Grainstone and mud-dominated fabrics are reasonably well-constrained permeability fields. Although grain-dominated packstone fabrics plot at an intermediate location between grainstones and mud-dominated limestones, they show more variability because of the large grain size difference. Grain size of grain-dominated packstone ranges from 400 microns for oncoid fabrics to 80-150 for fine peloid fabrics.

Class 1 ooid grainstone	Class 1 ooid grainstone
Class 2 peloid grain-dominated packstone	Class 2 oncoid grain-dominated packstone
Class 3 fossil mud-dominated packstone	Class 3 rudist floatstone
Class 3 Orbitolind wackestone
Figure7. Examples of nonvuggy limestone rock fabrics. (click on each image to see a larger version)

Despite the considerable scatter in the data, grainstone, grain-dominated packstone, and mud-dominated fabrics are reasonably well constrained to the three permeability fields. Whereas grain size and sorting define the permeability fields, the interparticle porosity defines pore-size distribution and thus the permeability within the field. Systematic changes in intergrain porosity by cementation, compaction, and dissolution processes will produce systematic changes in pore-size distribution and result in systematic changes in permeability. Therefore, permeability in nonvuggy limestones is a function of interparticle porosity, grain size, and sorting.

Figure 8. Porosity-air permeability cross plot for nonvuggy limestone fabrics compared with the three permeability fields.

Dolostone Rock Fabrics

Examples of nonvuggy dolostone petrophysical rock fabrics are illustrated in figure 9. Dolomitization can change the rock fabric significantly. In limestones, fabrics can usually be distinguished with little difficulty. If the rock has been dolomitized, however, the overprint of dolomite crystals often obscures the limestone fabric precursor. Precursor fabrics in fine-crystalline dolostones are easily recognizable. However, as the crystal size increases, the precursor fabrics become progressively more difficult to determine.

Class 1 peloid med xl dolograinstone	Class 1 ooid med xl dolograinstone
Class 2 peloid med xl g.d. dolopackstone	Class 2 peloid med xl g.d. dolopackstone
Class 3 f. xl dolowackestone	Class 2 med xl dolowackestone
Class 1 lg xl dolowackestone	Class 1 lg xl dolograinstone
Figure 9. Examples of nonvuggy dolostone rock fabrics. (click on each image to see a larger version)

Dolomite crystals (defined as particles in this classification) commonly range in size from several m to >200. Lime mud particles are usually <20 in size. Thus, dolomitization of a mud-dominated carbonate fabric can result in an increase in particle size from <20 to >200 (Fig. 9). The plot of interparticle porosity against permeability (Fig. 10a) illustrates the principle that, in mud-dominated fabrics, permeability increases as dolomite crystal size increases. Finely crystalline (average 15 m) mud-dominated dolostones plot within the class 3 permeability field. Medium crystalline (average 50) mud-dominated dolostones plot within the class 2 permeability field. Large crystalline (average 150 ) plot in the class 1 permeability field.

Figure 10. Porosity-air permeability cross plots of nonvuggy dolomite fabrics compared with the three permeability fields illustrated in Figure 5. (A) Mud-dominated dolostones with dolomite crystal sizes ranging from 10 to 150 microns. (B) Dolograinstones (average grain size is 200 microns) with dolomite crystal sizes ranging from 15 to 150 microns. (C) Grain-dominated dolopackstones with fine to medium dolomite crystal sizes.s.

Grainstones are usually composed of grains much larger than the dolomite crystal size (Fig. 9) so that dolomitization does not have a significant effect on the pore size distribution. This principle is illustrated in Fig. 10b where interparticle porosity is plotted against permeability measurements from dolograinstones. The grain size of the dolograinstones is 200. Fine crystalline dolograinstone, medium crystalline dolograinstone and large crystalline dolograinstone all plot within the class 1 permeability field. The large crystalline mud-dominated dolostones also plot in this permeability field, indicating that they are petrophysically similar to grainstones (Fig. 10a).

A cross plot of interparticle porosity and permeability measurements from fine to medium crystalline grain-dominated dolopackstones is presented in Fig. 10c. The average grain size is 200. The data plot in the class 2 permeability field. The medium crystalline mud-dominated dolostones also plot in this field (Fig. 10a).

Figure 11 illustrates all dolomite data compared with permeability fields. Dolograinstones and large crystal dolostones constitute the class 1 permeability field. Grains are very difficult to recognize in dolostones with a > 100 crystal size. However, because all large crystalline dolostones and all dolograinstones are petrophysically similar, whether the crystal size or the grain size is described makes little difference petrophysically. Fine and medium crystalline grain-dominated dolopackstones and medium crystalline mud-dominated dolostones constitute the class 2 permeability field. Fine crystalline mud-dominated dolostones constitute the class 3 field.

Figure 11. Composite porosity-air permeability cross plot for nonvuggy dolostone fabrics compared with the three permeability fields.

The dolomite permeability fields are defined by dolomite crystal size as well as grain size and sorting of the precursor limestone. Within the field, permeability is defined by interparticle porosity. Systematic changes in intergrain and intercrystal porosity by predolomite calcite cementation, dolomite cementation, and compaction will systematically change the pore-size distribution, resulting in a systematic change in permeability. Therefore, dolomite crystal size, grain size, and sorting define the permeability field, and interparticle porosity defines the permeability.

Figure 12. Composite porosity-air permeability cross plot for nonvuggy limestones and dolostones showing transforms for each class. See text for equations.

Limestone and Dolostone Combined

Data from limestone and dolostone rock fabrics are combined into one interparticle-porosity permeability cross plot in figure 12. The fabrics that make up the class 1 field are (1) limestone and dolomitized grainstones and (2) large crystalline grain-dominated dolopackstones and mud-dominated dolostones. An upper particle size limit of 500 is imposed but not well defined. The upper limit is imposed because as the grain size increases the slope of the porosity-permeability transform approaches infinity and permeability becomes independent of porosity.

Figure 13. Petrophysical/rock-fabric classes based on similar capillary properties and interparticle-porosity/permeability transforms.

Fabrics that make up the class 2 field are (1) grain-dominated packstones, (2) fine to medium crystalline grain-dominated dolopackstones, and (3) medium crystalline mud-dominated dolostones. The class 3 field is characterized by mud-dominated fabrics (mud-dominated packstone, wackestone, and mudstone) and fine crystalline mud-dominated dolostones. This arrangement of petrophysically similar rock fabrics is illustrated in figure 13.

Transforms are presented below for each combined permeability field (Fig. 12).