Datos para estimar el IGR

This section introduces the different pore types observed in the Carboniferous carbonate rocks exposed in the Bodón Unit during fieldwork and thin section petrography. Pores have only been observed in HTD rocks, and not in precursor limestones which are generally very tight and therefore not discussed in this section. The dolomite pore types are defined based on the classification system of Choquette and Pray (1970) and are listed in order of decreasing importance. Photographs of the different pore types have been assembled in figure 3.22. This section introduces each pore type, while section 3.6 focuses on the evolution of reservoir characteristics during the diagenetic history of the platform limestones, which has been elaborated in section 3.4. Section 3.7 describes the importance of each pore type for the dolomite matrix porosity in each of the three key outcrops.

3.5.1. Vuggy pores

Vuggy pores are defined by Choquette and Pray (1970) as mm- to m-sized non-fabric selective pores caused by early or late diagenetic dissolution. The origin of vugs in dolomite rocks has been assigned to the 13 % volume decrease upon dolomitization (see section 1.2.4), or to the occurrence of molds and intergranular pores present in the precursor limestones. According to Merino and Canals (2011), vugs in dolomite rocks are often generated through dissolution of calcite by dolomitizing fluids upon entering a limestone. In the current study, vuggy pores are defined as dissolutional vugs, following Merino and Canals (2011), associated with the dolomitization process. In contrast to the original definition of Choquette and Pray (1970), vuggy pores are found to be fabric selective in the Bodón Unit. They are typically associated with specific dolomitized facies such as microbial boundstones of the Valdeteja Fm. in KO 1 (figure 3.22A) and Fe-rich coarse-crystalline dolomite in KO 3 (figures 3.16C & 3.22B). Vugs can range from a few cm in size (figures 3.22A & B) to less than a mm (figure 3.22C). However, in case of small pores, it is often difficult to assign a precise origin.

3.5.2. Zebra pores

Diagenetic zebra textures, with associated zebra porosity, are made up of rhythmic alternations of replacive and void-filling dolomite and calcite (Nielsen et al., 1998). They form under high pore fluid pressures in low-permeability dolomite rocks (see chapter 4), and occur seldom in coarse-grained facies (Davies and Smith, 2006). The genesis of zebra textures has been assigned to focused flow of fluids at suprahydrostratic pressures, creating parallel fractures which are partly or completely filled with dolomite cement crystals (Nielsen et al., 1998; Vandeginste et al., 2005). Merino et al. (2006) introduced a new model of zebra dolomitization in which the textures are created by self- organization of layers of displacive dolomite and calcite crystals (instead of void-filling cement) that push aside replacive dolomite due to local induced stress generated by crystal growth. The calcite which occurs in between the dolomite crystals often dissolves leaving behind sheet-like pores, the so-called zebra pores. In the Bodón Unit, zebra textures and associated pores are omnipresent in the dolomitized mudstones of the Barcaliente Fm. (figure 3.22D) and occur locally in dolomitized microbial boundstones (figures 3.17E & 3.22E), where they are often associated with vuggy pores. Zebra pores can be up to several cm in length and a few mm in width.

3.5.3. Intercrystalline pores

Pores occurring in between more or less equal-sized crystals are referred to as intercrystalline pores and are typically associated with dolomitization (Flügel, 2004). Intercrystalline pores are encountered in most dolomite samples in the Bodón Unit and their size ranges from a few µm to > 100 µm (figures 3.22F & G). The intercrystalline pores can be partly or completely occluded with insoluble residue or calcite cement, but can also be enlarged by dissolution (figure 3.22H).

3.5.4. Biomoldic pores

Biomoldic pores are created by selective dissolution of fossils or bioclasts. Many large and irregular pores in the dolomitized microbial boundstones of the Valdeteja Fm. of KO 1 resemble biomolds of which the origin is difficult to determine (figure 3.22I). They do not represent stromatactis-like cavities which are typically associated with microbial boundstones (see section 3.3.1.2; Bahamonde et al., 2014), as cavities of this kind were fully cemented prior to Late-Variscan hydrothermal dolomitization (e.g. figure 3.6C). Rare examples of sponge (figure 3.22J) and crinoid molds (figure 3.22K) have been observed, but are limited contributors to overall petrophysical properties. 3.5.5. Crystal-moldic pores

Crystal-moldic pores are formed by selective leaching of crystals. In the Bodón Unit, numerous void- filling dolomite crystals have been affected by leaching, resulting in intracrystalline dissolution and the creation of crystal-moldic pores. Dissolution often affects the last (sometimes Fe-rich, see section 3.4.1.2) growth phase in dolomite crystals (figure 3.22L), and is regularly associated with dedolomitization and oxidation of Fe-rich components. A distinct type of crystal-moldic porosity is encountered in dolomitized boundstones in KO 1 and KO 2. Here, molds are created by dissolution of elongated quartz crystals (figure 3.22M), which likely represent recrystallized sponge spicules (see sections 3.3.1 and 3.3.2).

Figure 3.22 (previous page): Overview of dolomite pore types observed during fieldwork and petrographic observations. (A) Vuggy pores in white dolomitized microbial boundstone exposed along the Millaró road section in KO 1. Pencil for scale

(14 cm). (B) Vuggy pores on a weathered surface of Fe-rich dolomitized fault gouge in KO 3. Hammer for scale (32 cm). (C) PP photomicrograph of small vuggy pores (filled by yellow resin) in a dolomitized carbonate mound in KO 1 (sample SL16RH028). (D) Zebra textures and associated zebra pores in the Barcaliente Fm. exposed in the LCQ in KO 1. (E) Zebra textures and associated zebra pores in dolomitized microbial boundstones in KO 1. (F) PP photomicrograph of intercrystalline pores in dolomitized mudstones in KO 1 (sample SL16RH063). (G) FL photomicrograph of intercrystalline pores in a dolomitized mudstone. Sample SL16RH011 from the Barcaliente Fm. in KO 1. (H) PP photomicrograph showing intercrystalline porosity in dolomite from KO 2. The pores are partly occluded with calcite cement (left; red stained) and partly dissolution-enlarged (right; filled by yellow resin). (I) Large and irregular pores in pink dolomitized microbial boundstone exposed along the Millaró road section of KO 1. Many of these pores resemble biomolds, although the exact origin of the molds cannot be determined. (J) Molds resembling a dissolved sponge in microbial boundstone exposed along the Millaró road section of KO 1. (K) Molds of crinoid stems (white arrows) in dolomitized grainstone exposed along the Millaró road section in KO 1. (L) PP photomicrograph of small crystal-moldic pores (white arrows) which formed after dissolution of a specific cement phase in a void-filling dolomite crystal. Sample SL16RH065 from KO 1. (M) PP photomicrograph of crystal-moldic pores in dolomitized microbial boundstone in KO 2. Large parts of the original quartz crystals are preserved. (N) Fracture sets on an exposed bedding plane of dolomitized Barcaliente limestones in KO 3. These fractures are generally filled with calcite. Hammer for scale (32 cm). (O) Example of well-bedded subvertical dolomitized Barcaliente limestones exposed along the Millaró road section, with superimposed subhorizontal fractures. Note the thin dark lines parallel to the bedding of the beige dolomite rocks. Width of view is approximately 1.5 m.

3.5.6. Fractures

All pore types introduced above contribute to the matrix porosity of the dolomite rocks exposed in the Bodón Unit. Hydrothermal dolomite rocks are often affected by fracturing, given their brittle nature and typical fault-controlled settings (see chapter 1). Log data of KO 1 confirm a larger abundance of fractures in dolomite compared to precursor limestone rocks (see Appendix II figure A). Open fractures can strongly increase matrix porosity and many prolific hydrocarbon reservoirs in dolomite rocks are due to this association of matrix porosity and open fractures (Purser et al., 1994). The dolomite bodies exposed in the three key outcrops are crosscut by several fracture sets (figures 3.17H; 3.22N & O), which are discussed in more detail in section 3.7. Fractures are generally filled with calcite, or open and affected by dissolution resulting in channels.

3.5.7. Caverns and channels

Cavern and channel pores are large dissolutional vugs and elongated pores respectively, and are both typically associated with dissolution and karstification processes (Choquette and Pray, 1970). Caverns and channels are omnipresent in the Carboniferous platform limestones of the Bodón Unit, and contribute to spectacular cave systems (see section 3.4.2.3). Elongated channel pores are omnipresent in the limestone and dolomite rocks exposed in the Bodón Unit, and are caused by dissolution along stylolites (figure 3.19B) or fractures.