Dolomite in KO 2

The dark gray HTD geobodies exposed in KO 2 are narrow subvertical bodies of a few meters wide cutting through the light gray limestones of the Valdorria platform (Valdeteja Fm.; figures 3.30A & B). Some of the larger geobodies in KO 2 are stratabound and follow depositional contacts (figure 3.9A). The geobodies studied in this section are located at the eastern side of the Río Curueño valley (figure 3.8), in prograding slope deposits (peloidal micrite-rich boundstones) of growth phase 2 of the Valdorria platform (figure 3.9B; Chesnel et al., 2015). The geometry of the dolomite geobodies is controlled by subvertical fractures with a general orientation of 270/90 (figure 3.31A), similar to many important valleys and gorges cutting through the entire Valdorria carbonate platform (figure 3.9B). An additional fracture set, with subhorizontal orientations of 250/00, occurs throughout KO 2 (figure 3.31A).

3.7.2.2. Petrography

The exposed contact between dolomite and limestone rocks is very abrupt and sharp (figure 3.30B), and is locally bounded by subvertical fractures (270/90) and Variscan calcite veins on a cm- to dm-scale. Dolomite veins, with similar subvertical orientations, are observed in limestones bordering the dolomite geobodies (figure 3.10A). The crystal size of the dolomite samples is variable, ranging from unimodal fine-crystalline to coarse-crystalline. Coarse dolomite crystals show rare signs of a second dolomite phase overgrowing the first one. The dolomite samples are intensely fractured and brecciated, and cemented with hydrothermal calcite (figure 3.30C). Fracturing, brecciation and calcite cementation preferentially affected specific zones with similar subvertical orientations (270/90; e.g. figure 3.16A). The calcite cement partly dedolomitized dolomite clasts (figure 3.30D) and shows bright orange luminescence (figure 3.30E) which corresponds to the phase of hydrothermal calcite cementation following dolomitization (Gasparrini et al., 2006a). The calcite phase cementing fractures and breccias is locally associated with quartz. Calcite veins in the nearby precursor limestone rocks crosscut dolomite veins and are bounded by rims of quartz crystals sticking out of the limestone surface due to differential

Figure 3.30: Geometry and petrography of dolomite geobodies exposed in KO 2. (A) Eastern valley side of KO 2 showing

subvertical dolomite geobodies in dark gray cutting through light gray limestones. Photograph from Gasparrini et al. (2006a). (B) Exposed dolomite geobody approximately 1.5 m in width. Fieldbook for scale (22 cm). (C) Stained dolomite sample brecciated and cemented with calcite (red stained). (D) PP photomicrograph of brecciated dolomite sample cemented by calcite (red stained). Dolomite clasts have been partly dedolomitized and are delineated with black dashed lines. (E) CL photomicrograph of brecciated dolomite sample. Dolomite clasts (black arrow) float in a matrix of hydrothermal calcite (white arrows). A second fracturing event followed by meteoric calcite cementation affected the sample. (F) Vein of hydrothermal calcite crosscutting limestone and bounded by quartz crystals.

Figure 3.32: Overview of petrophysical data for limestone and dolomite plugs of the Valdeteja Fm. in KO 2. (A) Boxplots

showing the He porosity data of limestone and dolomite plugs. (B) Boxplots showing the gas permeability data of limestone and dolomite plugs. (C) Reconstructed and labeled pore network of a large core of dolomite (SL14RH122) obtained through medical CT scanning. Note preferential dissolution along a small fracture (black arrow). (D) Reconstructed and labeled pore network of a large core of dolomite (SL15JP016) obtained through medical CT scanning.

Figure 3.31: Stereographic projection of structural data. (A) Orientations of bedding planes and fractures measured in KO 2. (B) Orientations of bedding planes and fractures measured in KO 3.

weathering (figure 3.30F). The orientation of these calcite veins corresponds to both subvertical and subhorizontal fracture sets (figure 3.31A). Additional fracturing and cementation with meteoric calcite (figure 3.30E; cfr. Gasparrini et al., 2006a) further affected the brecciated dolomite bodies.

3.7.2.3. Matrix porosity and permeability

WS and He porosity measurements, as well as gas permeability measurements, were performed on 4 limestone plugs and 6 dolomite plugs from the Valdeteja Fm. in KO 2 (see Appendix III). Medical CT scanning was performed on 5 large cores.

The matrix porosity of the undolomitized peloidal micrite-rich boundstones ranges between 0.2 and 0.6 %, with an average porosity of 0.4 % (He porosimetry; figure 3.32A). The only visible pores in the limestone samples are caused by local dissolution along small fractures or stylolites. Matrix porosity values obtained for dolomite samples are significantly higher compared to the precursor limestone rocks (confirmed with ANOVA), and average around 1.8 % (1.1 – 2.8 % range; He porosimetry; figure 3.32A). These higher porosities are related to dissolution of dolomite crystals and calcite cement. Dissolution is apparent in all dolomite samples of KO 2 and creates dissolution-enlarged intercrystalline pores (figure 3.22H), crystal-moldic pores (figure 3.22M) or channel pores (figures 3.19B & 3.32C). The connectivity between these pore types is usually small, resulting in badly connected pore networks (figures 3.32C & D). The permeability of the dolomite samples is 0.025 mD on average, compared to 0.005 mD for the precursor limestone rocks (figure 3.32B). ANOVA results indicate that this difference is not significant. The skewing towards higher permeabilities in the limestone samples (figure 3.32B) is cause by a plug containing a dissolution- enlarged fracture.

3.7.2.4. Dolomitization model

The limestones of the Valdorria platform have been affected by dolomitization mostly along subvertical fractures predominantly N-S oriented and cutting perpendicular through the platform limestones. The pathways followed by circulating fluids were likely similar compared to KO 1 (i.e. fracture zones), but the available volume of dolomitizing fluids was less important in KO 2, resulting in relatively small fracture-controlled dolomite geobodies. Dolomitization, and associated hydrothermal calcite cementation, are likely caused by limited fluid expulsion related to separate episodes of fracture reactivation. This is indicated by the occurrence of dolomite- and calcite-cemented veins surrounding the fracture zones, which have similar subvertical orientations and were thus active fluid conduits at the moment of dolomitization. The coarse- crystalline dolomite samples showing overgrowths might have been recrystallized by a later dolomitization phase (see section 3.4.2.2). The fracture zones were shortly reactivated during their Post-Variscan history, and cemented by meteoric calcite (figure 3.30E). The timing of this reactivation is hard to constrain. Reactivation might have occurred in response to Alpine uplift, or in response to more recent tectonic activity. Current exposure of the dolomite geobodies results in dissolution along fractures and stylolites. Late- to Post-Variscan dolomitization along the fractures likely created vuggy porosity, which became occluded following limited dolomite overgrowth, brecciation and calcite cementation. The current pores are mainly the result of dissolution along fractures, which are more abundant in dolomite compared to the precursor limestone rocks.

3.7.3. Dolomite in KO 3