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Facies heterogeneity of a Kimmeridgian carbonate ramp (Jabaloyas, eastern Spain): a combined outcrop and 3D geomodelling analysis/ Heterogeneidad de facies en una rampa carbonatada del Kimmeridgiense (Jabaloyas, Este de Espana): analisis de campo y geomodelizacion tridimensional.

1. Introduction

Carbonate reservoirs from the Middle East are the most important hydrocarbon sources worldwide (BP Statistical Review, 2011). Among these hydrocarbon fields, some are reaching a mature stage where internal heterogeneity needs to be understood in order to optimize ultimate recovery (Enhance Oil Recovery -EOR- and Improved Oil Recovery-IOR-phases). The Upper Jurassic Arab D reservoirs are one example of these heterogeneous carbonate reservoirs. This work is focussed on the Upper Kimmeridgian carbonate strata which are well exposed in outcrops around the Jabaloyas village (Northeast Spain; Fig. 1). The Jabaloyas deposits share similarities with the Arab D carbonates in terms of age, lithofacies and depositional environments and could represent a valuable outcrop analogue.

The main purpose of this work is the characterization of the vertical and lateral facies distribution of the Upper Kimmeridgian strata in Jabaloyas outcrop with the main target of constraining the spatial distribution of carbonate lithofacies types and the sedimentary heterogeneity of the Arab D reservoirs. The use of outcrops has become one of the most important tools for documenting the heterogeneities at various scales (Jahn et al., 2003; Jennings, 2000) thanks to the continuous exposures in both vertical and horizontal directions. The examination of outcrop analogues provides a better understanding of the fine scale reservoir heterogeneity, as required for EOR of many mature oil fields in Arab D carbonates.

The Arab D deposits were accumulated in the southern realm of the Tethys. This unit mostly consists of well-sorted oolitic packstone-grainstone forming active shoals and patch reefs mainly composed by stromatoporoids in the foreshoal environment (Grostch et al., 2003). As the studied analogue of Jabaloyas, the Arab D deposits occurred in the shallow domains of an Upper Kimmeridgian carbonate ramp (Ayoub and En Nadi, 2000; Al-Saad and Ibrahim, 2005). However, a significant difference with the studied strata around Jabaloyas is the presence of interbedded evaporitic deposits, observed to be episodically deposited in the supratidal environments of the Arab D reservoir. The quality of this reservoir is due to the interparticle porosity in the peloidal and oolitic grainstones and the vuggy porosity resulting from the dissolution of stromatoporoids bioclasts. Consequently, it is crucial to reproduce the distribution of shoals and reef bodies in the 3D reservoir model if realistic simulation of the flow units is meant to be achieved in this heterogeneous reservoir (Lehman et al., 2008).

The study of the facies distribution in the Jabaloyas outcrops has been primarily based on a detailed sedimentological analysis including a thorough characterization of facies, depositional architecture and sedimentary environments of the Kimmeridgian carbonate ramp. These data have been used to construct a 3D geological model, which has allowed further understanding and quantification of the field information. These well documented 3D models have contributed to better understand and quantify facies distribution in a subsurface Arab D reservoir and therefore to reduce the geological uncertainties associated with facies heterogeneities in a similar carbonate ramp reservoir.

2. Geological setting

Extensive carbonate platforms covered the eastern part of the Iberian Plate during Late Jurassic times. These carbonate ramps were open to the Tethys towards the SE-ESE (Fig. 2). The studied Kimmeridgian unit corresponds to the Torrecilla Formation, which represents the shallow facies belts of the Iberian low angle carbonate ramp. This unit includes the local development of coral-microbial reef buildups which coexisted with sediments showing a large variety of non-skeletal components such as ooids, peloids, intraclasts and oncoids. This shallow facies belt progressively graded eastwards to the slightly deeper part of the carbonate ramp where lime mudstones and marls accumulated (e.g., Badenas and Aurell, 2001; Aurell and Badenas, 2004).

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The Torrecilla Formation forms part of a third-order sequence (i.e., Kim 2 Sequence), which was mostly developed during the Late Kimmeridgian (e.g., Badenas and Aurell, 2001; Aurell et al, 2003; see Fig. 2). This age assignment is based on the scarce ammonite and microfossil content (i.e., benthic foraminifera, calcareous algae; e.g., Fezer, 1988; Nose, 1995; Badenas and Aurell, 2001). Around Jabaloyas, the Upper Kimmeridgian Torrecilla Formation consists of five high-frequency sequences bounded by regional discontinuities, which were tentatively related to sea level fluctuations controlled by orbital cycles (high-frequency Sequences Ato E in Badenas and Aurell, 2010). The present work concentrates on the high-frequency Sequence C, which shows a variable thickness across the study area from 12 to 20 m. This sequence includes the largest development of coral-microbial reef buildups found around the Jabaloyas area.

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3. Methodology

The outcrops of the Torrecilla Formation studied around the Jabaloyas village are squared in the left part of figure 1. Fieldwork included the logging of 17 detailed sedimentological profiles, the geological mapping of the facies using photomosaics of the intermediate areas, and the georeferencing and detailed documentation of 274 reefbuildups along the studied outcrop (Fig. 3). All data were georeferenced over the Digital Elevation Model (DEM, 1:5000) in combination with the aerial photos (50 cm pixel resolution) from the Geographic Information System webpage of the Aragon Government.

Around 160 samples were collected to complete the facies description under binocular and petrographic microscope. Petrographic analysis allowed determining the semiquantitative proportion of skeletal and non-skeletal components (visual estimation with petrographical templates) as well as texture following Dunham (1962) and Embry and Klovan (1971) classifications. For the description of coated grains, the proposed nomenclature for oncoids (Dahanayake, 1977, 1978) and ooids (Strasser, 1986) was adopted. Cathodoluminescence tests were carried out over some thin sections. The results showed low luminescence in all the studied thin sections.

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A number of facies types were differentiated and interpreted, in order to reconstruct the different sedimentary environments across the studied carbonate ramp. Lateral and vertical facies trends are shown by different cross-sections. Sequence analysis was performed on the recognition of stacking patterns in the facies successions. Sharp facies changes associated with prominent surfaces have been considered as time-lines at outcrop-scale, and have been used to subdivide the studied high-frequency C into four sedimentary episodes.

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This set of stratigraphic and sedimentological data forms the basis for the reconstruction of the 3D outcrop model. The vertical and lateral facies distribution reconstructed from fieldwork was modelled by using Petrel 2010.2 E&P Software Platform[R] (Schlumberger Limited, 2011). Two 3D models were built. The complete modelling of the study area around Jabaloyas or Full-field Model had a grid increment of 20 x 20 m. This grid increment is suitable to show the overall distribution of the different inter- and post-reef facies but does not give enough resolution to include the coral-microbial reefs, because the cell size is bigger than most of the documented buildups. The cemented bed found at the top of the studied interval, which is a prominent cemented surface, traceable across the study area, was used for mapping, georeferencing and reconstructing in 3D the Sequence C along the gullies. A more specific area was modelled (i.e., the Sector Model) in order to define precisely the relationship between the coral-microbial reefs and the inter-reef facies.

4. Facies types

Nine facies types have been distinguished based on main components, texture and sedimentary structures. They have been grouped in three facies associations: coral-microbial reef facies, inter-reef facies, and post reef facies. Inter-reef facies 1 to 5 and post-reef facies 6 to 8 are numbered according to the distal-proximal relationship (i.e., from the relatively distal facies 1 to the proximal lagoonal facies 7 and 8). The description and interpretation of each facies type is provided below. Figure 4 illustrates two examples of facies distribution from field photomosaics.

4.1. Coral-microbial reef facies

The reef fabric basically consists of boundstones with variable proportion of colonial organisms (up to 90% of the components), microbial crusts and associated encrusting organisms (10-80%), and cavities filled by internal sediment (Fig. 5a,b). The observed proportion of colonial organisms and microbial crusts allowed to define two types of fabrics (Aurell and Badenas, 2004) according to the classification of Leinfelder (1993): coral-microbial reefs (colonial organisms forming the largest proportion of the fabric); and coral-bearing thrombolites (colonial organisms and microbial crust found in similar proportion). The most common coral genera are Thamnasteria, Cosmoseris and Microsolena. Calamophylliopsis, Stylina excelsa, Ovalastrea delgadoi, Milleporidium formosum, Axosmilia infundibuliformis, Ovalastrea delgadoi occur in lower proportion (Fezer, 1988; Nose, 1995). Stromatoporoids, chaetetids, sponges, Solenopora sp., bivalves and echinoderms are also present. The microbial crust shows dense micritic and micro-peloidal fabric, and associated encrusting organisms (serpulids, bryozoans, Koskinobullina, Lithocodium, Tubiphytes, Bacinella and solenoporacean algae). Most of the internal cavities were early filled by either mud- or grain-supported sediment derived from inter-reef sediments described below. Geopetal fillings are also common.

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The coral-microbial reefs form isolated buildups that have vertical to sub-vertical walls, with variable size across the studied area, between 4-19 m in thickness and 3-43 m in width. Some trends have been found from proximal (N) to distal (S-SE) localities. Most of them rely in the 1:1 line if the relationship between their height and width is plotted (Fig. 6). However, they can be amalgamated, forming "ribbons" up to 50 m long in more proximal domains (i.e., close to BD1 section, see Fig. 3), while they form pinnacle cylindrical buildups in distal areas where they tend to grow more vertically than laterally. The average linear distance between buildups is 50 m. Nevertheless, as a general rule, buildup density increases progressively towards proximal localities.

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The buildups may locally occur as metre-decimetre scale laterally continuous patches (i.e., biostromes), dominated by branching corals in growth position and large stromatoporoids (i.e., decimetre-size). This particular type of coral-stromatoporoid reef has been found in proximal areas with some degree of temperature and salinity fluctuations (C3 episode, see below). The increasing proportion of stromatoporoids towards the proximal domains of the ramp (compared to corals) agrees with previous observations and models for the NW Tethyan realm (e.g., Leinfelder et al, 2005; Aurell et al, 2011).

4.2. Facies 1: Burrowed bioclastic mudstone-wackestone

The facies is formed by tabular to irregular (nodular) limestone beds (up to 0.4 m thick), interbedded with centimetre- to decimetre-thick marls (Fig. 8a). Cemented beds richer in large-size bioclasts are also found forming "aprons" around the reefs (coarser textures surrounding the buildups, i.e., rudstone and floatstone). The dominant bioclasts are bivalves (including ostreids), echinoderms (up to centimetre-size) and corals. Less common are gastropods, solenoporacean algae, chaetetids and foraminifera (lituolids). Irregular and poorly rounded peloids and intraclasts mostly corresponding to small-size debris of microbial crusts may occasionally represent up to 70% of the components.

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The high proportion of lime mud and clay (i.e., marls) indicates the low-energy more distal environment of the studied ramp sector. The thickness of this facies varies from 3 m in proximal areas to 14 m in distal ones, and it indicates that deeper depositional setting occurred towards the SE.

4.3. Facies 2: Intraclastic-bioclastic packstone-grainstone

The facies consists of tabular to irregular beds (up to 0.4 m thick) with frequent hummocky cross-stratification and cross-bedding (Fig. 8c,d). Burrowed bedding surfaces are frequently found. Reef-derived intraclasts dominate (up to 90% of the components). They are up to 5 mm in diameter and mostly correspond to fragments of microbial crusts and reworked micrite (Fig. 5e,f). Peloids are also common and may form in some cases up to 50% of the components. The bioclasts (up to 70% of the components) are bivalves, foraminifera (lituolids), echinoderms and serpulids. Less frequent are corals, gastropods, solenoporacean, Cayeuxia and miliolids. Type I oncoids are present in low proportion.

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The grain-supported textures point to a more energetic and proximal environment compared to muddy facies 1. Hummocky cross-stratification indicates storm-induced flows probably below the fair-weather base level.

4.4. Facies 3: Peloidal grainstone-packstone

The facies is characterized by tabular to irregular beds (up to 0.5 m thick) with frequent cross-lamination and local hummocky and planar cross-bedding. Thalassinoides traces are usually found (Fig. 8b) and Planolites traces are also common. The peloids usually form up to 90% of the components (Fig. 5c,d). They are ovoid and irregular in shape, and up to 0.2 mm in mean diameter (up to 0.4 mm occasionally). Most of them correspond to small size-irregular, well-sorted and poorly rounded lithic peloids (according to Flugel, 2004), with similar origin to the boundstone-derived intraclasts described in facies 2 (Fig. 5d). The bioclasts (up to 20% of the components) consists of bivalves, echinoderms and foraminifera (lituolids, miliolids and textularids). Less common are gastropods and corals. Type 1 ooids may be locally present (up to 20% of the components). They are ovoid and spherical in shape and up to 0.7 mm in diameter.

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The cross-laminated, well-sorted fine-grained facies would correspond to isolated bars migrating in the mid-ramp area. The common presence of burrowed intervals indicates frequent sedimentary interruptions in mid-ramp domains.

4.5. Facies 4: Bioclastic-oolitic packstone-grainstone

This facies is arranged in tabular to irregular beds (up to 0.3 m thick) with local planar cross-bedding and channelized bases. Skeletal grains (up to 70% of the components) mainly correspond to bivalves, corals, solenoporacean and chaetetids. Less common are echinoderms, foraminifera (lituolids), gastropods and dasycladacean algae. Type 1 ooids (up to 1 mm in diameter) may form up to 60% of the components. Frequently they are superficial ooids, with spherical to ovoidal shapes. Type I oncoids, peloids and intraclasts are occasional.

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This facies corresponds to higher energy environments compared to the previously described facies 2 and 3. Micritic ooids would be formed in the shoal environment, under the continuous agitation of the sea floor (Strasser, 1986). The existence of fragments of corals and stromatoporoids floating in the oolitic sediment indicates the proximity of coral reefs to the shoal environment.

4.6. Facies 5: Oolitic packstone-grainstone with Marinella

Facies 5 is arranged in irregular beds (up to 0.4 m thick) with local planar cross-bedding. The facies is characterized by the presence of fragments of the red algae Marinella lugeoni and spherical to ovoid type 3 and type 4 ooids (Fig. 7a). The nuclei of these ooids are miliolids, lituolids, algae, echinoderms and quartz grains. Besides the red algae Marinella, the main skeletal components are gastropods and bivalves. Foraminifera (miliolids, textularids and lituolids), Cayeuxia, echinoderms and solenoporacean are less abundant. Peloids and aggregates are also occasionally found. Type II oncoids and intraclasts (fragments of peloidal and burrowed bioclastic inter-reef facies) are also present.

Similar facies showing Marinella bioclasts associated with oolites were described in the Portuguese Upper Jurassic (Leinfelder et al., 1996). These were formed in high-energy settings, where the algae Marinella had the ability of regenerate from broken fragments. Marinella was able to colonize environments that were not previously occupied by other encrusting organisms such as microbial forms, Bacinella or Lithocodium. The facies can be divided into two textural groups, according to the matrix proportion, indicating a progressive energy increase from relatively distal wackestone-packstone to proximal packstone-grainstone textures.

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4.7. Facies 6: Burrowed peloidal-oolitic wackestone-packstone

The facies is arranged in tabular beds up to 1.3 m thick. The main components are ovoid and irregular peloids (up to 70% of the components) up to 0.4 mm in diameter (Fig. 7e,f). Ovoid and spherical type 3 and 4 ooids up to 1 mm in diameter form up to 40% of the components. The main bioclasts (up to 20%) are foraminifera (miliolids, Nautiloculina oolithica, lituolids, including Alveosepta jaccardi) and gastropods. Bivalves, other benthic foraminifera (textularids) and Marinella lugeoni are occasionally found. Type II oncoids are present in lower proportion. The presence of muddy textures and peloids reflect low-energy depositional environments probably located in the back side of the active shoal.

In contrast with the previously described facies 1 to 5, facies 6 was deposited after the coral-microbial reef buildups growth (i.e., post-reef facies). The post-reefal character of the facies is indicated by the absence of bioclasts of colonial organisms and by its onlapping geometry over the reefs (Fig. 4 and 8f). Hence, the decrease of the energy can be explained by the "barrier" formed by the oolitic-bioclastic shoals (in combination with the C2 reef palaeo-relief; see section 5). The presence of Alveosepta jaccardi (Lituoloidea superfamily, Fig. 7f) highlights the deposition of this facies in the shallow inner ramp setting (Flugel, 2004).

4.8. Facies 7: Oolitic-peloidal packstone-grainstone

The facies is arranged in irregular beds up to 0.6 m thick with planar cross-lamination (Fig. 7c). It shows up to 50% of ovoidal, sub-spherical and spherical fibrous-radial type 3 and type 4 ooids, up to 1 mm in diameter (Fig. 7d). Peloids (up to 45% of the components) are irregular, ovoid and spherical and no larger than 0.4 mm in diameter. Bioclasts are dominated by bivalves and foraminifera (miliolids, lituolids, Lenticulina, textularids). Gastropods, echinoderms, Marinella lugeoni and Cayeuxia occasionally occur. Type II oncoids, aggregate grains, compound ooids and quartz grains are occasionally found.

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The existence of type 4 ooids and planar cross-lamination and the abundance of bivalves and foraminifera indicate deposition during episodic high-energy events in a non-restricted interior lagoon.

4.9. Facies 8: Oolitic grainstone

Facies 8 consists of tabular beds (up to 0.5 m thick) with frequent planar cross-bedding (Fig. 8e). The ooids (95% of the components) are spherical, up to 1.5 mm in diameter (Fig. 7b). Type 3 and type 4 ooids are found in larger proportion than high-energy type 1 ooids. Gastropods and foraminifera (miliolids and lituolids) are often found, most of them surrounded by incipient oolitic coatings. The bioclasts are mainly bivalves. Quartz grains are observed in low proportion, forming the nuclei of the ooids.

Facies 8 represents an oolitic sand bar formed in very shallow domains (i.e., the shoreface) which occurred parallel to the shoreline. Palaeocurrent measurements indicate a longshore driven current (i.e., SW-NE).

5. Facies distribution and sedimentary model

Extensive fieldwork and facies analysis, including lateral tracing of some key surfaces, analysis from photomosaics and detailed logging, resulted in a precise understanding of the lateral and vertical distribution of coral reefs and associated inter- and post-reef facies that have been described above. The information is summarized in figure 9, showing the correlation between the 17 logged profiles. The northern and southern correlation panels show the facies distribution from proximal to distal ramp areas, whereas the eastern correlation panel would be almost parallel to the defined facies belts. These panels also show the progressive thickness increase of the studied high-frequency Sequence C from proximal to distal localities, reaching up to 20 m in more distal stratigraphic profiles (BB). The studied Sequence C is bounded by planar and well-cemented discontinuity surfaces that can be traced at regional scale (see Aurell and Badenas, 2004; and Badenas and Aurell, 2010, for details).

The sequence displays a deepening-shallowing upward trend, and was described by Aurell and Badenas (2004) as a catch-down type sequence (nomenclature according to Hillgartner and Strasser, 2003). Four sedimentary episodes have been established within Sequence C, named C1, C2, C3 and C4 episodes. They are bounded by discontinuity surfaces (i.e., sharp burrowed and ferruginous bedding planes), linked to abrupt vertical facies changes (see Fig. 9).

The performed sedimentological analysis allows a precise reconstruction of the different facies belts in the low angle carbonate ramp where the facies of the studied Upper Kimmeridgian high-frequence sequence were deposited. There are no clear evidences pointing to the deposition in intertidal environments. All the described facies have been interpreted as deposited in subtidal environments, in an almost pure carbonate system. The siliciclastic input in Sequence C is very low and sand-size quartz grains are mostly found as nuclei of the different types of ooids, mostly in episodes C3 and C4. Occasional quartz concentrations in distal facies were possibly caused by storm-induced density flows.

As it is indicated by the reconstructed lateral and vertical facies distribution (see Fig. 9), there is a progressive transition from proximal facies 6 to 8 deposited in the protected interior ramp areas (i.e., back barrier lagoon) to relatively open marine domains (i.e., proximal middle ramp) represented by the mud-dominated facies 1. The observed relationship between the coral-microbial buildups and the coeval inter-reef facies 1 to 4 shows that the development of the buildups occurred mostly in the midramp domain. Some smaller size patches or biostromes have been observed during the episode C3, time-equivalent of the Marinella facies 5.

The reconstructed sedimentary model illustrated in figure 10 shows the distribution of coral reef facies and inter- and post-reef facies 1 to 8. This is an idealized model, because any of the four episodes C1 to C4 includes all the facies: episodes C1 and C2 encompass facies 1 to 4; episode C3 is formed by facies 1 and 5, and episode C4 only includes the more proximal facies 6 to 8. The relative proportion of the key components found in the different facies types is indicated in figure 11, showing the down-dip gradation of the different non-skeletal grains. The extent of the coral-microbial reefs across the ramp during episodes C1 and C2 is also shown.

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6. Sedimentary evolution

The four episodes C1-C4 document the sedimentary evolution of the Sequence C (see in figure 12 successive facies distribution maps for each episode). Episode C1 represents the Transgressive Deposits (TD) of the studied Sequence C, as indicated by the retrogradation of the low-energy distal facies 1 until the maximum flooding surface (mfs, green dashed line in Fig. 9). This mfs is a borrowed and ferruginized surface, which coincides in proximal areas with a sharp vertical facies change from grain-supported to mud-supported facies (S2 surface in Aurell and Badenas, 2004; see S2 in Fig. 9). Additionally, this surface correlates with a condensed surface within the buildups, which is followed by a thin level where microbial crusts predominate over the colonial organisms.

The episode C2 is interpreted as the early Highstand Deposits (HD) and is dominated by the sedimentation of the low-energy mud-supported facies 1 through the most part of the study area. The deposition of low-energy facies occurred in response to the relative high sea level. A sharp surface associated with a sudden facies change that can be traced across most of the study area separates the C2 and C3 episodes (see Fig. 9).

Regarding the main components of the lower episodes Cl and C2, type 1 ooids dominated in more proximal areas, passing gradually basinwards into peloidal-intraclastic facies (Fig. 11). The reef-derived debris (peloids and intraclasts) and skeletal grains (colonial organisms, solenoporacean algae) are widespread over all the studied domains, coeval to the growing of coral-microbial reefs. The reefs grew from high-energy to low-energy midramp areas, mostly during the episodes of rapid gain of accommodation. The presence of bivalves, echinoderms and serpulids tends to increase towards relatively deep waters. Benthic foraminifera locally appear close to the active shoal environment probably as resedimented particles.

The upper two episodes C3 and C4 correspond to the late HD of the Sequence C, defined by a rapid facies progradation (Fig. 9). Episode C3 is characterized by the progressive onlap of facies 5 (which was filled by Marinellaoolitic facies) over the top of the reef pinnacles, involving the eventual filling of the irregular topography left by the reef palaeohighs at the sea floor. In addition, facies 5 does not include significant amount of reef debris, indicating that the coral-microbial reefs were not productive when facies 5 was deposited. The proximity of inner ramp domains is outlined by the abundance of sparitic ooids and benthic foraminifera. However, the Marinella-rich facies were deposited in open environments, as indicated by the gradual change to the offshore mud-supported facies 1. It is remarkable that there was also a direction change of the facies belts from episodes C1 and C2 to episode C3 (from NE-SW to N-S; Fig. 12).

The uppermost episode C4 is characterized by the widespread deposition in backshoal environment, that is the widespread progradation of the inner ramp facies 6 to 8. Facies 6 represents relatively low-energy environments behind the active shoal with high proportion of lime mud. These conditions allowed the formation of larger type 4 ooids, dominating over the other types in these backshoal areas. The characteristic massive bioturbated beds of facies 6 also contain high amount of benthic foraminifera and the proportion of bioclasts from Marinella algae has been considerably decreased. Resedimented ooids from the active shoals are also present in this facies. Peloids could be formed by micritization processes in these low agitation environments. Gastropods and miliolid foraminifers are common in this low-energy, back barrier depositional setting. The presence of the ooid-rich facies 7 and 8 found at the top of the sequence marks the last episode with the progradation of the ooid sand bar. The sand bars represented by facies 7 and 8 are almost completely composed by type 3 and type 4 ooids.

7. 3D Modelling

Two 3D models of the vertical and lateral facies distribution around the Jabaloyas area have been built using Petrel software: the complete modelling of the study area around Jabaloyas or Full-field Model and a more specific area close to the BH stratigraphic profile (i.e., the Sector Model), in order to draw in detail the relationship between the coral-microbial reefs and the inter-reef facies.

7.1. Full-field Model

Studying the distribution and the geometries of the main sedimentological bodies within the Sequence C has provided an adequate amount of information to create a 3D Full-field Model, with a size of 12 [km.sup.2] and 2 x [10.sup.8] [m.sup.3]. Exported pictures from the model have been added in figure 13. The selected grid increment for this model was 20 x 20 m.

The cemented bed found at the top of the studied interval was used as reference surface in the 3D modelling. The key-bounding surfaces between the episodes defined in Sequence C were also incorporated in the 3D model. These surfaces were adjusted to some minor fault planes which were not incorporated as 3D objects. The Facies modelling was carried out with the Truncated Gaussian algorithm with trends. The facies maps reconstructed from the field data were adjusted layer by layer during the performed simulation. The resulted facies belts that have been reconstructed from field observations are approximately 1-3 km width (Fig. 12).

The exported pictures of the Full-field Model in figure 14 and the cross-sections in figure 15 show the adjustments of the modelled facies distribution to the facies maps and correlation panels respectively, which were interpreted directly from the stratigraphic profiles. Regarding the E-W cross-section (Fig. 15, above), it would correspond to the northern section where it encompasses the most important lateral facies change from proximal to distal areas (see Fig. 9). The N-S cross-section reproduced in figure 15 (below) was adjusted to the eastern section (see Fig. 9), where the muddy facies 1 dominates.

The relative proportion of each facies for each episode and the total proportion for the whole Sequence C is given in figure 16. Facies 1 represents the major sedimentary body with a total volume of 8.55 x [10.sup.7] [m.sup.3] (few cubic metres in C1 and C3 but 6.32 x [10.sup.7] [m.sup.3] in C2). Facies 4, located only in proximal areas, represents a body of 6.31 x [10.sup.6] [m.sup.3]. The other facies form sedimentary bodies between 1-3 x [10.sup.7] [m.sup.3], excepting the ooid sand bar (facies 8) that only has 2.51 x [10.sup.6] [m.sup.3]. Similar workflow with petrophysical properties could be also interesting for simulation purposes by assigning mainly porosity and permeability data into the different facies types.

7.2. Sector Model

The Sector Model was necessary to incorporate the reef bodies in the 3D model. The selected area represents a 0.12 [km.sup.2] surface on a modern structural platform where the Sequence C is weathered at a certain height. It entails the visualization of the real geometry of the individual boundstone buildups in the aerial picture (Fig. 17). The Object Modelling was adjusted to these spots. The selected Sector Model grid increment was 0.5 x 0.5 m and its 3D building was created based on the same limiting surfaces from the Full-field Model. The total volume of the sector model is 1 x [10.sup.7] [m.sup.3].

The coral-microbial reefs of the area selected for the Sector Model are located between the BH and BS2 logs and correspond to the relatively distal ramp domains (see Fig. 9 and 12). The buildups have a relatively low spatial density, forming isolated pinnacle bodies up to 19 m thick. Reef buildups have the spherical and elliptical geometries that could be seen from aerial view (see "pinnacle projection in surface" in the upper part of Fig. 17).

8. Conclusion and perspectives

The 3D reconstruction of the facies heterogeneities within a high-frequency sequence (Sequence C, 12 to 20 m thick) has been carried out. This sequence encompasses the inner to middle facies of the Late Kimmeridgian carbonate ramp. Facies analysis allowed the definition of eight facies from proximal to distal settings, with different proportion of skeletal and non-skeletal components (ooids, peloids, intraclasts and oncoids). Based on the facies distribution, the studied sequence was further divided into four episodes. The lower episodes (C1, C2) correspond to the development of mud to grain-supported mid-ramp facies (intraclastic, skeletal, peloidal, oolithic) and coral-microbial reefs. The upper episodes (C3, C4) cover the rapid progradation of proximal facies. Episode C3 is characterized by the widespread presence of the red algae Marinella lugeoni, which marks the transition area between the higher energy inner ramp and the mid-ramp domain.

The studied outcrops allowed the identification and georeferencing of 274 coral-microbial buildups, showing conical to pinnacle cylindrical geometry up to 19 m thick. These buildups accumulated during the main episodes of accommodation gain, corresponding to the transgressive and early highstand stages of Sequence C (i.e., episodes C1, C2). Most of coral-microbial reef growth occurred in the mid-ramp. Towards the inner-middle ramp transition, local amalgamations and eventual development of reef ribbons upto50m long are observed.

The quality of Jabaloyas outcrops provides precise data for the creation of 3D models. Two models were developed: a Full-field Model allowing calculation of volume and proportion of inter-reef and post-reef facies; and a Sector Model, where the reef bodies were incorporated. These models have been used as a template for further test on diagenesis overprint and resulting distribution of reservoir bodies and connectivity for sector simulation (phenomenology purposes) with real reservoir/field data. The precise reconstruction of reefal and inter-reef facies might be used as a template for further tests on diagenesis. For this purpose, integrated analysis within different techniques (Electromagnetic multifrequency broadband survey and Magnetometry) and Ground Penetrating Radar resolutions has exhibited a potential approach to develop a high-resolution 3D reconstruction of pinnacles. Preliminary results have been published in Pueyo-Anchuela et al. (2011).

http://dx.doi.org/10.5209/rev_JIGE.2013.v39.n1.41761

Acknowledgements

This research has been funded by TOTAL, S.A. Financial support was also provided by the H54 Research Group "Reconstrucciones paleoambientales" funded by the Gobierno de Aragon and the European Social Fund. We warmly thanks to the editor Javier Martin Chivelet and to Giovanna Della Porta for very helpful comments and suggestions on the original version of the manuscript.

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G. San Miguel (1) *, M. Aurell (1), B. Badenas (1), V. Martinez (1), B. Caline (2), C. Pabian-Goyheneche (2), J.P. Rolando (2), N. Grasseau (3)

(1) University of Zaragoza, c/Pedro Cerbuna, 12, 50009--Zaragoza, Spain galosanmiguel@gmail.com; maurell@unizar.es; bbadenas@unizar.es; vicmartinezgarcia@hotmail.com

(2) Total E&P, Technology Centre, CSTJF, Avenue Larribau, F-64018 Pau Cedex, France bruno.caline@total.com; cecile.pabian-goyheneche@total.com; jean-paul.rolando@total.com

(3) EGID, University of Bordeaux, 3-1. 33607 Pessac Cedex-France nicolas.grasseau@free.fr

* corresponding author

Received: 04/09/2012 / Accepted: 02/04/2013
Fig. 16.--Relative facies proportions
based on volume calculation in each
episode. Once the Full field Model has
been created, the volume calculation
was automatically obtained for three
dimensional bodies that can be split by
assigning sedimentological properties.

Fig.16.--Proporcion relativa de las facies
en el modelo total y separada por estadios
evolutivos. Una vez creado el
modelo 3D, la separacion y calculo de
volumenes de cuerpos por propiedades
es automatica.

Facies Proportion

Sequence C

Facies 6       7%
Facies 8       1%
Facies 4       3%
Facies 3       7%
Facies 2      16%
Facies 1      43%
Facies 5      11%
Facies 7      12%

Facies Proportion

Seq C1

Facies 1      19%
Facies 4       9%
Facies 3      23%
Facies 2      49%

Facies Proportion

Seq C2

Facies 4       1%
Facies 2       1%
Facies 1      98%

Facies Proportion

Seq C3

Facies 1      68%
Facies 5      32%

Facies Proportion

Seq C4

Facies 8       6%
Facies 6      34%
Facies 7      60%

Note: Table made from pie chart.
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Title Annotation:texto en ingles
Author:San Miguel, G.; Aurell, M.; Badenas, B.; Martinez, V.; Caline, B.; Pabian-Goyheneche, C.; Rolando, J
Publication:Journal of Iberian Geology
Date:Jul 1, 2013
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